Calcitroic acid (1α-hydroxy-23-carboxy-24,25,26,27-tetranorvitamin D3) is a metabolite of 1α,25-dihydroxyvitamin D3 (calcitriol). Its formation is catalyzed by the enzyme calcitriol 24-hydroxylase. Calcitroic acid is soluble in water and excreted in urine. Calcitroic acid is known to be the major water-soluble metabolite produced during the deactivation of 1 alpha,25-dihydroxyvitamin D(3). This deactivation process involves a series of oxidation reactions at C(24) and C(23) leading to side-chain cleavage and, ultimately, formation of the calcitroic acid. Like 1 alpha,25-dihydroxyvitamin D(3), 1 alpha,25-dihydroxyvitamin D(2) is also known to undergo side-chain oxidation; however, to date there has been no evidence suggesting that 1 alpha,25-dihydroxyvitamin D(2) undergoes side-chain cleavage. Calcitroic acid is a major terminal product for the deactivation of 1 alpha,25-dihydroxyvitamin D(2).
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