title: Kinetic properties of 25-hydroxyvitamin D- and 1,25-dihydroxyvitamin D-24-hydroxylase from chick kidney.
abstract: 1,25-Dihydroxyvitamin D3 induces both 25-hydroxyvitamin D3- and 1,25-dihydroxyvitamin D3- 24-hydroxylase activities. However, whether 24-hydroxylation of these substrates is catalyzed by a single enzyme is unknown. We have examined the substrate specificity of the enzyme using the solubilized and reconstituted chick renal mitochondrial 24-hydroxylase enzyme system. The soluble enzyme catalyzes 24-hydroxylation of both substrates. The apparent Km of the 24-hydroxylase for 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 were 1.47 and 0.14 microM, respectively. Kinetic studies demonstrated that 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 act as competitive inhibitors with respect to each other. 1,25-Dihydroxyvitamin D3 inhibited the production of 24,25-dihydroxyvitamin D3 with an apparent Ki of 0.09 microM and 25-hydroxyvitamin D3 inhibited the production of 1,24,25-trihydroxyvitamin D3 with an apparent Ki of 3.9 microM. These results indicate that chick 24-hydroxylase preferentially hydroxylates 1,25-dihydroxyvitamin D3 and support the idea that the 24-hydroxylation of these substrates is catalyzed by a single enzyme.
authors.affiliation: Department of Biochemistry, University of Wisconsin-Madison.
authors.forename: M
authors.initials: M
authors.lastname: Burgos-Trinidad
authors.forename: H F
authors.initials: HF
authors.lastname: DeLuca
mesh_terms: D015650:24,25-Dihydroxyvitamin D 3 / Q000378:metabolism; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000378:metabolism*; D000818:Animals; D002112:Calcifediol / Q000378:metabolism*; D002645:Chickens; D003577:Cytochrome P-450 Enzyme System; D007668:Kidney / Q000201:enzymology*; D007700:Kinetics; D008928:Mitochondria / Q000201:enzymology; D012995:Solubility; D013250:Steroid Hydroxylases / Q000378:metabolism*; D013379:Substrate Specificity; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D015650:24,25-Dihydroxyvitamin D 3; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; D002112:Calcifediol