Metabolism of 22-oxacalcitriol by a vitamin D-inducible pathway in cultured parathyroid cells

Abstract

Catabolism of 22-oxacalcitriol (OCT) in parathyroid cells was compared to that of the parent hormone, 1,25-dihydroxyvitamin D 3 [1,25-(OH) 2D 3]. Catabolism of both compounds was greatly accelerated by pretreatment of the cells with 1,25-(OH) 2D 3 or OCT. The rate of degradation of OCT was slightly greater than that of 1,25-(OH) 2D 3. Excess unlabeled OCT or 1,25-(OH) 2D 3 inhibited metabolism of both tritiated substrates. Ketoconazole, a cytochrome P450 inhibitor, blocked catabolism of both compounds. The major OCT metabolite appeared to be 1,20-dihydroxy-22,23,24,25,26,27-hexanor-vitamin D 3 which was not active in suppressing PTH secretion. We conclude that OCT appears to be metabolized by the same vitamin D-inducible side chain oxidation pathway that catabolizes other vitamin D compounds and that its higher than expected suppression of PTH secretion is not due to slower cellular metabolism.