1980 年 88 巻 1 号 p. 87-95
Tritium labeled 1 α, 24 (R)-dihydroxyvitamin D3 and 1 α, 24 (S)-dihydroxyvitamin D3 were synthesized and the metabolism and distribution of these compounds in the rat were studied. The 1 α, 24 (R)-dihydroxyvitamin D3 was assumed to be gradually metabolized to 1 α, 24 (R) 25-trihydroxyvitamin D3 in the liver and released into the blood, since relatively large amounts of the administered compound appeared in the serum as 1 α, 24 (R) 25-trihydroxyvitamin D3. In contrast, large amounts of the unchanged compound and only very small amounts of 1 α, 24 (R) 25-trihydroxyvitamin D3 were found in other tissues. The distribution pattern of 1 α, 24 (S)-dihydroxyvitamin D3 was similar to that seen with 1 α, 24 (R)-dihydroxyvitamin D3; the major portion was recovered as the 25-hydroxylated metabolite in the serum whereas in the small intestine and bone large amounts of the unchanged compound and a trace amount of the metabolite were found. It was also clarified by liver perfusion experiments that 25-hydroxylations of these compounds take place in the liver and the resulting products are released into the blood. The 25-hydroxylations of 1 α, 24 (R)-dihydroxyvitamin D3 and 1 α, 24 (S)-dihydroxyvitamin D3 in the liver were not inhibited by the addition of each metabolite, 1 α, 24 (R) 25-trihydroxyvitamin D3 or 1 α, 24 (S) 25-trihydroxyvitamin D3, suggesting that the hydroxylation reaction is catalyzed by the enzyme similar to that for 1 α-hydroxyvitamin D3 25-hydroxylation. From these results, it was suggested that 1 α, 24 (R)-dihydroxyvitamin D3 and 1 α, 24 (S)-dihydroxyvitamin D3 perform their activities for intestinal calcium transport and bone calcium mobilization without undergoing the 25-hydroxylation reaction.
