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Kinetic and thermodynamic studies of the conversion of previtamin D3 to vitamin D3 in human skin

1993; Elsevier BV; Volume: 268; Issue: 20 Linguagem: Inglês

10.1016/s0021-9258(18)82416-4

1083-351X

Xiao Quan Tian, Tai C. Chen, Lois Y. Matsuoka, Jacobo Wortsman, Michael F. Holick,

Biotin and Related Studies

The thermoisomerization of previtamin D3 to vitamin D3 is the last step in the synthesis of vitamin D3 in human skin.Kinetic and thermodynamic studies of this reaction in human skin and an organic solvent revealed that not only the equilibrium of the reaction was shifted in favor of vitamin Ds formation in human skin (equilibrium constant K at 37 "C = 11.44)compared to hexane ( K = 6.15), but also the rate of the reaction was increased by more than 10-fold in human skin ( T I 1 2 at 37 "C = 2.5 h) when compared to hexane (Tip = 30 h).This extraordinarily fast reaction rate was also confirmed in vitro in chicken skin and in vivo in human subjects.The enthalpy change for the reaction determined by the van't Hoff plot was D = -21.58kJ mol" in human skin and D = -15.60kJ mol" in hexane.Arrhenius plots showed that the activation energies for both the forward and the reverse reactions were lower in human skin (Ea1 = 71.05kJ mol-' and Ee2 = 92.63kJ mol") than in hexane (Ea1 = 84.90kJ mol-' and Ea2 = 100.5 kJ mol").Activation parameters for the reaction in human skin and in hexane were also reported.Subcellular fractionation of human epidermal tissue revealed that most epidermal 7-dehydrocholesterol and previtamin D3 were in the membrane fraction, while only 20% were in the cytosol.The interaction of previtamin D3 with intracellular lipids and/or proteins in skin may be responsible for the increased vitamin D3 formation rate in the skin.Of all the vitamins, vitamin D3 is unique in that it can be photosynthesized in the skin.When human skin is exposed to sunlight, the high energy ultraviolet (UVB)' (290-315 nm) radiation penetrates the epidermis and photolyzes epidermal 7-dehydrocholesterol (7-DHC) to previtamin D3 (preDJ and other photoisomers (Holick, 1987).Once formed, preD3 immediately begins to isomerize to vitamin D3 by a temperaturedependent process (Velluz et al., 1949; Holick et al., 1980).The reaction rate of this isomerization is enhanced as the temperature increases.Early studies found that the preD3 isomerization rate could not be influenced by acids, bases, catalysts, and inhibitors of radical chain processes