Micronutrients (Metals and Iodine)

Iron-deficiency Anemia

Estimates by the World Health Organization are that 30% of the world’s population is anemic, and of these, half is due to the most common form of anemia, iron-deficiency anemia ( IDA ). IDA is the result of inadequate consumption of dietary iron. The diminished capacity of persons with IDA results in lowered efficiency of job performance, indirectly affecting the economy. IDA in children inhibits growth and learning.

During menstruation , from 4 to 100 mg of iron is lost. A woman loses about 500 mg of iron with a pregnancy , and the newborn consumes iron-deficient mother’s milk. Infants and young children consuming cow’s milk have a risk of IDA because the high level of calcium in milk competes with iron for intestinal absorption . There may be conditions that result in lowered efficiency in iron absorption due to disease ( Crohn’s disease or celiac disease ), and there are other situations leading to decreased iron absorption, such as overuse of antacids containing calcium, gastric bypass surgery , a strict vegetarian diet , and poor general nutrition, especially in older adults. With IDA, there is a reduction in the number of red blood cells that require hemoglobin (containing heme iron) to carry oxygen to the tissues.

In addition to inadequate dietary consumption of iron, there are other causes of anemia: internal bleeding (e.g., from ulcers), tumors (e.g., colon cancer), heavy menstruation, and others. Small blood losses from gastrointestinal (GI) bleeding can cause anemia that could evade detection through testing hemoglobin in the stool, a test that might not be highly sensitive. When there is a condition of ulcers, aspirin or other nonsteroidal antiinflammatory drugs can cause internal bleeding. Dietary IDA is most common in women (20%) and 50% in pregnant women compared to 3% of males (men have larger stores of iron than women).

The uptake of iron from the diet is regulated by absorptive cells of the small intestine. The need for iron is about 20 mg/day. Only 10% of dietary iron is actually absorbed and about 1 mg is absorbed in a day from a typical diet containing 10–20 mg. Given the 120-day lifespan of a red blood cell , about 0.8% of red blood cells are degraded and replaced in a day; 2.5 g of iron are incorporated into hemoglobin with a daily turnover of 20 mg for synthesis of hemoglobin plus 5 mg for other functions. A few tenths of a gram of iron are bound to myoglobin and about 20 mg is distributed to proteins involved in electron transfer, especially those in the electron transport chain that generate adenosine triphosphate (ATP). About 1 g of iron is stored in ferritin for future use. Iron is reused through the plasma and amounts of iron in excess of usage are deposited in the iron stores, ferritin , or hemosiderin . Nonheme plant sources of iron in the ferric state are poorly absorbed by the small intestine. Most of the absorbed iron from the intestine is in the form of myoglobin and hemoglobin ; these forms account for about 60% or more of the iron in the body.

A person who is anemic usually feels cold because iron plays a role in temperature regulation . There is a limit to the amount of oxygen that can be delivered to the mitochondria for electron transport and ATP production because in IDA there is a decrease in muscle myoglobin, causing an increase in the production of lactic acid and a reduced content of hemoglobin in red blood cells. Enzymes that require nonheme iron as cofactors, such as NADH dehydrogenase and succinate dehydrogenase of mitochondrial metabolism, are affected as is ribonucleotide reductase , an enzyme involved in the synthesis of DNA. Other enzymes requiring iron, besides the mitochondrial enzymes, are heme-containing catalase and peroxidase that protect cells from reactive oxygen species (ROS) derived from hydrogen peroxide.

Bodily temperature is a reflection of the ability of tissues to form ATP energy, and iron (and copper) may affect the functioning of the thyroid gland that plays a role in the regulation of body temperature.

