The 1 alpha-hydroxylation of 24-hydroxyvitamin D3 by chick kidney homogenates

Tritium-labeled 24(R)-hydroxyvitamin D3 and 24(S)-hydroxyvitamin D3 were chemically synthesized and the 1 alpha-hydroxylation of these compounds by chick kidney homogenates was studied. A marked stereospecific preference with regard to the orientation of the hydroxyl functionality on carbon-24 was noted: while the 24(R)-epimer could be 1 alpha-hydroxylated in readily detectable amounts, the 24(S)-epimer was not hydroxylated. Thus, 1.2 micrograms of 1 alpha,24(R)-dihydroxyvitamin D3 was isolated and its structure confirmed by mass spectrometry. The relative rate of 1 alpha-hydroxylation of 125 nM substrate tritiated 24(R)-hydroxyvitamin D3 and 25-hydroxyvitamin D3 (the presumed natural substrate for the renal 1 alpha-hydroxylase) was 1:6.7.