Serum enzymes

Leakage of enzymes from cells

Enzymes are retained within their cells of origin by the plasma membrane surrounding the cell. The plasma membrane is a metabolically active part of the cell, and its integrity depends on the cell’s production of adenosine triphosphate (ATP). Any process that impairs ATP production by depriving the cell of oxidizable substrates or by reducing the efficiency of energy production by restricting the access of oxygen (ischemia or anoxia) promotes deterioration of the cell membrane. The earliest sign of impaired energy metabolism is the efflux of potassium with influx of sodium; water thus accumulates within the cell, causing it to swell. The next and most serious stage is the entry of calcium, which stimulates intracellular enzymes, leading to both cell damage and disruption of the cell membrane. Finally, free radicals formed during these processes may cause further damage. The membrane becomes leaky; if cellular injury becomes irreversible, the cell will die, although enzyme loss may also occur without the occurrence of irreversible injury. Small molecules are the first to leak from damaged or dying cells followed by larger molecules, such as enzymes and other proteins. Cytosolic proteins appear early on in the plasma followed much later by mitochondrial and membrane-bound enzymes. It appears that ATP must decline to below a certain level before substantial enzyme release occurs. Ultimately, the complete content of the necrotic cells is discharged.

Because of very high concentrations of enzymes within the cells—thousands or even tens of thousands times greater than concentrations in extracellular fluid—and because extremely small amounts of enzyme can be detected by their catalytic activity, increased enzyme activity in the extracellular fluid or plasma is an extremely sensitive indicator of even minor cellular damage, some causes of which are listed in Table 32.1 .

A reduction in the supply of oxygenated blood perfusing any tissue will promote enzyme release, such as occurs in myocardial infarction (MI). Cells of the affected region rapidly begin to deteriorate and die, releasing their protein and enzyme contents to the systemic circulation, which accounts for the rapid rise in serum biomarkers that is characteristic of this condition. The liver is also very sensitive to hypoxia, which can result from diminished cardiac output (heart failure) or other causes. Direct attack on the cell membranes by such agents as viruses or organic chemicals also causes enzyme release, which is particularly important in the case of the liver. Skeletal muscles also contribute enzymes to blood. Again, the cause may be poor perfusion, hypothermia, or direct trauma to the muscles (crush injuries). Infection, inflammation (polymyositis), degenerative changes (dystrophies), drugs (e.g., statin-induced myotoxicity), and alcohol (alcoholic myopathy) may also cause enzyme leakage from myocytes.

Acute pancreatitis results when digestive enzymes of the exocrine pancreas after a pancreatitis-causing noxious stimulus find their way into the parenchyma, and autodigestion of pancreatic tissue ensues. Key among these digestive enzymes is trypsin, which activates other proteolytic enzymes leading to pancreatic injury. Mutations in trypsin and other genes thought to be associated with pathologic enzyme activation (such as serine protease inhibitor 1) have been found in familial forms of acute pancreatitis.

Efflux of enzymes from damaged cells

When conditions for leakage of enzymes from cells have occurred, the speed and extent to which the process is reflected in enzyme changes in the blood depend on several factors.

The driving force of enzyme release is the steep concentration gradient that exists between the interior and the exterior of the cells. The rate of escape of enzyme molecules is presumably controlled to some extent by diffusion; therefore smaller enzyme molecules might be expected to appear in the extracellular fluid earlier than larger ones.

The way in which released enzyme molecules are transferred from the interstitial fluid to the blood varies from one tissue to another; they may pass directly through the capillary wall, or lymphatic transfer may occur. Direct transfer largely occurs in the liver, which is a highly vascular tissue with many permeable capillaries, although evidence suggests that liver enzymes may also be subject to lymphatic transfer. On the other hand, the capillaries of skeletal muscle are relatively impermeable, and in this tissue it is probable that released enzymes mainly reach the circulatory system through drainage from the lymphatic system. Lymph drainage is also important in transporting enzymes released from damaged intestinal, pancreatic, and myocardial cells to the circulation, although after MI, a minor proportion of myocardial enzymes reaches the circulation also by direct capillary transfer.