Vitamin deficiencies of folic acid and vitamin B12 (cobalamin) play a role in certain anemias. Oxidized iron (Fe ³⁺ ) in the diet is reduced to ferrous iron (Fe ²⁺ ) by ascorbic acid (vitamin C), and an absorbable ascorbic acid–iron complex is formed that enhances the intestinal absorption of nonheme iron . Oxidized iron is not absorbed as such and must be reduced to ferrous iron to be absorbed . Stomach acid is needed to release the Fe ³⁺ in ingested foods, so a condition in which there is a lack of hydrochloric acid in digestive juices ( achlorhydria ) can lead to the poor absorption of iron. Vegetarians who refrain from eating red meat (rich in iron) may also ingest low levels of vitamin B 12 . Cereals, soybeans (tofu and tofu derivatives), and other plant foods contain phytic acid that binds iron and other metals rendering them unabsorbable by the intestine. Phytic acid contains six phosphate groups providing a metal sequestering agent ( Fig. 19.1 ).

Table 19.1 lists some common foods that contain iron.

In addition to the foods listed in Table 19.1 , broccoli, spinach, and egg yolks are good sources of iron.

The recommended daily allowances for dietary iron are listed in Table 19.2 .

Mild anemia does not produce noticeable symptoms but intense anemia will produce many symptoms: pale skin color, shortness of breath, fatigue, palpitations on climbing stairs, leg cramps, dizziness, irritability, sore tongue, weakness, atrophy of taste buds, sores at corners of mouth, brittle nails, frontal headache, difficulty in sleeping, and decreased appetite. In advanced IDA, there can be problems in swallowing due to the formation of webs of tissue in the throat and esophagus ( Plummer–Vinson syndrome ). Iron-deficient persons may consume nonfood items like starch or clay, known as the condition of Pica , a behavioral disturbance of iron deficiency. Measurement of the blood hemoglobin level generates the diagnosis of iron deficiency. These measurements can be expressed as hematocrit values that are measurements of the volume of red blood cells as a percentage of the total blood volume where the normal values for males are 43%–49% and for females, 37%–43% ( Table 19.3 ).

Other measurements that can be made to define iron deficiency are red blood cell size , serum level of iron , and the iron-binding capacity of blood . In the differentiation between IDA and anemia caused by chronic disease, the measurements of serum ferritin and sustainable iron in tissue stores can be useful. The saturation of ferritin with iron does not become abnormal until the tissue stores of iron have been depleted. When the tissue stores of iron have been depleted, the hemoglobin concentration decreases but values of red blood cells may not become abnormal for several months (red blood cell half-life=120 days) after the tissue stores have been depleted. Because conditions of rapid growth require high levels of iron, infants, children, adolescents, and pregnant women are particularly vulnerable to iron deficiency . Especially, in children in poor conditions, parasitic worms can cause intestinal bleeding that leads to iron deficiency.

The treatment of IDA involves the oral administration of ferrous iron (Fe ²⁺ ) that corrects the deficiency. Normalization of the iron level is determined by the measurement of the blood hemoglobin level. Intravenous transfusion could be required where iron cannot be absorbed, or there are acute attacks of bleeding.

Measurements of serum ferritin are the most sensitive tests that can be carried out in the laboratory. Normally, the ferritin complex store houses about 4500 iron atoms in the ferrous form (Fe ²⁺ ) that is stored as the hexahydrate (Fe[H ₂ O] 62+ ). When there is less than 5% saturation of iron in ferritin storage, this always indicates iron deficiency. Normal value for females is >12% saturation of iron storage in ferritin and for males the value is >15%. Fig. 19.2 shows various aspects of ferritin and the storage of ferrous iron atoms.

Disease conditions, such as rheumatoid arthritis, inflammatory bowel disease, HIV, and heart failure, are accompanied by anemia.

Uptake of Iron During Digestion

In the diet, iron is in the form of both heme and nonheme iron bound to proteins. In the intestinal cells the endoplasmic reticulum (ER) heme oxygenase enzyme converts heme (heme, by itself, can be toxic) to biliverdin , carbon monoxide (the only reaction in the body producing CO expired by the lungs), and reduced iron, releasing the iron from heme, and the released iron is transferred into the body as nonheme iron. The ferrous iron can be incorporated into ferritin or exported from the cell. Biliverdin is converted to bilirubin by biliverdin reductase . These reactions are recorded in Fig. 19.3 .

