Conversion of vitamin D-3 to 1 alpha,25-dihydroxyvitamin D-3 in human skinequivalents

These results demonstrate for the first time that human keratinocytes under in vivo-like conditions have the capacity of the enzymatic hydroxylation o f vitamin D-3 to hormonally active calcitriol (1 alpha,25(OH)(2)D-3). Suppl ementation of the culture medium with bovine serum albumin (BSA) up to 1.5% (w/v) amplifies the conversion of vitamin D-3 to 1 alpha,25(OH)(2)D-3. The maximum turnover rate of this reaction at 780 nM vitamin D-3 in presence o f 1.0% (w/v) BSA amounts to approximately 3 pmol 1 alpha,25(OH)(2)D-3 per 1 0(6) cells after 6 h of incubation. The hydroxylation of vitamin D-3 to 1 a lpha,25(OH)(2)D-3 is inhibited by the P-450 oxidase inhibitor ketoconazole. The generation of 1 alpha,25(OH)(2)D-3 from vitamin D-3 has an apparent Mi chaelis constant (K-m) of 2.3x10(-6) M. The intrinsic conversion of vitamin D-3 to biologically active 1 alpha,25(OH)(2)D-3 may be of importance for t he regulation of proliferation and differentiation of keratinocytes.