The production of 1,25-dihydroxycholecalciferol, the biologically active form of cholecalciferol (vitamin D3), was studied using homogenates or subcellular fractions incubated with 25-hydroxy-[26,27-3H]-cholecalciferol. It was found that only the kidney in the chick was capable of producing 1,25-dihydroxycholecalciferol. Also, kidney tissue obtained from the rat, dog and man produced, under in vitro conditions, a radioactive sterol which cochromatographed with standard 1,2 5-dihydroxycholecalciferol. The enzymatic activity was found to be located in the mitochondrial, but not the nuclear, microsomal or supernatant fractions of chick kidney. In the dog, the enzymatic production of 1,25-dihydroxycholecalciferol was found only in the kidney cortex and not in the medulla. It appears that in organisms utilizing cholecalciferol, the vitamin is converted to its active form by the kidney. Thus, the implication of the kidney in cholecalciferol metabolism may be of relevance to the pathogenesis of disorders of calcium metabolism in chronic uremia.
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Midgett et al. (1973) studied this question.