title: Rat cytochrome P450C24 (CYP24) does not metabolize 1,25-dihydroxyvitamin D2 to calcitroic acid.
abstract: 1alpha-Hydroxy-23 carboxy-24,25,26,27-tetranorvitamin D(3) (calcitroic acid) is known to be the major water-soluble metabolite produced during the deactivation of 1,25-(OH)(2)D(3). This deactivation process is carried out exclusively by the multicatalytic enzyme CYP24 and involves a series of oxidation reactions at C(24) and C(23) leading to side-chain cleavage and, ultimately, formation of the calcitroic acid. Like 1,25-(OH)(2)D(3), 1alpha,25-1,25-(OH)(2)D(2) is also known to undergo side-chain oxidation and side-chain cleavage to form calcitroic acid (Zimmerman et al. [2001]. 1,25-(OH)(2)D(2) differs from 1,25-(OH)(2)D(3) by the presence of a double bond at C(22) and a methyl group at C(24). To date, there have been no studies detailing the participation of CYP24 in the production of calcitroic acid from 1,25-(OH)(2)D(2). We, therefore, studied the metabolism of 1,25-(OH)(2)D(3) and 1,25-(OH)(2)D(2) using a purified rat CYP24 system. Lipid and aqueous-soluble metabolites were prepared for characterization. Aqueous-soluble metabolites were subjected to reverse-phase high-pressure liquid chromatography (HPLC) analysis. As expected, 1,23(OH)(2)-24,25,26,27-tetranor D and calcitroic acid were the major lipid and aqueous-soluble metabolites, respectively, when 1,25-(OH)(2)D(3) was used as substrate. However, when 1,25-(OH)(2)D(2) was used as substrate, 1,24(R),25-(OH)(3)D(2) was the major lipid-soluble metabolite with no evidence for the production of either 1,23(OH)(2)-24,25,26,27-tetranor D or calcitroic acid. Apparently, the CYP24 was able to 24-hydroxylate 1,25-(OH)(2)D(2), but was unable to effect further changes, which would result in side-chain cleavage. These data suggest that the presence of either the double bond at C(22) or the C(24) methyl group impedes the metabolism of 1,25-(OH)(2)D(2) to calcitroic acid by CYP24 and that enzymes other than CYP24 are required to effect this process.
authors.affiliation: National Animal Disease Center, ARS-USDA, Ames, Iowa, USA. rhorst@nadc.ars.usda.gov
authors.forename: R L
authors.initials: RL
authors.lastname: Horst
authors.forename: J A
authors.initials: JA
authors.lastname: Omdahl
authors.forename: S
authors.initials: S
authors.lastname: Reddy
mesh_terms: D000818:Animals; D002117:Calcitriol / Q000031:analogs & derivatives* / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004872:Ergocalciferols / Q000378:metabolism*; D051381:Rats; D013250:Steroid Hydroxylases / Q000378:metabolism*; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D004872:Ergocalciferols; C003564:1,25-dihydroxyergocalciferol; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; C021640:calcitroic acid; D002117:Calcitriol