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glutathione and ascorbate are closely linked. In humans, who are dependent on dietary vitamin C intake, this link remains to be clarified.

Prevalence in Skin Although a number of studies are available on glutathione (GSH) and glutathione disulfide (GSSG), absolute values obtained for levels in total skin, epidermis, and dermis are highly variable (Table 2). However, comparing the relative levels, most studies demonstrated higher glutathione levels in the epidermis than in the dermis. Furthermore, the epidermis reveals a higher ratio of GSH/GSSG than the dermis, indicating either a lower oxidative challenge or a better antioxidative protection. Since the epidermis is more directly exposed to the environment, it seems also possible that the pathways leading to the endogenous formation of epidermal glutathione are upregulated by chronic environmental factors, as was shown for glutathione peroxidase in ozone-exposed lung epithelium (26). It must be considered that the cell turnover rate in the epidermis is very high, as well as cellular differentiation processes; since GSH is an important substrate for essential enzymes and GSSG can inactivate enzymes by forming disulfides (27), a high GSH/GSSG ratio could be essential for the stratified and keratinized epidermis.

Urate

Antioxidant Properties Uric acid (deprotonated form: urate) is a small water-soluble molecule (Fig. 1) that accumulates in human tissues as the end-product of purine metabolism. In blood plasma, urate has been shown to be a powerful scavenger of singlet oxygen, peroxyl-, and hydroxyl radicals (28). Further studies have demonstrated that urate scavenges ozone (15) and hypochlorous acid (29). In addition to its radical-scavenging potential, urate was proposed to stabilize reduced vitamin C in serum. This stabilizing effect appears to be due to inhibition of iron-catalyzed oxidation of ascorbate, which largely results from the formation of a stable, noncatalytic urate-iron complex (30). Unlike radical-scavenging reactions, this protective effect provided by iron chelation is not associated with depletion of urate. Direct free-radical attack upon urate generates allantoin, which has therefore been proposed as a marker molecule for free-radical reactions in vivo (31).

Prevalence in Skin Only little data are available on urate levels in cutane- ^

ous tissues. Lopez-Torres et al. reported values of 147 ± 5 nmol g_1 tissue in |

the epidermis, and 75 ± 9 nmol g_1 in the dermis of hairless mice (32). In humans, js

Shindo et al. reported levels of 1071 ± 242 nmol g_1 tissue in the epidermis, -a and 182 ± 24 nmol g_1 tissue in the dermis, respectively (18). Thus, as found < for other antioxidants, the highest cutaneous urate levels are present in epidermal tissue.

Table 2 Physiological Levels of Glutathione in Cutaneous Tissues

Skin layer

Species

Concentration

References

Year

Epidermis

Human

1.8 (imol/g tissue (GSH)

Halprin and Ohkawara (200)

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