Nutritional Antioxidants

α-Lipoic acid

R-Alpha-lipoic acid (αLA) is synthesized in the mitochondria of plants and animals, including humans. Natural αLA is covalently bound to proteins via lysine; thus only minimal free αLA enters the circulation after biosynthesis or eating αLA-rich food. The lipoamide is a required cofactor for two enzymes in the citric acid cycle. It is also essential for the formation of a cofactor required in nucleic acid synthesis and for the metabolism of branched chain amino acids.

With oral supplements of free αLA, unbound αLA is transported to tissues. Free αLA is rapidly metabolized by the liver, so that the half-life in blood after absorption is only about 30 minutes, limiting the amount delivered. High tissue levels are short lived since most free αLA is rapidly reduced to dihydrolipoic acid (DHLA), as shown in Figure 15.1 .

Notwithstanding this transient availability, free αLA has been shown to be therapeutic for autoimmune liver disease by binding autoantibodies, for heavy metal intoxication by trapping circulating metals, for diabetic polyneuropathy by preventing oxidative damage, and for mushroom poisoning. Although not normally found in significant amounts in the skin, αLA is a good candidate for topical application:

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  As a small, stable molecule, it could successfully be percutaneously absorbed

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  As a potent antioxidant it might protect from ultraviolet (UV) and other free radical environmental changes

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  Because it is soluble in both aqueous and lipid environments, it can interact with oxidants and antioxidants in many cellular compartments.

Indeed, αLA has been found to penetrate rapidly into murine and human skin to dermal and subcutaneous layers. Two hours after application of 5% αLA in propylene glycol, maximum levels of αLA were attained in the epidermis, dermis, and subcutaneous tissue. The stratum corneum concentration of αLA predicted the penetration and levels in the underlying skin. 5% of the αLA was converted to DHLA in both the epidermis and dermis, leading the researchers to conclude that both keratino­cytes and fibroblasts reduce αLA.

DHLA, unlike αLA, has the capacity to regenerate the endogenous antioxidants vitamin E, vitamin C, glutathione, and ubiquinol, as illustrated in Figure 15.2 . This is clearly of great importance for skin, since UV exposure directly depletes ubiquinone (coenzyme Q ₁₀ (CoQ ₁₀ )) and vitamin E in particular, as well as vitamin C, thereby stressing the other linked antioxidants. Regeneration of these major membrane and cytosol antioxidants gives cascading protection. Increases in the other important antioxidants (intracellular glutathione and extracellular cysteine) are noted when αLA is added to cell cultures. Vitamin E-deficient animals do not show symptoms (weight loss, neuromuscular abnormalities) when supplemented with αLA.

Topical αLA with its metabolite DHLA might directly protect the skin from oxidative stress in several ways. Both αLA and DHLA are highly effective antioxidants as summarized in Table 15.1 . DHLA is actually the more potent form. Both successfully scavenge reactive oxygen species (ROS) in vitro and in vivo. However, pro-oxidant activity has been observed. This occurs when an antioxidant reacts with a ROS scavenger, forming a product that is more harmful than the scavenged ROS. Fortunately, αLA can act as an antioxidant against the pro-oxidant activity of DHLA ( ). Both αLA and DHLA further provide antioxidant activity by chelating Fe ²⁺ and Cu ²⁺ (αLA) and Cd ²⁺ (DHLA).

Also, αLA (but not DHLA) acts as an anti-inflammatory agent by reducing the production and inhibiting the binding of transcription factors such as nuclear factor kappa B (NF-κB), thereby indirectly affecting the gene expression of inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukins. DHLA (but not αLA) can repair oxidatively damaged proteins, which in turn regulate the activity of proteinase inhibitors such as α ₁ -AP, an inflammatory modulator. In vitro, both αLA and DHLA inhibit lipolysaccharide-induced nitric oxide (NO) and prostaglandin E ₂ (PGE ₂ ) formation and suppress inducible NO synthase (iNOS) but do not affect the expression of cyclooxygenase-2 (COX-2). In a mouse model, topical DHLA inhibits chemically induced activation of skin inflammation with a concomitant decrease in inflammatory modulators. Furthermore, topical DHLA (but not oral αLA) reduces chemically induced skin tumor incidence and multiplicity, and inhibits iNOS and COX-2 in a dose-dependent manner. As antioxidants, both αLA and DHLA are directly anti-inflammatory by virtue of their quenching oxidants secreted by leukocytes and macrophages at sites of inflammation.

