Rat intestinal 25-hydroxyvitamin D3- and 1alpha,25-dihydroxyvitamin D3-24-hydroxylase.

The 24-hydroxylation of 25-hydroxyvitamin D3 and 1alpha,25-dihydroxyvitamin D3 occurs in vitro in rat intestinal homogenates. 1alpha,24,25-Trihydroxyvitamin D3 obtained by incubating 1alpha,25-dihydroxyvitamin D3 with rat intestinal homogenates was isolated and then identified by co-chromatography of the biologically generated material with synthetic 1alpha,24R,25-trihydroxyvitamin D3 on a high pressure liquid chromatography system and by ultraviolet spectroscopy, periodate cleavage, and mass spectroscopy. 24,25-Dihydroxyvitamin D3 obtained by incubating 25-hydroxyvitamin D3 with intestinal homogenates was identified by co-chromatography of the biologically generated material with authentic 24R,25-dihydroxyvitamin D3 on a high pressure liquid chromatography system and by periodate cleavage. Tissue distribution studies indicate that the process occurs in rat intestinal homogenates but not in rat muscle, liver, or fetal calvaria homogenates. Nephrectomy does not diminish the production of 1alpha,24,25-trihydroxyvitamin D3 in vivo while it reduces markedly, but does not eliminate, production of 24,25-dihydroxyvitamin D3.