The intracellular location of the leaking enzymes affects the rates at which they appear in the circulation. As would be expected, the most sensitive indicators of cell damage are the molecules that are present in the soluble fraction of the cell. Release of structurally bound membrane proteins requires both a leaky cell membrane and a dissociation or degradation, which is a slower process. Enzymes associated with subcellular structures, such as mitochondria, are less readily released into the circulation and often indicate irreversible cellular injury. This fact has been used in attempts to distinguish reversible leakage, presumed to reflect damage only to the cell membrane, from necrotic lesions, in which intracellular structures are destroyed.

The relation between tissue injury and the appearance of enzymes in the circulation is most clearly seen in MI, in which a relatively short episode of damage is followed by a rapid transfer of enzymes to the circulatory system. About 24 hours after an MI, the pattern of relative activity of various enzymes in the circulatory system closely resembles that in myocardial tissue. These relationships are less clearly recognized in other conditions, such as chronic liver disease, in which enzyme release is a process that continues over a period of time. The pattern of relative enzyme activities in serum in chronic disease may also become distorted by differential rates of removal of enzymes from the circulation and possibly by differential changes in rates of enzyme synthesis in affected tissue.

Release of enzymes from damaged or dying cells and changes in the rate of enzyme production constitute the most important mechanisms by which changes in enzyme activity in the serum or plasma are produced. However, other possibilities exist and appear to account for some changes of diagnostic importance. For example, much of the γ-glutamyltransferase (GGT) activity of liver cells is located on their exterior surfaces. Ectoenzymes such as this may be eluted from the surfaces, especially when the detergent action of bile salts due to their accumulation following cholestasis is increased. This process does not involve cell damage in the sense of increased membrane permeability, as evidenced by lack of correlation between activities in the serum of GGT and the aminotransferases in liver disease of different types.

Altered enzyme production

Small amounts of intracellular enzymes physiologically present in the plasma can be assumed to result from wear and tear of cells or leakage of enzyme from healthy cells. This contribution of enzymes to the circulating blood may decrease as the result of a genetic deficiency of enzyme production (e.g., as is the case for ALP in hypophosphatasia, due to loss-of-function mutation(s) of the gene that encodes the tissue nonspecific isoenzyme of ALP, or in individuals homozygous for the “silent” gene for serum cholinesterase) or when enzyme production is depressed as a result of disease (e.g., cholinesterase in liver disease). However, cases in which enzyme production is increased are generally of more interest in diagnostic enzymology. For example, an increase in the number and activity of ALP–producing osteoblasts of bone is responsible for the increased concentration of ALP in the serum of normally growing children. Increased osteoblastic activity also accounts for increased concentrations of this enzyme in the serum in various types of bone disease.

The process of enzyme induction also increases enzyme production. An example of such induction is the increased activity of GGT in serum, which results from administration of drugs such as barbiturates or phenytoin, and from intake of ethanol.

Clearance of enzymes

Significant evidence is available about the way in which enzymes are cleared from the circulation. Few enzyme molecules are small enough to pass through the glomerulus of the kidney; therefore urinary excretion is not a major route for elimination of enzymes from the circulation. An exception to this is α-amylase (molecular weight, 54–62 kDa), which is the only plasma enzyme physiologically found in urine; increased concentrations of this enzyme in the blood (e.g., after acute pancreatitis) are accompanied by increased excretion in the urine.

Most enzymes are not inactivated in the plasma but are rapidly removed, probably by the reticuloendothelial system, such as the bone marrow, spleen, and liver (Kupffer cells), or, to a lesser extent, by nearly all cells in the body. The mechanism appears to consist of receptor-mediated endocytosis (the process of recognition, specific accumulation, and uptake of protein by specific cell surface receptors followed by fusion with lysosomes, digestion of ingested protein, and recycling of the receptor back to the cell membrane). For example, hepatic Kupffer cells have been shown to take up several tissue-derived enzymes—such as creatine kinase, adenylate kinase (AK), cytoplasmic and mitochondrial aspartate aminotransferase, and malate and alcohol dehydrogenases—by receptor-mediated endocytosis, which may have affinity for lysine residues on these enzymes. The adult isoform of intestinal ALP is a galactosyl-terminal glycoprotein that reacts with a galactosyl-specific receptor on the hepatocyte membrane and undergoes subsequent endocytosis. This process is rapid, accounting for the extremely short plasma half-life of this isoform. However, in hepatic cirrhosis, in which considerable reduction in parenchymal cell mass often occurs, the plasma concentration and half-life of the isoform increase. Other ALP isoenzymes and isoforms are sialoglycoproteins that do not react with the galactosyl receptor and therefore are protected from rapid uptake from blood. Indeed, examples are known of excessive sialylation of ALPs produced by malignant cells, prolonging their plasma half-lives and facilitating their detection. This example illustrates the importance of understanding the processes by which enzymes are cleared from plasma.