As a potential antioxidant with an anti-inflammation activity, αLA might be an excellent topical cosmeceutical. Unfortunately this molecule is not stable because of its low melting point and the distorted dithiolane ring, which absorbs light around 330 nm (low wavelength UVA), causing decomposition. However, when αLA is exposed to UVB in the presence of thiol compounds, such as cysteine and homocysteine but not methionine, this decomposition of the αLA molecule is retarded and, instead, increased recovery of αLA ensues. Thus possibly formulating αLA with cysteine or homocysteine might improve its photochemical stability. The dithiolane ring structure further renders αLA quite vulnerable to thermal stimuli, causing decomposition and formation of interlocking polymers. This problem is more difficult to surmount when making a product with enough stability to assure an effective shelf-life.

Thus whether αLA can provide effective protection against UVB-induced damage is not clear. A single topical application of αLA (0.5 µmol/cm ² ) on pig skin was shown to reduce UVB-radiation-induced oxidative stress and lipid peroxidation, thereby reducing apoptosis. In contrast, other research reported that topical application of 5% LA is ineffective at suppressing UV-induced sunburn cells as evaluated by apoptotic markers in keratinocytes. αLA was shown to protect against ionizing radiation-induced lipid and protein oxidation in mice and to decrease significantly the formation of malondialdehyde (MDA, a marker of oxidative stress) in all tissues examined, especially in the brain, most probably due to its free radical quenching ability.

αLA and DHLA have been shown to be effective depigmenting agents. Both depigment dark-skinned swine, inhibit tanning of light-skinned swine, and inhibit chemical and UVB-induced tyrosinase activity in melanocyte cultures. A recent new derivative of αLA has been proven to be an effective depigmenting agent in melanoma cells in vitro. This depigmentation is achieved by formation of DAPA conjugate products.

αLA may prove to retard and correct both intrinsic and extrinsic aging of the skin as well as other organs. By damaging DNA, the ROS continuously formed in normal metabolism may be largely responsible for the functional deterioration of organs with aging. A decrease in cellular protein and DNA as well as in αLA levels has been measured in aged rat liver, kidney, and spleen. Supplementation with αLA increases nucleic acid and protein levels in the elderly organs. Similarly, the age-related decrease of mitochondrial function in cardiac and brain cells can be improved with αLA supplementation. Clearly, aging skin might similarly benefit.

To evaluate possible improvement of photodamage, a split face study was done on 33 women. Topical application twice daily of 5% lipoic acid cream for 12 weeks decreased skin roughness by 50.8% (as measured by laser profilometry) when compared with the placebo. Clinical and photographic evaluation showed reduction in lentigines and fine wrinkles. In another study, twice daily oral intake of αLA combined with other proteins, vitamins, and minerals improved wrinkles, roughness, and telan­giectasias after 4–6 months in humans, as assessed clinically and by measurements of skin thickness and elasticity. Experiments with fibroblasts in vitro showed increased collagen synthesis when high concentrations of αLA were added to culture media.

Clearly, topical αLA should be further studied by quantitative techniques to confirm these results and to elucidate mechanisms of action. Whether αLA modulates signaling pathways by scavenging ROS as an antioxidant or by direct inhibition of a signal transduction like an enzyme is difficult to determine. The benefits of avoiding the UV-related skin inflammatory damage by αLA may be accompanied with risks such as induction of skin cancer. In fact, there are no reports of the inhibitory effect of LA on photocarcinogenesis. Careful application of αLA for skin aging must be recommended to avoid unwanted adverse effects. Further basic data on αLA is absolutely necessary to assure stability for long and safe topical use of αLA.