23,25-Dihydroxyvitamin D3: a natural precursor in the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone.

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Abstract
To elucidate the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone, 2 known metabolites of 25-hydroxyvitamin D3.sbd.23,25-dihydroxyvitamin D3 and 25,26-dihydroxyvitamin D3.sbd.were incubated individually with kidney homogenate prepared from vitamin D-supplemented chickens, a preparation known to produce the lactone from 25-hydroxyvitamin D3. The 25-hydroxyvitamin D3-26,23-lactone produced in vitro was then separated, purified, identified and quantitated by consecutive straight-phase and reverse-phase high-performance liquid chromatography. 23,25-Dihydroxyvitamin D3 is a far better substrate for production of 25-hydroxyvitamin D3-26,23-lactone than is 25,26-dihydroxyvitamin D3. Production of lactone is highly selective for the natural 23(S)-hydroxy-23,25-dihydroxyvitamin D3 while both epimers of 25,26-dihydroxyvitamin D3 resulted in small amounts of product comigrating with naural lactone. It appears that 23(S),25-dihydroxyvitamin D3, but not 25,26-dihydroxyvitamin D3, is a natural precursor in the synthesis of 25-hydroxyvitamin D3-26,23-lactone; this result also implies that the configuration of the lactone at C-23 is S.