The half-lives of enzymes in plasma vary from a few hours to several days, but in most cases, the average half-life ( t _1/2 ) is 6 to 48 hours. Rates of decay may also be expressed as k _d values with units of hour ⁻¹ —the fractional disappearance rate—and the relationship to t _1/2 values is as follows:

Typical disappearance rates from human blood for clinically relevant enzymes are shown in Fig. 32.1 .

Clinical significance

Creatine kinase determination is the preferred laboratory test in case of suspected muscle damage and any clinical setting where symptoms or history suggest possible skeletal muscle damage should be investigated with its measurement. Serum CK is increased in nearly all patients when injury, inflammation, or necrosis of skeletal (or heart) muscle occurs.

Increase of serum CK activity may be the only sign of subclinical neuromuscular disorders. In case series, 30 to 44% of asymptomatic subjects with persistently elevated CK [up to fivefold the upper reference limit (URL)] have myopathy. Serum CK activity is greatly elevated in all types of muscular dystrophy. In progressive muscular dystrophy (particularly Duchenne sex-linked muscular dystrophy), enzyme activity in serum is highest in infancy and childhood (7 to 10 years of age) and may be increased long before the disease is clinically apparent. Serum CK activity characteristically falls as patients get older and as the mass of functioning muscle diminishes with progression of the disease. Of asymptomatic female carriers of Duchenne dystrophy, 50% to 80% show three- to sixfold increases in CK activity.

High values of CK (up to 50-fold the URL in active disease) are noted in viral myositis, polymyositis, immune-mediated and other inflammatory myopathies. However, in neurogenic muscle diseases, such as myasthenia gravis, multiple sclerosis, poliomyelitis, and Parkinsonism, serum enzyme activity is not increased. Very high activity is also encountered in malignant hyperthermia, an inherited life-threatening condition characterized by high fever and brought on by administration of inhalation anesthesia (usually halothane) to the affected individual. Molecular genetic investigations have confirmed the skeletal muscle type ryanodine receptor to be the major malignant hyperthermia locus with more than 70% of families carrying a mutation in this gene.

In acute rhabdomyolysis caused by crush injury, with severe skeletal muscle destruction, serum CK activities exceeding 200 times the URL may be found. In this condition, a very high serum CK, mirroring myoglobinuria and its heme-induced mechanism of renal injury, has been associated with the risk of development of acute kidney injury (AKI). If the CK remains below 5000 U/L (about 30 times the URL) during the first 3 days after the insult, the probability of developing AKI appears to be low.

A transitory increase of CK may help distinguish generalized tonic-clonic seizures from syncope or psychogenic non-epileptic seizures, but given the moderate sensitivity, the test is not useful in ruling out epileptic seizures. The mechanism causing muscle damage after generalized epileptic seizures is thought to be similar to that of exertional muscle damage. Serum CK can also be mildly increased by other direct trauma to muscle, including intramuscular injection and surgical intervention. Finally, a number of drugs, when given at pharmacologic doses, can increase serum CK activities. The drugs principally responsible are statins, fibrates, antiretrovirals, and angiotensin II receptor antagonists. Up to 5% of statin users develop CK elevation. The clinical spectrum of statin-induced myotoxicity includes asymptomatic rise in serum CK activity, myalgia, myopathy/myositis, and rhabdomyolysis. Rarely, a statin-associated autoimmune myopathy (anti-3-hydroxy-3-methylglutaryl-coenzyme A reductase antibodies) is also possible ( Table 32.4 ). A single-nucleotide polymorphism in the SLCO1B1 gene on chromosome 12, which encodes a transport protein involved in regulation of hepatic statin uptake, has been associated with a higher risk of statin-induced myopathy. Routine monitoring of CK in asymptomatic patients is not recommended; however, CK must be assessed in patients presenting with muscle pain and weakness and statin treatment stopped if values increase more than fourfold the URL, afterward monitoring normalization of CK.