In quiescent macrophages the breakdown products of heme are released into the soluble cytoplasm for incorporation into ferritin or for recycling from the cell to ferroportin for export. Ferroportin (SLC40A1) is a transporter that plays a role in intestinal absorption of iron and also in the release of iron from the interior of a cell. Major functions of ferroportin occur in intestinal cells for uptake, in hepatocytes, and in macrophages for release of cellular iron. A regulatory peptide ( hepcidin , produced in the liver) binds to ferroportin, causing ferroportin to be internalized and degraded. Ferroportin has been found to be essential in early development. Ferroportin-deficient experimental animals accumulate iron in intestinal enterocytes (involved in intestinal transport of iron), macrophages (where heme breakdown occurs), and hepatocytes (iron storage) that are the chief sites of ferroportin action. Hephaestin is highly expressed in the intestine and is required for the removal of iron from the enterocyte to the extracellular space and eventually into the bloodstream. Membrane-bound hephaestin is a ceruloplasmin (Cp) homolog and is a multicopper ferrooxidase converting Fe ²⁺ , which has been removed from the enterocyte on the basolateral side, to the Fe ³⁺ form. The roles of ferroportin and hephaestin in macrophage cells and in enterocytes are pictured in Fig. 19.4 .

Thus copper (in Cp ) is important in the oxidation of ferrous iron to ferric iron so that it can be transported in the plasma bound to transferrin . Hephaestin is a membrane-bound homolog of Cp that is required for iron transport in the intestine. The level of Cp is greatly reduced in certain hereditary diseases, such as Wilson’s disease , the Melk syndrome , and hereditary hemochromatosis . Mutations in the gene for Cp can lead to aceruloplasminemia , a rare human genetic disease. In this unusual case, iron is overloaded in brain, liver, pancreas, and retina. Iron overload in cells, mostly a result of hereditary hemochromatosis, an autosomal recessive condition, causes an accelerated rate of iron absorption in the intestine and progressive tissue iron deposition. This disease affects 0.5% of those of European ancestry mostly in the age range of 30–50 s, although there is occasional occurrence in children. The pathological effect of iron accumulation in a given organ is in the form of hemosiderin . Hemosiderin is a particle representing an iron storage complex that is formed by the breakdown of hemoglobin or an abnormal metabolic pathway of ferritin . Hemosiderin granules are yellowish in color and have diameters in the range of about 10–75 Å, and some contain iron hydroxide micelles of ferritin molecules in the form of ferritin and apoferritin and partially hydrated α-Fe ₂ O ₃ . In cases of iron overload, bloodletting is used as a treatment until the level of iron reaches a normal range. There are iron-chelating agents , such as the drug Deferoxamine that binds plasma iron and increases the elimination of iron in the urine and feces. One of the unfortunate effects of iron overload is that it enhances the rate of cancer cell growth and the rate of growth of infectious organisms.

Normally, heme is converted to bilirubin ( Fig. 19.3 ), and in hepatocytes, bilirubin is rendered more water soluble by the formation of bilirubin diglucuronide , a product of uridine diphosphate glucuronyl transferase ( Fig. 19.5 ).

Bilirubin diglucuronide is conjugated to cholesterol and excreted as a bile acid pigment through the intestine.

The absorption of dietary iron across intestinal epithelial cells is facilitated by the divalent metal ion transporter 1 ( DMT1 ). DMT1 is a 12-transmembrane domain protein consisting of about 586 amino acid residues. The fourth transmembrane domain is critical for the function of the transporter. Iron in the diet is transported from the intestinal lumen across the villus cell and exported from the cell at the basolateral side, mediated by transferrin. Transferrin regulates transport of iron from the villus and crypt cells to body tissues. The transferrin molecule (20%–45% saturated with iron) binds two iron atoms. During iron deficiency, crypt cells tend to move toward the villi to increase the efficiency of iron transfer to internal tissues. The transport of iron into the intestinal cells is reviewed in Fig. 19.6 , and the overall fates of dietary iron are illustrated in Fig. 19.7 .

