Calcitroic Acid Is a Major Catabolic Metabolite in the Metabolism of 1α-Dihydroxyvitamin D2

Calcitroic acid (1α-hydroxy-23 carboxy-24,25,26,27-tetranorvitamin D3) is known to be the major water-soluble metabolite produced during the deactivation of 1α,25-dihydroxyvitamin D3. This deactivation process involves a series of oxidation reactions at C24 and C23 leading to side-chain cleavage and, ultimately, formation of the calcitroic acid. Like 1α,25-dihydroxyvitamin D3, 1α,25-dihydroxyvitamin D2 is also known to undergo side-chain oxidation; however, to date there has been no evidence suggesting that 1α,25-dihydroxyvitamin D2 undergoes side-chain cleavage. To investigate this possibility, we studied 1α,25-dihydroxyvitamin D2 metabolism in HPK1A-ras cells as well as the well characterized perfused rat kidney system. Lipid and aqueous-soluble metabolites were prepared for characterization. Aqueous-soluble metabolites were subjected to reverse-phase HPLC analysis. The major aqueous-soluble metabolite from both the kidney and cell incubations comigrated with authentic calcitroic acid on two reverse-phase HPLC columns of different chemistry. The putative calcitroic acid from the cell and kidney incubations was methylated and found to comigrate with methylated authentic standard on straight-phase and reverse-phase HPLC columns. The identity of the methylated metabolite from cell incubations was also confirmed by mass spectral analysis. These data show, for the first time, that calcitroic acid is a major terminal product for the deactivation of 1α,25-dihydroxyvitamin D2. Intermediates leading to the formation of the calcitroic acid in the 1α,25-dihydroxyvitamin D2 metabolism pathway are currently being studied.