Hypothyroidism is a common cause of endocrine myopathy. Up to 60% of hypothyroid subjects show an average increase of CK activity fivefold greater than the URL, so that one setting where thyroid hypofunction should be considered is represented by patients with unexplained persistent serum CK elevation. The major isoenzyme present is CK-MM, suggesting muscular involvement. Serum CK activity falls within the reference range after a few weeks from the start of replacement hormone therapy. Skeletal muscle that is diseased or chronically damaged (e.g., by extreme exercise) may contain significant proportions of CK-MB owing to the phenomenon of “fetal reversion,” in which fetal patterns of protein synthesis (in the case of CK, the B monomer) reappear. Thus serum CK-MB isoenzyme may increase in such circumstances. This explanation may also account for the increased CK values sometimes observed in chronic renal failure (uremic myopathy).

Changes in serum CK and its MB isoenzyme after acute MI were the mainstay of diagnosis for many years. However, it is now more clinically appropriate to use cardiac-specific troponin I or T (see Chapter 48 ).

During physiologic childbirth, a sixfold increase in maternal serum CK activity occurs. Surgical intervention during labor further increases the activity of CK in serum. CK-BB may be increased in neonates, particularly in brain-damaged or very-low-birth-weight newborns. The presence of CK-BB in blood, usually at low concentrations, may, however, represent a physiologic finding in the first days of life.

Determination of creatine kinase activity

Numerous photometric, fluorometric, and coupled enzyme methods have been developed for the assay of CK activity using the forward (Cr → CrP) or the reverse (Cr m CrP) reaction. Currently, all commercial assays for total CK are based on the reverse reaction, which proceeds about six times faster than the forward reaction.

Creatine kinase catalyzes the conversion of CrP to Cr with concomitant phosphorylation of ADP to ATP. The ATP produced is measured by hexokinase (HK)/glucose-6-phosphate dehydrogenase (G6PD)–coupled reactions that ultimately convert oxidized nicotinamide adenine dinucleotide phosphate (NADP ⁺ ) to reduced nicotinamide adenine dinucleotide phosphate (NADPH), which is monitored spectrophotometrically at 340 nm. The assay is optimized by adding (1) N -acetylcysteine to activate CK, (2) ethylenediaminetetraacetic acid (EDTA) to bind Ca ²⁺ and to increase the stability of the reaction mixture, and (3) adenosine pentaphosphate (Ap ₅ A) in addition to AMP to inhibit AK. In 2002, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) developed a reference procedure based on this reaction principles for the measurement of CK at 37 °C, acting as the basis for standardization of enzyme measurements (see Chapter 25 ). Twelve years later, the accuracy of CK results was assessed in a study involving four European countries. The authors concluded that CK measurements are now satisfactorily standardized among different commercial assays.

Specimens for CK analysis include serum and heparin plasma. Anticoagulants other than heparin should not be used in collection tubes because they inhibit CK activity. CK activity in aliquoted serum is relatively unstable and is rapidly lost during storage. Average stabilities are less than 8 hours at room temperature, 48 hours at 4 °C, and 1 month at −20 °C. Therefore the serum specimen should be chilled to 4 °C if the sample is not analyzed immediately and stored at −80 °C if analysis is delayed for longer than 30 days. Hemolyzed specimens are unsatisfactory because enzymes and intermediates (AK, ATP, and G6P) liberated from the erythrocytes may affect the lag phase and side reactions occurring in the assay system. To this regard, commercially available measuring systems may show a different degree of interference depending on the optimization of the reagent in terms of AK inhibition.

