title: Kinetics and regulation of 25-hydroxycholecalciferol 1 alpha-hydroxylase from cells isolated from human term decidua.
abstract: Kinetics and regulation of 25-hydroxycholecalciferol 1 alpha-hydroxylase from cells isolated from term human decidua were studied. The production of 1 alpha,25-dihydroxycholecalciferol (calcitriol) was linear with time for up to 6 h and was directly proportional to the number of cells up to 20 X 10(6)/dish at a substrate concentration of 100 nM. Under these conditions the apparent Km was 88 nM and the Vmax 3.0 pmol/10(6) cells. The production of [3H]calcitriol was inhibited by 0.1 nM (P less than 0.01) and 1 nM (P less than 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.
mesh_terms: D000818:Animals; D002117:Calcitriol / Q000096:biosynthesis; D002417:Cattle; D053493:Cholestanetriol 26-Monooxygenase; D003656:Decidua / Q000166:cytology / Q000201:enzymology*; D005260:Female; D006801:Humans; D007700:Kinetics; D010281:Parathyroid Hormone / Q000494:pharmacology; D010710:Phosphates / Q000494:pharmacology; D013250:Steroid Hydroxylases / Q000378:metabolism*; D013997:Time Factors
chemical_list: D010281:Parathyroid Hormone; D010710:Phosphates; D013250:Steroid Hydroxylases; C506831:CYP27A1 protein, human; D053493:Cholestanetriol 26-Monooxygenase; D002117:Calcitriol