The metabolism of 1, 25-dihydroxyvitamin Da₃

VITAMIN D plays a major role in the maintenance of extracellular fluid calcium and phosphorus concentrations, the mineralization of bone, and the maintenance of skeletal integrity (13). The vitamin increases active calcium and phosphorus transport in the duodenum and proximal intestine; rachitic animals and humans have increased feeal losses of calcium that are reversed by the administration of vitamin D. Furthermore, there is evidence that the vitamin or its metabolites have direct effects upon muscle metabolism and on isolated bone cells, and upon the mobilization of calcium from isolated bone (1). Finally, the vitamin has pronounced effects upon the maintenance of adequate growth in experimental animals and in man (1, 3). The present review will focus upon some of the more recent findings concerning the regulation of 25-hydroxyvitamin D₃ lα-hydroxylase activity and upon the peripheral metabolism of vitamin D₃ homologs.