AJP - Endocrinology and Metabolism, Vol 254, Issue 4 E402-E406, Copyright © 1988 by American Physiological Society
Biological activity of 1,25-dihydroxyvitamin D2 and 24-epi-1,25-dihydroxyvitamin D2
H. F. DeLuca, R. R. Sicinski, Y. Tanaka, P. H. Stern and C. M. Smith
Department of Biochemistry, University of Wisconsin-Madison 53706.
The biological activity of 1,25-dihydroxyvitamin D2 [1,25(OH)2D2] and
24-epi-1,25-dihydroxyvitamin D2 [24-epi-1,25(OH)2D2] has been determined in
vitamin D-deficient rats. The biological effectiveness of 1,25(OH)2D2 is
equal to that reported previously for 1,25-dihydroxyvitamin D3
[1,25(OH)2D3] (15) in intestinal calcium transport, mineralization of bone,
mobilization of bone calcium, and elevation of plasma inorganic phosphorus
of rachitic rats. However, 24-epi-1,25(OH)2D2 is at best one-half as active
as 1,25(OH)2D2 in stimulating intestinal calcium transport and in the
mineralization of rachitic bone. The 24-epi-1,25(OH)2D2 is one-third as
active as 1,25(OH)2D3 in binding to the chick intestinal receptor for
1,25(OH)2D3. Thus receptor discrimination may account for the twofold
difference in intestinal calcium transport activity. 24-Epi-1,25(OH)2D2
appeared inactive in in vivo mobilization of bone calcium or bone
phosphorus. On the other hand, in fetal rat bone in culture, the epi
compound was only five times less active than 1,25(OH)2D2 in inducing
resorption. Short-term experiments on bone mineral mobilization in vivo
show that the 24-epi-1,25(OH)2D2 does induce bone calcium mobilization but
that its activity in this respect is short lived. It is suggested that
24-epi-1,25(OH)2D2 and, as a result, it shows preferential activity on
intestine whose response to a single dose of 1,25(OH)2D2 remains for
several days, whereas the short-lived bone system does not remain
stimulated during the 24-h period between doses.