Precisely 75% of the iron absorbed into the body becomes bound to proteins, such as hemoglobin that are involved in the transport of oxygen. The iron storage pool takes up 10%–20% of the absorbed iron, and this pool can be used in the formation of red blood cells (erythropoiesis). One molecule of ferritin can store as many as 4000 iron atoms in its mineral core ( Fig. 19.2 ). When excess iron is absorbed in the diet, more ferritin is produced to store the excess. Tissue cells have membrane receptors for transferrin–iron complexes. The transferrin receptors (TfRs) engulf and internalize transferrin–iron complexes.

There exists an iron-sensing mechanism for both TfR and ferritin at the level of messenger RNA (mRNA) (posttranscriptional mechanism). There are iron-responsive elements ( IREs ) in mRNA sequences that code for the translation of ferritin and TfR. When there is excess iron over what is in the immediate stores, iron binds to the IRE-binding protein ( IREBP ), which is an aconitase , and this binding event causes a change in the conformation of the IREBP so that it no longer binds to TfR mRNA . Consequently, the cell produces more ferritin ( Fig. 19.8 ).

ApoIREBP (without iron) binds to the IRE on the mRNA coding for TfR . The binding of IRE without iron stabilizes the mRNA-encoding TfR and permits its translation to proceed.

When cells become low in iron content, they continue to produce TfRs allowing more transferrin–iron complexes to enter the cells. As the level of iron accumulates in the cells, iron binds to IREBPs; as a result, they change their conformation and dissociate from the TfR mRNA. In the absence of bound IREBP, TfRs are degraded, and cells no longer produce TfRs. In the condition of low cellular iron , IREBPs bind to the IRE of ferritin mRNA and cause reduced translation of ferritin but the binding of apoIREBPs to TfR mRNA causes an increase in the translation of TfRs increasing the cellular capacity for the acquisition of more iron as it becomes available.

There are at least two IREBPs, named IREBP1 and IREBP2 . IREBP1, under conditions of elevated cellular content of iron, binds with an iron cluster (4Fe-4S) and adopts the conformation of aconitase that is not permissive for binding to the IRE. IREBP2, on the other hand, is degraded under conditions of high levels of iron. The affinities of different IREBPs for different IREs vary.

The IRE is located in untranslated regions of the mRNAs that translate proteins involved in iron metabolism, such as ferritin and TfRs. In ferritin mRNA a single IRE exists in its 5′ untranslated region. In the case of the TfR mRNA, several IREs exist in the 3′ untranslated region. The IREs in different mRNAs, that are bound by IREBPs, are conserved short stem-loops, shown in Fig. 19.9 .

The posttranscriptional regulation of translation by regulatory proteins is a new area and one that is sure to open up new pharmaceutical approaches.

Hepcidin, A Peptide Hormone, Is the Principal Regulator of Iron Homeostasis

The principal regulator of systemic iron homeostasis is the peptide hormone hepcidin. Hepcidin is produced in hepatocytes through the pathway: preprohepcidin (84 amino acids)→ prohepcidin (60 amino acids)→hepcidin (25 amino acids). The last conversion of prohepcidin to hepcidin is catalyzed by the convertase, furin . Secretion of hepcidin produced in the liver is regulated by the proteins: HFE (high iron) protein (encoded by the HFE gene ), hemojuvelin (HJV), and TfR2 . The HFE protein, the protein product of HFE gene (chromosome 6.22.2), also referred to as the hemochromatosis protein, appears to regulate the interaction between the TfR and transferrin . HFE protein is located mainly on the surface of liver and intestinal cells.

In iron deficiency the amount of hepcidin secreted from the liver is reduced, allowing for the normal functioning of ferroportin in releasing iron from the intestinal enterocytes and from iron stores in macrophages into the bloodstream. Conversely, when iron is in excess, more hepcidin is released from the liver resulting in the blockage of ferroportin so that iron is neither absorbed into the bloodstream from iron in enterocytes nor is it released into the bloodstream from macrophage stores. These events are illustrated in Fig. 19.10 .