Reference intervals

Serum CK activity is subject to a number of physiologic variations. It is influenced by sex, age, ethnicity, muscle mass, and physical activity. Males have higher values than females, and blacks have higher values than nonblacks. Additionally, there is a CK decline with aging in males. The distributions of CK activity are notably skewed with a tail toward higher values in reference populations. In white subjects, the reference interval was preliminarily found to be 46 to 171 U/L for men and 34 to 145 U/L for women when measured with an assay traceable to the IFCC 37 °C reference procedure. Some studies also show Asian male individuals have significantly higher CK values (58 to 261 U/L) than Caucasian males. Other authors suggested an upward adjustment of CK URLs. Accordingly, one set of proposed reference intervals, based on European residents from different backgrounds, was: 47 to 322 and 29 to 201 U/L for white men and women, 47 to 641 and 37 to 313 U/L for Asian men and women, and 71 to 801 and 48 to 414 U/L for black men and women, respectively. Newborns generally have higher CK activity (up to 10 times the URL in adults) resulting from skeletal muscle trauma during birth. Serum CK in infants decreases to the adult reference interval by 6 to 10 weeks of age.

CK activity in the serum of healthy people is due almost exclusively to CK-MM activity (although small amounts of CK-MB may be present) and is the result of physiologic turnover of muscle tissue. Physical exercise, particularly if unaccustomed, can increase serum CK activity, which may ascend as high as 10 times the URL within 24 hours of activity. Accordingly, in assessing asymptomatic CK elevations, the test should be repeated after a week of rest.

Methods for separation and quantification of creatine kinase isoenzymes

Even if not routinely used, being labor intensive and requiring interpretative skills, electrophoretic methods are useful for the separation of all CK isoenzymes. The isoenzyme bands are visualized by incubating the support (e.g., agarose or cellulose acetate) with a concentrated CK assay mixture using the reverse reaction. NADPH formed in this reaction is then detected by observing the bluish-white fluorescence after excitation by long-wave (360 nm) ultraviolet light. NADPH may be quantified by fluorescent densitometry, which is capable of detecting bands of 2 to 5 U/L. The mobility of CK isoenzymes at pH 8.6 toward the anode is BB > MB > MM, with the MM remaining cathodic to the point of application. The discriminating power of electrophoresis also allows the detection of abnormal bands (e.g., macro-CK) (see Fig. 32.3 ).

Immunoassays are applicable to the direct measurement of CK-MB. Immunoinhibition techniques measuring the catalytic activity of the B subunit of CK dimer were first introduced. Currently, the recommended approach is to measure concentrations of the CK-MB protein (“mass”) by using “sandwich” immunoassays with monoclonal antibodies specifically recognizing the MB dimer. Mass assays are more sensitive than activity-based methods with a limit of detection for CK-MB of less than 1 μg/L. Other advantages include sample stability, noninterference with hemolysis, anticoagulants or other catalytic activity inhibitors, and full automation. With CM-MB mass assays, the URL for males is 5.0 μg/L, with values for females being less than male values.

Clinical significance

Serum ALD determinations have been of some clinical interest in primary diseases of skeletal muscle, such as progressive muscular dystrophy and polymyositis. In general, however, measurement of ALD activity in the serum of subjects with suspected muscle disease does not add information to that available more readily from measurement of CK. Accordingly, ALD measurement is now considered redundant and obsolete, and should be discouraged.

Methods for measurement of aldolase activity

All assay methods are based on the forward ALD-catalyzed reaction. In the analytical approach on which all commonly used procedures and kits are based, the ALD reaction is coupled with two other enzyme reactions. Triosephosphate isomerase (EC 5.3.1.1) is added to ensure rapid conversion of all GLAP to DAP. Glycerol-3-phosphate dehydrogenase (EC 1.1.1.8) is added to reduce DAP to glycerol-3-phosphate, with NADH acting as hydrogen donor. The decrease in NADH concentration is then measured.

Aldolase activity in serum is quite stable. Activity is unchanged at ambient temperatures for up to 48 hours and at 4 °C for several days. Hemolyzed specimens should not be used; plasma is preferred over serum because of the possible release of platelet enzyme during the clotting process.

