抄録
Vitamin D itself is inactive on the target tissues of bone and intestine, but is converted to the 25-hydroxy derivative (25-OH-D) in the liver. The conversion which is an O_2,Mg^<2+> and NADPH requiring system is strongly product-inhibited by 25-OH-D. This inhibition controls the rate of formation and "secretion" of 25-OH-D into the blood. The 25-OH-D acts more rapidly in inducing intestinal calcium transport and bone mineral mobilization, demonstrating that vitamin D must be hydroxylated in the 25 position before it is effective. The 25-OH-D is converted very rapidly to more polar metabolites which appear in the target tissues. Two such metabolites were isolated from hog's plasma and identified as 21,25-(OH)_2-D_3 and 25,26-(OH)_2-D_3. These two metabolites, however, were not more active than 25-OH-D. Another metabolite which is more potent and acts more rapidly than 25-OH-D was isolated from the chicken intestine and identified as 1,25-(OH)_2-D_3,suggesting it to be the metabolically active form in the intestine. This metabolite is generated from 25-OH-D_3 in the kidney. Dietary calcium concentration has a profound effect on the in vivo production of 1,25-(OH)_2-D_3 and 21,25-(OH)_2-D_3. These alterations in metabolite balance correlate with changes in serum calcium.