Hemojuvelin, An Anchored Membrane Protein Stimulates Hepcidin Transcription Through Bone Morphogenic (Morphogenetic) Proteins and Smad

HJV (also named RGMc or HFE2) is a regulator of hepcidin production. It is tethered to the cell membrane by a glycosylphosphatidylinositol anchor . It complexes with bone morphogenic proteins ( BMPs ) that when activated by HJV signals Smads to form a complex that enters the nucleus and stimulates the transcription of hepcidin. HJV that is released from the cell to the extracellular fluid lasts there for more than 24 hours, whereas the predominant membrane-associated form (two chains stabilized by disulfide bonds) is not found in the extracellular fluid and disappears from the cell surface with a half-life of less than 3 hours. HJV consists of bound and soluble forms and three isoforms: RGMa (repulsive guidance molecule a) and RGMb are found in the nervous system, and RGMc is located in skeletal muscle and liver. The knockout of HJV in experimental animals produces a depression in the expression of hepcidin. Whereas the membrane-bound form of HJV stimulates the expression of hepcidin, the soluble form may suppress hepcidin expression. There appear to be two soluble isoforms and two membrane isoforms of HJV. The action of HJV as a coreceptor of BMP operating through Smads is summarized in Fig. 19.11 .

Heme Synthesis

Heme is a cofactor for several heme-containing proteins that function in oxygen binding or metabolism as well as in electron transport. Hemoglobins are the primary oxygen-binding proteins, and oxygen metabolism is carried out by oxidases, peroxidases, catalases, hydroxylases, and other heme proteins contain the heme prosthetic group, such as nitric oxide synthase and guanylic cyclase . Cytochromes , containing heme, are involved in the electron transport chain and as cofactors in hydroxylation reactions. The biosynthetic pathway for heme is located in the soluble cytoplasm and in mitochondria, and this pathway is identical in all mammalian cells; however, the regulation of heme biosynthesis may differ between erythroid and nonerythroid cells. Heme, the final product of the biosynthetic pathway, feeds back negatively on the synthetic process but the specific step at which the negative feedback operates may differ between different cell types.

Cellular levels of heme are carefully controlled so that there is a balance between the biosynthesis of heme and its catabolism by heme oxygenase . Developing erythroid cells synthesize about 10 times more heme than the liver hepatocytes, the second major producer of heme. Iron metabolism in a specific cell type determines the regulation and the rate of heme biosynthesis. The first enzyme in the heme biosynthetic pathway is 5-aminolevulinic acid synthase ( ALAS ). There are two genes specifying ALAS, one that encodes ALAS1 that is expressed ubiquitously and one that encodes ALAS2 that is specific to erythroid cells . The mRNA for the translation of ALAS2 contains an IRE in its 5′ untranslated region; this controls the induction of translation. The availability of iron controls the level of protoporphyrin IX in hemoglobin-synthesizing cells.

In nonerythroid cells the rate-limiting step in heme biosynthesis is catalyzed by ALAS1, synthesis of which is the object of negative feedback by heme (not in the case for erythroid cells). However, the concentration of heme decreases the access of iron from transferrin without affecting its use in heme synthesis. In erythroid cells, heme stimulates the transcription of the globin gene so that hemoglobin can be formed. The enzyme controlling the degradation of heme, heme oxygenase, turns out to be a major enzymatic antioxidant system in that it provides bilirubin that is an antioxidant.

The biosynthesis of heme begins with succinyl-coenzyme A (CoA) and glycine in the mitochondria to form γ- ALA through the ALAS reaction. ALA is exported to the cytoplasm where ALA dehydratase converts it to porphobilinogen . Porphobilinogen is then converted to uroporphyrinogen III (or uroporphyrinogen I) by uroporphyrinogen I synthase and uroporphyrinogen II cosynthase . Uroporphyrinogen III is converted to coproporphyrinogen III by uroporphyrinogen decarboxylase , and this product enters the mitochondria where it is converted to protoporphyrinogen IX by coproporphyrinogen III oxidase . Protoporphyrinogen IX is converted to protoporphyrin IX by protoporphyrinogen IX oxidase . In the final step, protoporphyrin IX is converted to heme by the action of ferrochelatase that incorporates iron into the molecule. The outline of the biosynthetic pathway is shown in Fig. 19.12 , and the chemical reactions are shown in Fig. 19.13 .

