Vitamin C

Ascorbic acid or vitamin C is one of the most important water soluble antioxidants and present in high amounts in the skin. While most species are able to produce ascorbic acid, humans lack the enzymes necessary for its synthesis. Deficiency in ascorbic acid causes scurvy, a disease already described in the ancient writings of the Greeks [28]. Apart from the pure antioxidant function ascorbic acid is an essential co-factor for different enzymes. The antioxidant capacity of vitamin C is related to its unique structure (Fig. 2). Due to its pKa1 of 4.25 it is present as a monoanion at physiological pH, which can undergo a

Figure 2 Structural formula of vitamin C as the monoanion ascorbate.

one electron donation to form the ascorbyl radical with a delocalized electron and can be further oxidized to result in dehydroascorbic acid. Dehydroascorbic acid is relatively unstable and breaks down if it is not regenerated (see antioxidant network). In vitro ascorbic acid can scavenge many types of radicals including the hydroxyl- (OH^), the superoxide-(O/~) and water soluble peroxyl- (ROOO radicals as well as other reactive oxygen species such as O3, and quenches singlet oxygen. Due to their relative reduction potentials, ascor-bate can reduce Fe(III) to Fe(II), which in turn can decompose hydrogen peroxide (H2 O2) to the dangerous hydroxyl radical. Therefore, vitamin C can exert pro-oxidant effects in the presence of unbound iron (fenton chemistry).

In the skin, vitamin C is found in all layers. In SC it forms a similar gradient as vitamin E with decreasing concentrations towards the outside. Vitamin C is depleted by O3, UV radiation, and BPO. One of the earliest discoveries of vitamin C benefits in the skin was the observation that it stimulates collagen synthesis in dermal fibroblasts [29]. Recently a pretranscriptional role of vitamin C had been described [30]. Also, vitamin C is essential in the formation of competent barrier lipids in reconstructed human epidermis [31].

Several studies have investigated protective effects of vitamin C against oxidative stress. UVB-induced immunotolerance, as a marker of damage to the immune system, could be abrogated by topical application of vitamin C to murine skin [32]. UVB-induced sunburn cell formation was mitigated by vitamin C in porcine skin [33]. While one study reported a postadministrative protective effect of vitamin C-phosphate against UV-induced damage in mice [34], another study found no such effect in humans [35]. Systemic application of vitamin C in combination with vitamin E protected against UV-induced erythema in humans [21]. In a keratinocyte cell culture system vitamin C reduced UVB-induced DNA damage [18]. In mice, an anticarcinogenic effect of vitamin C was described [36]. However, no data regarding such benefits exists in humans.

Since vitamin C is not very stable, it is difficult to incorporate it into topical formulations. Esterification with phosphate is used to circumvent this limitation. In vitro experiments demonstrated that Mg-ascorbyl-2-phosphate penetrates the murine skin barrier and is bioconverted into free ascorbate [37].

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