Kinetics and regulation of 25‐hydroxycholecalciferol 1α‐hydroxylase from cells isolated from human term decidua

Abstract

Kinetics and regulation of 25‐hydroxycholecalciferol 1α‐hydroxylase from cells isolated from term human decidua were studied. The production of 1α,25‐dihydroxycholecalciferol (calcitriol) was linear with time for up to 6 h and was directly proportional to the number of cells up to 20x106/dish at a substrate concentration of 100 nM. Under these conditions the apparent Km was 88 nM and the Vmax 3.0 pmol/106 cells. The production of [3H]calcitriol was inhibited by 0.1 nM (P<0.01) and 1 nM (P<0.005) unlabeled calcitriol. Unlike the kidney enzyme and for reasons that remain unclear, neither inorganic phosphate salts nor parathyroid hormone had any acute effect on the calcitriol production. Further studies are required to delineate the regulatory mechanism of this enzyme. Copyright © 1987, Wiley Blackwell. All rights reserved