Metabolism of 3 H‐1α,25‐dihydroxyvitamin D 3 in cultured human keratinocytes

In the present investigation we studied the metabolism of 1α,25‐dihydroxy‐[1β‐ 3 H] vitamin D 3 ( 3 H‐1,25(OH) 2 D 3 ) in culture‐grown human keratinocytes (CHK). Our results showed that the cellular uptake of 3 H‐1,25(OH) 2 D 3 , upon incubation with CHK, occurred very rapidly; and it paralleled a decrease in the concentration of 3 H‐1,25(OH) 2 D 3 in the medium. The amount of 3 H‐calcitroic acid, on the other hand, increased slowly in the medium, while the concentration of 3 H‐calcitroic acid in the cell remained undetectable during the whole period of incubation. When the cells were preincubated with 1,25(OH) 2 D 3 (10 −8 M), conversion of 3 H‐1,25(OH) 2 D 3 to 3 H‐calcitroic acid increased almost twofold, indicating that 1,25(OH) 2 D 3 catalyzed its own catabolism. © 1995 Wiley‐Liss, Inc.