24,25-Dihydroxycholecalciferol, also known as 24,25-dihydroxyvitamin D3 (), are metabolites closely related to calcitriol, the active form of vitamin D3. Like vitamin D3 itself and calcifediol (25-OH D3), it is inactive as a hormone both in vitro and in vivo. It was first identified in 1972 in the laboratory of Hector DeLuca and Michael F. Holick.
It can be further distinguished into two epimers hydroxycalcidiol and hydroxycalcidiol (abbreviated as and ). The (24R) epimer is also known as secalciferol.
Formation 24(R),25-(OH)2D3 is formed from 25-hydroxyvitamin D3 by the action of CYP24A1 (25-hydroxyvitamin D3-24-hydroxylase). CYP24A1 appears to be "a multicatalytic enzyme catalyzing most, if not all, of the reactions in the C-24/C-23 pathway of 25-OH-D3 metabolism." 24(S),25-(OH)2D3 is derived from reduction of 25-OH-24-oxo-D3, a metabolite of 24(R),25-(OH)2D3.
The proportion of serum 24,25-(OH)2D3 that is the 24(R) isomer was found to be 96.8% in rats under standard conditions, decreasing to 89.5% several days after injection with a large dose of vitamin D3.
Biological role It has been proposed that 24,25-(OH)2D3 is a metabolite of 25-OH-D3 which is destined for excretion.
It is not known whether the compound have any physiologically significant activity. Some evidence of a possible receptor has been obtained.
The 24(R) form is a substrate for CYP27B1, which converts it to calcitetrol.
Interactive pathway map
References
Other articles