Reference intervals

The reference interval for the activity of ALD in adults is 2.5 to 10 U/L, measured at 37 °C. However, a definite sex difference has been noted, with men having higher values. Serum ALD activity in neonates is fourfold that seen in adults, and in children is twice that in adults. Adult values are attained by the time the child reaches puberty.

Clinical significance

Liver disease is the most important cause of increased aminotransferase activity in serum and represents the indication for the measurement requests (see Chapter 51 ). Although serum activities of both AST and ALT become increased whenever disease processes affect liver cell integrity, ALT is the more liver-specific enzyme. Serum increases of ALT activity are rarely observed in conditions other than parenchymal liver disease ( Table 32.6 ). Moreover, increases of ALT activity persist longer than those of AST activity. Thus the incremental benefit of routine determination of AST, in addition to ALT, is limited, making the joint order an important source of redundant duplication. Laboratories reporting abnormal ALT results (e.g., >URL) may choose to offer AST as an automatic reflex test and calculate the AST-to-ALT ratio (AAR, also known as the De Ritis ratio) because it provides useful diagnostic and prognostic information in clinical hepatology. In most types of liver disease, ALT activity is higher than that of AST; exceptions may be seen in alcoholic hepatitis, hepatic cirrhosis, and liver neoplasia. An AAR of ≥2 is suggestive and an AAR of ≥3 is highly suggestive of alcoholic liver disease, as mitochondrial injury by alcohol results in more AST leakage. Aminotransferase activities observed in cirrhosis vary with the status of the cirrhotic process and range from the URL to four times higher, with an AAR greater than 1. This appears to be attributable to a reduction in ALT production in a damaged liver, associated with reduced clearance of AST in advancing liver fibrosis. An AAR of 1 or greater has an approximately 90% positive predictive value for diagnosing the presence of advanced fibrosis in patients with chronic liver disease. Furthermore, the amount of increase in the AAR can reflect the grade of fibrosis in these patients. Two- to fivefold increases of aminotransferases may occur in patients with primary or metastatic carcinoma of the liver, with AST usually being higher than ALT, but activities are often normal in the early stages of malignant infiltration of the liver. It is worth mentioning that before interpreting the AAR, contributions to AST from other nonhepatic tissues should be excluded.

In viral hepatitis and other forms of liver disease associated with acute hepatic necrosis, serum AST and ALT activities are increased even before the clinical signs and symptoms of disease (e.g., jaundice) appear. Activities for both enzymes may reach values as high as 100 times the URL, although 10-fold to 40-fold increases are most frequently encountered. The most efficient aminotransferase threshold for diagnosing acute liver injury lies at seven times the URL (sensitivity and specificity >95%). In acute viral hepatitis, peak values of aminotransferase activity occur between the 7th and 12th days; activities then gradually decrease, reaching physiologic concentrations by the 3rd to 5th week if recovery is uneventful. Peak activities bear no relationship to prognosis and may fall with worsening of the patient’s condition due to lack of further functional hepatocytes to continue enzyme release.

Persistence of increased ALT for longer than 6 months after an episode of acute hepatitis is used to diagnose chronic hepatitis. Most patients with chronic hepatitis have maximum ALT less than five times the URL. ALT has been reported persistently normal in 15 to 50% of patients with chronic hepatitis C, but the likelihood of continuously normal ALT decreases with an increasing number of measurements. Therefore in patients with acute hepatitis C, ALT should be measured periodically over the next 1 to 2 years to determine if it becomes and stays normal.

The picture in toxic or ischemic hepatitis is different from that in infectious hepatitis. In the hepatic injury induced by acetaminophen poisoning, the aminotransferase peak is more than 85 times the URL in 90% of cases—a value rarely seen with acute viral hepatitis. Furthermore, AST and ALT activities typically peak early and fall rapidly.

