Abstract
Stereospecific side-chain hydroxylations of 5β-cholestane-3α, 7α-diol were studied in mitochondrial and microsomal fractions of human liver. Incubation of 5β-cholestane-3α, 7α-diol resulted in hydroxylations at C-12, C-24, C-25, and C-26. Hydroxylations at C-24 and C-26 were accompanied by the introduction of additional asymmetric carbon atoms at C-24 and C-25 respectively, that led to the formation of two distinct pairs of diastereoisomers, namely 5β-cholestane-3α, 7α,24-triols (24R and 24S) and 5β-cholestane-3α, 7α,26-triols (25R and 25S). A sensitive and reproducible radioactive assay to measure the formation of the different biosynthetic 5β-cholestanetriols was developed. Optimal assay conditions for human mitochondrial and microsomal systems were tentatively established.
The mitochondrial fraction was found to predominantly catalyze the 26-hydroxylation of 5β-cholestane-3α, 7α-diol with the formation of the 25R-diastereoisomer of 5β-cholestane-3α, 7α,26-triol as the major product. In the microsomal fraction, on the other hand, 25-hydroxylation was more efficient than 26-hydroxylation and accounted for 6.4% of the total hydroxylations. The microsomes catalyzed the formation of both diastereoisomers of 5β-cholestane-3α, 7α,26-triol (25R and 25S, 4.2 and 1.6% respectively).
These experiments suggest that the initial step in the degradation of the steroid side chain during the biosynthesis of chenodeoxycholic acid in man is mediated by the mitochondria, and involves the formation of the 25R-diastereoisomer of 5β-cholestane-3α, 7α,26-triol. The role of the microsomal 25- and 26-hydroxylated intermediates requires further exploration.
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Selected References
These references are in PubMed. This may not be the complete list of references from this article.
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