title: End product inhibition of hepatic 25-hydroxyvitamin D production in the rat: specificity and kinetics.
abstract: The role of vitamin D metabolites in the regulation of hepatic 25-hydroxyvitamin D production was investigated by examining the effects of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and 24,25-dihydroxyvitamin D on the synthesis of [25-3H]hydroxyvitamin D by rachitic rat liver homogenates. Production of [25-3H]hydroxyvitamin D was inhibited by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, but not by 24,25-dihydroxyvitamin D. 25-Hydroxyvitamin D increased the Km of the vitamin D-25-hydroxylase enzyme(s), while 1,25-dihydroxyvitamin D decreased the Vmax with a Ki of 88.7 ng/ml. Inhibition of hepatic 25-hydroxyvitamin D production by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D may be another control mechanism to regulate circulating vitamin D levels.
mesh_terms: D015650:24,25-Dihydroxyvitamin D 3; D000818:Animals; D002112:Calcifediol / Q000494:pharmacology*; D002117:Calcitriol / Q000494:pharmacology*; D004100:Dihydroxycholecalciferols / Q000494:pharmacology*; D006887:Hydroxycholecalciferols / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D012279:Rickets / Q000378:metabolism; D014316:Tritium; D014808:Vitamin D Deficiency / Q000378:metabolism*
chemical_list: D004100:Dihydroxycholecalciferols; D006887:Hydroxycholecalciferols; D014316:Tritium; D015650:24,25-Dihydroxyvitamin D 3; D002117:Calcitriol; D002112:Calcifediol