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of aminotransferase increases other than viral and alcoholic hepatitis. NAFLD includes a spectrum of liver pathology, from simple steatosis without inflammation to nonalcoholic steatohepatitis (NASH), in which inflammatory changes and focal necrosis may progress to liver fibrosis, cirrhosis, and hepatic failure. NAFLD is now considered to be an additional feature of the “metabolic syndrome.” Indeed, serum aminotransferase elevation in NAFLD is associated with higher body mass index, waist circumference, serum triglycerides, and fasting insulin and lower high-density lipoprotein (HDL) cholesterol—all features characteristic of this syndrome. Given the high prevalence of NAFLD and its potential morbidity, early identification of this condition by screening programs using ALT has been advocated. However, which enzyme threshold should be applied is still a matter of debate. As a low diagnostic sensitivity (<50%) is obtained using URL, some expert groups recommended lowering the ALT cutoffs to avoid missing cases with disease present. However, some have argued against the universal application of these lower ALT cutoffs due to the risk of overdiagnosis and unnecessary further evaluation.

Slight or moderate increases of AST and ALT activities have been observed after administration of various prescribed medications, such as nonsteroidal anti-inflammatory drugs, antibiotics, antiepileptic drugs, statins, or opiates. Over-the-counter medications and herbal preparations are also implicated. It should be noted that some drug-induced increases in serum aminotransferases resolve despite continued treatment with the offending drug—a process termed “adaptation.” A helpful resource that is available to ascertain whether a drug or supplement may be hepatotoxic is the website livertox.nih.gov. In patients with increased aminotransferases, negative viral markers, and a negative history for drugs or alcohol ingestion, the diagnostic workup should include less common causes of chronic hepatic injury, such as autoimmune hepatitis, primary biliary cholangitis, sclerosing cholangitis, celiac disease, hemochromatosis, Wilson disease, and α1-antitrypsin deficiency. In pregnancy-associated liver disorders (i.e., intrahepatic cholestasis of pregnancy and acute fatty liver of pregnancy) and other diseases unique to pregnancy with possible liver involvement (i.e., hyperemesis gravidarum and preeclampsia/eclampsia) aminotransferase activities in serum may vary from mild to 20-fold elevations.

As might be expected from the high AST concentration in muscles, AST activity in serum also is increased after acute MI and in muscular dystrophy and myositis, although it is normal in muscular disease of neurogenic origin. Pulmonary emboli can increase AST to two to three times the URL, and slight to moderate increases are noted in acute pancreatitis, crushed muscle injury, and hemolytic disease (including HELLP syndrome in pregnancy, where an AST activity higher than two times the URL is one of the diagnostic criteria).

Several authors have described AST linked to immunoglobulins, or macro-AST. Typical findings include a persistent increase in serum AST activity, ranging from 2 to 50 times the URL, with normal ALT concentrations in an asymptomatic subject, with absence of any demonstrable pathology in organs rich in AST. Increased AST activity might reflect decreased clearance of the abnormal complex from circulation. Macro-AST has no known clinical relevance. However, identification is important to avoid unnecessary diagnostic procedures in these subjects. The laboratory demonstration of macro-AST in serum is obtained by differential precipitation with PEG 6000 (see the Amylase section later in this chapter). AST recovery in the supernatant after precipitation lower than 27% should be considered suggestive of the presence of macro-AST, although the optimal cutoff should be confirmed by the testing laboratory.

Methods for measurement of aminotransferase activity

The assay system for measuring aminotransferase activity contains two amino acids and two oxo-acids. Because no convenient method is available for assaying amino acids, formation or consumption of the oxo-acids is measured. Continuous-monitoring methods are commonly used to measure aminotransferase activity by coupling aminotransferase reactions to specific dehydrogenase reactions. The oxo-acids formed in the aminotransferase reaction are measured indirectly by enzymatic reduction to corresponding hydroxy acids, and the accompanying change in NADH concentration is monitored spectrophotometrically. Thus oxaloacetate, formed in the AST reaction, is reduced to malate in the presence of malate dehydrogenase (MD).

Pyruvate formed in the ALT reaction is reduced to lactate by LDH. The substrate, NADH, and an auxiliary enzyme, MD or LDH, are present in sufficient quantity, so that the reaction rate is limited only by the amounts of AST and ALT, respectively. As the reactions proceed, NADH is oxidized to NAD ⁺ . The disappearance of NADH is followed by measuring the decrease in absorbance at 340 nm. The change in absorbance per minute (Δ A /min) is proportional to the micromoles of NADH oxidized and in turn to micromoles of substrate transformed per minute. A preliminary incubation period is necessary to ensure that NADH-dependent reduction of endogenous oxo-acids in the sample is completed before 2-oxoglutarate is added to start the aminotransferase reaction. After a brief lag phase, the change in absorbance (Δ A ) is monitored. As already mentioned, supplementation with P-5′-P ensures that all aminotransferase activity of the sample is measured.

