25-Hydroxycholecalciferol: High Affinity Substrate for Hepatic Cytochrome P-450
In vitro studies with hepatic microsomes showed that 25-hydroxy-vitamin D3 (25-OH-D3) is bound tightly by the microsomal cytochrome P-450 system, with the spectral dissociation constant of 84 nM the lowest reported to-date for a natural or xenobiotic compound. Vitamin D2 and dihydrotachysterol also were bound, but their dissociation constants were 200–300 times higher. Aminopyrine demethyl-ation was competitively inhibited in the presence of 25-OH-D3 and NADPH cytochrome. P-450 reductase activity was doubled by 25-OH-D3 addition. Taken together these findings suggest that liver microsomes are involved in the transformation and degradation of 25-OH-D3 and other vitamin D congeners. However, the enzyme systems are not vitamin D-dependent. Moreover, even though phenobarbital treatment led to a doubling of enzyme activity and, in animals undergoing vitamin D depletion, to a 40% faster time-dependent drop of 25-OH-D3 plasma levels, as compared to untreated controls, the two groups had the same levels of intestinal calcium-binding protein (CaBP). If CaBP is one expression of the active metabolite, 1,25-dihydroxy vitamin D3 (1,25-(OH) 2-D3), then alterations of a substrate (25-OH-D 3)-product (1,25-(OH)2-D3) relationship at a single control point (liver), brought about by phenobarbital treatment, did not lead to detectable changes in hormone expression. The experiments underscore the stability of the calcium regulating system and point to the existence of multiple control loops.
KeywordsLiver Microsome Maximal Binding Capacity Plasma Calcium Level Phenobarbital Treatment High Affinity Substrate
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