Metabolism of Non-Enzymatically Derived Oxysterols: Clues from sterol metabolic disorders.

ABSTRACT

Cholestane-3ß,5α,6ß-triol (3ß,5α,6ß-triol) is formed from cholestan-5,6-epoxide (5,6-EC) in a reaction catalysed by cholesterol epoxide hydrolase, following formation of 5,6-EC through free radical oxidation of cholesterol. 7-Oxocholesterol (7-OC) and 7ß-hydroxycholesterol (7ß-HC) can also be formed by free radical oxidation of cholesterol. Here we investigate how 3ß,5α,6ß-triol, 7-OC and 7ß-HC are metabolised to bile acids. We show, by monitoring oxysterol metabolites in plasma samples rich in 3ß,5α,6ß-triol, 7-OC and 7ß-HC, that these three oxysterols fall into novel branches of the acidic pathway of bile acid biosynthesis becoming (25R)26-hydroxylated then carboxylated, 24-hydroxylated and side-chain shortened to give the final products 3ß,5α,6ß-trihydroxycholanoic, 3ß-hydroxy-7-oxochol-5-enoic and 3ß,7ß-dihydroxychol-5-enoic acids, respectively. The intermediates in these pathways may be causative of some phenotypical features of, and/or have diagnostic value for, the lysosomal storage diseases, Niemann Pick types C and B and lysosomal acid lipase deficiency. Free radical derived oxysterols are metabolised in human to unusual bile acids via novel branches of the acidic pathway, intermediates in these pathways are observed in plasma.