Because of the large numbers of AST and ALT activity measurements performed daily in medical laboratories throughout the world, standardization of aminotransferase measurements is a priority need for patient care. As discussed in Chapter 25 , the reference measurement system approach, based on the concepts of metrological traceability and the hierarchy of analytical measurement procedures, gives clinical laboratories and the medical community universal means of creating and ensuring the equivalence of results. In this system, the reference measurement procedure forms the highest metrological level and thereby constitutes the definition of the respective measurable enzyme quantity. Primary IFCC reference procedures are available for the measurement of catalytic activity concentrations of AST and ALT. ^, Values assigned to the manufacturer’s product calibrators and measurement results obtained with commercial measuring systems in daily practice should be traceable to these top-level reference measurement procedures, thus improving the equivalence of aminotransferase results. It should be remembered that the concept of the reference system is valid only if the reference procedure and corresponding commercial procedures have identical, or at least very similar, selectivity for the measured enzyme. Thus it will not be possible to calibrate procedures for aminotransferases that do not incorporate P-5′-P using a procedure that does, such as the IFCC reference procedure, because the ratio of preformed holoenzyme to apoenzyme differs among specimens. The lack of P-5′-P addition in the employed commercial reagents is the more frequent cause of unacceptably biased results of aminotransferases. ^, A recent assessment of ALT measurements among Canadian laboratories revealed a between-laboratory CV for ALT of 25%, which is likely not compatible with an interchangeability of results from different marketed assays.

AST activity in serum is stable for up to 48 hours at 4 °C. Specimens should be stored frozen if they are to be kept longer. ALT activity should be assayed on the day of sample collection because activity is lost at room temperature, 4 °C, and at −25 °C. ALT stability is better maintained at −70 °C. Hemolyzed specimens should not be used, especially when AST is measured, because of the large amount of this enzyme present in red cells.

Reference intervals

Using assays traceable to the IFCC reference procedure, the AST URL for adults, calculated as the 97.5th percentile of the reference distribution, was 34 U/L, with no significant sex-related differences. Conversely, a clear difference in ALT activities has been noted between men and women. By enrolling only reference individuals with body mass index less than 25 kg/m ² , the resulting URLs were 49 U/L for men and 33 U/L for women, respectively. ALT does not reveal a distinct age dependency during childhood, but serum AST activity in neonates and in children younger than 3 years old is twice that in adults. Adult values are attained by the time the child reaches puberty. A circadian variation has been observed in ALT activity, and serum concentrations of both aminotransferases tend to increase in the winter.

Clinical significance

Glutamate dehydrogenase is increased in the serum of patients with hepatocellular damage. Four- or fivefold increases are seen in chronic hepatitis; in cirrhosis, increases are only up to twofold. Very large rises in serum GLD occur in halothane toxicity, and notable increases are seen in response to some other hepatotoxic agents. A potential role of GLD for risk stratification of drug-induced liver injury (DILI) has been recently advocated.

Glutamate dehydrogenase potentially offers differential diagnostic potential in the investigation of liver disease, particularly when interpreted in conjunction with other enzyme test results. The key to this differential diagnostic potential is to be found in the intraorgan and intracellular distribution of the enzyme as discussed earlier in this chapter. As an exclusively mitochondrial enzyme, GLD is released from necrotic cells; therefore compared with hepatic disorders with extensive necrosis, release is less in diffuse inflammatory processes, and in these conditions, the release of cytoplasmic enzymes, such as ALT, is quantitatively more pronounced. GLD can therefore be of value in estimating the severity of liver cell damage.

Glutamate dehydrogenase is more concentrated in the central areas of the liver lobules than in the periportal zones. This pattern of distribution is the reverse of that of ALT. Pronounced release of GLD therefore is to be expected in conditions in which centrilobular necrosis occurs (e.g., as a result of ischemia, in halothane toxicity).