title: Naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of both C-24R and C-24S epimers.
abstract: Tritium-labeled 24,25-dihydroxyvitamin D3 was prepared both in vitro, by using chick kidney homogenates, and in vivo in rats from [26,27-methyl-3H]25-hydroxyvitamin D3. These compounds were mixed with synthetic 24(R),25- and 24(S),25-dihydroxyvitamin D3, converted to the corresponding trimethylsilyl ether derivatives, and analyzed by a high-pressure liquid chromatography procedure that separates the derivatized isomers. The tritium-labeled 24,25-dihydroxyvitamin D3 derivatives were found to be a mixture of both the 24(R) and 24(S) epimers; the ratio was found to be 96.4:3.6 in chick kidney homogenates and 96.8:3.2 in the serum of rats under physiological conditions. In addition, nonradioactive 24,25-dihydroxyvitamin D3 isolated from the serum of rats given large doses of vitamin D3 was shown to be an 89.5:10.5 mixture of the 24(R) and 24(S) isomers. When 25-hydroxy-24-oxo-vitamin D3 was utilized as a substrate, it was found to be more selectively reduced to 24(S),25-dihydroxyvitamin D3 than 24(R),25-dihydroxyvitamin D3 by the renal enzyme. The 24(S),25-dihydroxyvitamin D3 has been identified by ultraviolet absorption spectrophotometry, cochromatography with an authentic standard, and mass spectrometry. The reduced metabolites of 25-hydroxy-24-oxo-vitamin D3 were a 1:50 mixture of the 24(R) and 24(S) epimers. There are two known metabolic pathways leading to 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3; one is 24(R)-hydroxylation of 25-hydroxyvitamin D3 and the other is reduction of 25-hydroxy-24-oxo-vitamin D3. In contrast, 24(S),25-dihydroxyvitamin D3 is produced only by reduction of 25-hydroxy-24-oxo-vitamin D3 in the kidney. Therefore, naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of the 24(R) and 24(S) isomers, and not just the 24(R) isomer as reported previously.
mesh_terms: D015650:24,25-Dihydroxyvitamin D 3; D000818:Animals; D002645:Chickens; D002762:Cholecalciferol / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D004100:Dihydroxycholecalciferols / Q000097:blood / Q000302:isolation & purification / Q000378:metabolism; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008297:Male; D013058:Mass Spectrometry; D051381:Rats; D011919:Rats, Inbred Strains; D013057:Spectrum Analysis; D013237:Stereoisomerism; D014297:Trimethylsilyl Compounds / Q000032:analysis
chemical_list: D004100:Dihydroxycholecalciferols; D014297:Trimethylsilyl Compounds; D002762:Cholecalciferol; D015650:24,25-Dihydroxyvitamin D 3