Mechanism of Microbial Transformation of Cholesterol into Coprostanol

Abstract

The conversion of cholesterol into coprostanol by cecal contents from rats under anaerobic conditions was studied using deuterated water, [4β‐3H,4‐14C]‐ and [3α‐3H,4‐14C]cholesterol. On the basis of the following experimental findings, it was concluded that the conversion of cholesterol into coprostanol proceeds to at least 50% by means of the intermediate formation of Δ4‐cholestenone. Conversion of [3α‐3H,4‐14C]cholesterol into coprostanol occurred with loss of 50% of the tritium. The tritium retained in coprostanol was located in the C‐3α position. However, part of the tritium retained might have been introduced in a reductive step following an initial oxidative step. Evidence for this possibility was provided by the finding that coprostanol formed from [4‐14C]cholesterol in the presence of [3α‐3H]β‐sitosterol contained significant amounts of tritium in the C‐3α position. After conversion of part of [3α‐3H,4‐14C]cholesterol into coprostanol, unchanged cholesterol had a higher 3H/14C ratio than the cholesterol added to the incubation mixture, indicating that oxidation of the 3β‐hydroxyl group is at least partly rate limiting in the over‐all conversion of cholesterol into coprostanol. After incubation of cholesterol in the presence of deuterated water, the coprostanol isolated had a deuterium content of about 1.4 atoms at C‐2, C‐3 and C‐4, about 0.5 atoms at C‐5 and about 0.1 atoms at C‐6, indicating extensive oxido‐reduction at C‐3 during or after conversion of cholesterol into coprostanol. Coprostanol isolated after incubation of [4β‐3H,4‐14C]cholesterol retained 60% of the tritium. The main part of this tritium had been transferred to the C‐6 position, showing that the conversion of cholesterol into coprostanol involves isomerization of the Δ5 double bond to a Δ4 double bond. Incubation of Δ4‐[4‐14C]cholestenone resulted in efficient formation of coprostanol.

Elimination of Cholesterol in Macrophages and Endothelial Cells by the Sterol 27-Hydroxylase Mechanism

“…It is noteworthy that the sterol 27-hydroxylase is located at the inner membranes of the mitochondria (21). The nature of the mechanism behind transfer of cholesterol from the outer to the inner mitochondrial membranes in the present cells is not known.…”

“…Two of the patients had a deletion of thymine in exon 4, and one had a splice junction mutation in intron 4 as described previously (11). The patients (20,21, and 15 years, respectively) had xanthomas and elevated serum cholestanol levels typical for the disease (12). Blood was drawn from these patients and from healthy volunteers for isolation of monocytes (see below).…”

Propionate Enhances Synthesis and Secretion of Bile Acids in Primary Cultured Rat HepatocytesviaSuccinyl CoA

“…17 ) This mitochondrial hydroxylase requires NADPH generated by the NADP-dependent isocitrate dehydrogenase or by transhydrogenation of NADH formed during oxidation of organic acids in the citric acid cycle. 18) Therefore, it remains possible that succinyl CoA and its precursors caused an elevated production of bile acids via mitochondrial cholesterol 70:-hydroxylase. However, since additions of oxaloacetate and malate, which can easily penetrate into mitochondria, did not increase the bile acid secretion, the contribution of the mitochondrial 70:-hydroxylase to the bile acid synthesis appears to be small.…”

A novel pathway for biosynthesis of cholestanol with 7 alpha-hydroxylated C27-steroids as intermediates, and its importance for the accumulation of cholestanol in cerebrotendinous xanthomatosis.

“…Thus the coprostanol formed from the mixture of 7a-3H-and 4-'4C-labeled cholesterol had the same 3H/'4C ratio as the precursor. Since the biosynthesis of coprostanol also involves 4-cholesten-3-one as an intermediate (29), an isotope effect in the conversion of cholesterol into 4-cholesten-3-one should have been reflected by a lowered 3H/ 14C ratio in coprostanol.…”