Geographic influence of vitamin D synthesis in the skin

Seasonal variation in vr high at end of with a negative irtfluei

Photolytic conversion of 7-dehyi into previtamin D and subsequei (calclol) occurs year round betw

Seasonal variation in vitamin D synthesis; high at end of summer/low at end of winter with a negative influence of industrial pollution

Figure 2.2. Vitamin D is synthesized in our skin as a consequence of exposure to a specific wavelength of UV light (290-312 nm). /As UV exposure varies with latitude, only people within 40 ° N and S of the equator synthesize this critical vitamin year round. Outside of these latitudes, vitamin D synthesis is seasonal. It has been suggested that this may have acted as an evolutionary pressure for skin depigmentation.

Figure 2.3. It has been suggested that global variation in exposure to UV light may have acted as a selection pressure for skin pigmentation. Protection against the photolytic effect on labile folate, a molecule required for cell growth, division, and reproduction may have favored enhanced pigmentation as one moves toward the equator, whereas the reverse may be true for the necessary action of UV light on vitamin D synthesis, which could have led to depigmentation as one travels toward the poles. The figure shows the contrasting effect of UV light on the molecular structures of both folate and vitamin D.

Figure 2.3. It has been suggested that global variation in exposure to UV light may have acted as a selection pressure for skin pigmentation. Protection against the photolytic effect on labile folate, a molecule required for cell growth, division, and reproduction may have favored enhanced pigmentation as one moves toward the equator, whereas the reverse may be true for the necessary action of UV light on vitamin D synthesis, which could have led to depigmentation as one travels toward the poles. The figure shows the contrasting effect of UV light on the molecular structures of both folate and vitamin D.

laboratory shows C9-N10 bond scission of the intracellular polyglutamyl form of 5-methyltetrahydrofolate may be even more pronounced. Furthermore, another vitamin— ascorbic acid (vitamin C)—is crucial for optimizing native folate bioavailabilty (Figure 2.5) (24), whereas dietary riboflavin (vitamin B2) and cobalamin (vitamin B12) are also crucial to the one-carbon transfer reactions that folate facilitates in its role of de novo methionine, purine, and pyrimidine synthesis (25). Indeed, the most important of all folate's covitamins, vitamin B12, is highly UV sensitive, as is riboflavin. Clearly, many factors affect folate status and, hence, are likely to indirectly modulate the skin pigmentation evolutionary paradigm.

Figure 2.4. A simple schematic of vitamin D and calcium homeostasis.

A more indirect effect may lie in the role of closely related biopterin cofactors and melanin biosynthesis. Folate and biopterin coenzymes are structurally and functionally similar (Figure 2.6). Metabolic overlap is thought to occur between their respective pathways; man cannot synthesize folate de novo, but bacteria can. GTP cyclohydrolase 1 is used by bacteria for folate production, but it is also used by man for tetrahydrobiopterin biosynthesis (26). A common metabolic locus in both pathways, which is distinct in evolutionary terms, indicates a close metabolic relationship exists between both groups of cofactors and their dependent enzymes. Indeed, a reciprocal use of substrates has been reported. Melanin is formed from tyrosine; the first step is the formation of DOPA from tyrosine, a process that requires tetrahydrobiopterin (see below—Figure 4.1). Interestingly, folate and biopterin are known to interact in a synergistic manner at such sites (20,27-30), and they both may therefore play a role in melanogenesis, although this is unproven.

Nonmammalian vertebrate pigmentation may also advertise a capacity to deploy resources in a way that optimizes survival and reproductive success. Carotenoid precursors of vitamin A are used by birds to provide sexual coloration that advertises superior health as

Figure 2.5. Schematic showing that vitamin C is crucial for optimizing native folate bioavailabilty.

conferred by the antioxidant properties of carotenoids (31). However, several micronutrients with specific epigenetic and antioxidant capacity also undoubtedly help maintain genetic integrity and thus reproductive success in humans (see below).

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