In Fig. 19.14 is shown a space-filling model of heme , and in Fig. 19.15 is shown a space-filling model of hemoglobin .

The final step in the synthesis of heme is the insertion of iron by ferrochelatase ( Fig. 19.13 part G ) into protoheme IX . Because all of the intermediates in the biosynthesis of heme are tetrapyrroles or porphyrins , the overall reactions can be classified as porphyrin synthesis , somewhat analogous to the synthesis of vitamin B12 in certain other (than mammalian) species. The enzyme ( ALA synthase ) catalyzing the initial step of heme synthesis is regulated by the intracellular concentrations of both heme and iron . When the cell concentration of iron is low from a deficient diet, especially in bone marrow cells and liver cells, there is a decrease in the synthesis of porphyrin to eliminate its toxicity if it accumulates.

Mutations in the genes for ALA synthase or for other enzymes in the heme biosynthetic pathway cause inability to form heme ( genetic porphyria ). A list of genetic porphyrias appears in Table 19.4 .

If the single gene for ferrochelatase is mutated, this can lead to erythropoietic protoporphyria , a disease that would be carried in the germ line (X chromosome). Porphyrias also may be acquired, for example, in liver damage. Hematin or heme arginate can be administered intravenously to limit a severe attack. Other medications to limit pain and other symptoms are in use.

ferrochelatase, the final enzyme in heme synthesis, is found in the inner mitochondrial membrane in such a position that its catalytic center faces the space of the mitochondrial matrix . The prosthetic group of the enzyme is an iron–sulfur cluster (2Fe-2S) in the form of a chelate of iron with cysteine residues. The binding of iron to ferrochelatase involves a histidine group (H207) and a glutamate residue (E87) functional in the catalytic center. The enzymatic reaction at the inner mitochondrial membrane involves another protein, frataxin , which acts as the iron donor for the iron–sulfur cluster. Holofrataxin (without its binding to the iron–sulfur cluster) binds to ferrochelatase on the inner mitochondrial membrane. It also binds to the iron–sulfur cluster synthetic unit . The dimer of ferrochelatase (active form) contains a frataxin-binding site on its matrix side. Holofrataxin is a high affinity–binding partner for holoferrochelatase. Holofrataxin can deliver iron to ferrochelatase and is the mediator for the final step in the synthesis of heme. Frataxin is involved in the biosyntheses of both heme and iron–sulfur clusters. Shown in Fig. 19.16 is the iron-binding site of frataxin where the iron atom is bound through coordination with residues of histidine and aspartate.

In Fig. 19.17 the function of frataxin within mitochondria is presented.

Holofrataxin (frataxin bound with iron) is used as an iron donor for the biosynthetic pathways of both heme and iron–sulfur cluster. Fig. 19.18 shows models for the regulation of frataxin in the cell when the holofrataxin (Hftx) level is low ( top figure ) or when the holofrataxin level is high ( bottom figure ).

Heme synthesis responds to oxygen deficiency (hypoxia) by an increase in ferrochelatase mRNA. There exists a hypoxia-inducible transcription factor, hypoxia-inducible factor 1 (HIF-1) . Two HIF-1-binding motifs have been found in the gene promoter of the ferrochelatase gene. When cellular oxygen levels are low, HIF-1 (HIF-1α) binds to the ferrochelatase gene promoter resulting in increased transcription of the enzyme. ROSs negatively affect HIF-1, as might be expected.

Hemoglobin Formation

Heme interacts directly with globin to form a specific heme–globin complex . Then, the heme pocket collapses around the porphyrin, and a bond is formed between the proximal histidine residue (of globin) and the heme iron atom. This interaction is (nonenzymatically) governed by the concentrations of the reactants, heme and globin. Hemoglobin is a heterotetramer containing two α-globin molecules and two β-globin molecules, each of which has a heme associated with it (four hemes as shown in Fig. 19.15 ). The globins are encoded by different genes: the α-globin gene is located on chromosome 16, while the β-globin gene is located on chromosome 11. α-Globin is a protein of 141 amino acids, while β-globin is a protein of 146 amino acids. Other globin variants also exist, and there is one gene for each of the other globin structures. The α-globin gene promoter functions independently, while the β-globin gene promoter is dependent on an enhancer.

