• keratinocytes;
  • vitamin D3;
  • calcitriol;
  • metabolism

Abstract: These results demonstrate for the first time that human keratinocytes under in vivo-like conditions have the capacity of the enzymatic hydroxylation of vitamin D3 to hormonally active calcitriol (1α,25(OH)2D3). Supplementation of the culture medium with bovine serum albumin (BSA) up to 1.5% (w/v) amplifies the conversion of vitamin D3 to 1α,25(OH)2D3. The maximum turnover rate of this reaction at 780 nM vitamin D3 in presence of 1.0% (w/v) BSA amounts to approximately 3 pmol 1α,25(OH)2D3 per 106 cells after 6 h of incubation. The hydroxylation of vitamin D3 to 1α,25(OH)2D3 is inhibited by the P-450 oxidase inhibitor ketoconazole. The generation of 1α,25(OH)2D3 from vitamin D3 has an apparent Michaelis constant (Km) of 2.3×10−6 M. The intrinsic conversion of vitamin D3 to biologically active 1α,25(OH)2D3 may be of importance for the regulation of proliferation and differentiation of keratinocytes.