Hemoglobin synthesis takes place as erythroid cells are differentiating from immature red blood cells to mature red blood cells. When the red cells mature, the synthesis of hemoglobin stops and the hemoglobin already formed must survive for 120 days, the lifetime of the erythrocyte. When the formation of β-globin is insufficient, there is a significant excess of α-globin over β-globin generating a group of thalassemias called β-thalassemias. α-Globin, by itself, is unstable and forms aggregates on the membranes of red blood cells, damaging them and causing anemia. Under normal conditions, there is a small excess of α-globin over β-globin without generating a thalassemia. This small excess of α-globin becomes bound to a chaperone-like protein, the α-hemoglobin stabilizing protein ( AHSP ). The interaction between α-globin and AHSP (association constant=10 7 M −1 ) is weaker than the interaction between α-globin and β-globin (association constant=10 10 M −1 ). Thus when a molecule of β-globin appears, it can displace the AHSP from the α-globin–AHSP complex, and it can interact with the released α-globin. The two globins interact about 1000 times more avidly (comparing association constants) than the interaction between α-globin and AHSP. AHSP forms a triple helix bundle, and α-globin has a surface region for the binding of AHSP, whereas β-globin does not have a surface domain that interacts with AHSP. Thus the appearance of β-globin molecules causes the dissociation of the α-globin–AHSP dimer by competition, and the hemoglobin tetramer can be formed (one dimer of α-globin+two hemes; one dimer of β-globin+two hemes). In the hemoglobin tetramer, the AHSP-binding surface (of the α-globin component) is buried, and since there is no such surface on the β-globin component, AHSP cannot bind.

The normal red blood cell contains about 4-mM hemoglobin with a normal excess of α-globin that is about 10%–20% (about 0.4–0.8 mM of the hemoglobin concentration). The cellular concentration of AHSP is about the same, 0.4 mM, so the excess of α-globin is in a complex with AHSP. In β-thalassemia the large excess of α-globin forms aggregates and causes the release of the iron atom from heme . The free iron atom can generate toxic oxygen-free radicals (ROS), and the free Fe ²⁺ can interact with peroxide (ROS) to form Fe ³⁺ and hydroxyl ions (OH+OH − ). Fe ³⁺ can interact with H ₂ O ₂ to form Fe ²⁺ and another radical (OOH+H + ). Experimentally, the addition of AHSP can reduce free radical formation generated by the excess of α-globin. It does this by trapping the iron associated with α-globin in the Fe ³⁺ state so that it does not interact with peroxide and cycle back to Fe ²⁺ (left side of Fig. 19.19A ).

Table 19.5 summarizes the characteristics of thalassemias.

α-Thalassemias also can occur when there is a defective synthesis of α-globin. In β-thalassemia, there is a decreased production of β-globin chains. β-Thalassemia is an autosomal recessive disease. There is a condition called thalassemia minor in which heterozygotes are carriers with only mild-to-moderate microcytic anemia . Only a single gene is involved in β-thalassemia; however, two genes are involved in α-thalassemia. Heterozygotes with a defect in a single gene do not express symptoms (α-thalassemia-2). Heterozygotes with defects in two genes express anemia (α-thalassemia-1), and the clinical symptoms are similar to β-thalassemia.

In the normal red blood cell, hemoglobin is formed with four reduced iron atoms in the hemes, and hemes with oxidized iron ( Fig. 19.19B ) will not be produced nor should there be an appearance of ROS (e.g., •OOH). Thus the reaction of A and C , in Fig. 19.19 , will form D followed by the additional reactions to form the normal hemoglobin (following the reductase action). When β-globin is formed, it binds heme immediately and forms hemoglobin with the slight excess of α-globin. β-Globin synthesis, therefore, seems to be the rate-limiting event in the normal formation of hemoglobin in the immature red blood cell.

Trace Elements