Inverse relationship between plasma cholestanol concentrations and bile acid synthesis in sitosterolemia.

We investigated the relationship between plasma cholestanol (5a-dihydrocholesterol) concentrations and the ac- tivity and mRNA levels of cholesterol 7a-hydroxylase, the rate- controlling enzyme for bile acid synthesis, in three female sitosterolemic homozygotes. In this lipid storage disease, large amounts of plant sterols and cholestanol accumulate because of hyperabsorption and endogenous synthesis, respectively. Plasma cholestanol concentrations were 14 times greater in the three sitosterolemic homozygotes than the mean for five control sub- jects. To investigate the cholestanol biosynthetic pathway, tracer doses of two putative precursors, (1,2-3H)4-cholesten-3-one and (4-14C)7a-hydroxycholesterol were injected intravenously into a homozygote, and radioactivity was sought in cholestanol, bile acids, cholesterol, and sitosterol fractions isolated from plasma and bile. Tritium was concentrated only in cholestanol; neither cholesterol, sitosterol nor bile acids were derived from (1,2-3H)4- cholesten-3-one. In contrast, bile acids were labeled exclusively with I4C from (4-'4C)7a-hydroxycholesterol; no 14C radioac- tivity was detected in cholestanol. Mathematical analysis of specific activity versus time curves for (3H)cholestanol revealed very slow decay, large exchangeable pools, and enhanced synthe- sis in the sitosterolemic homozygote. Measurements of choles- terol 7a-hydroxylase activity were 39% lower in whole liver microsomes from three sitosterolemic homozygotes that con- tained 19% plant sterols as compared to the mean value for six control microsomal specimens that contained 0.1% plant sterols. Removal of the excess plant sterols from the microsomes, in vitro, normalized microsomal cholesterol 7a-hydroxylase ac- tivity in the homozygotes but did not affect enzyme activity in the controls. Equal amounts of cholesterol 7a-hydroxylase mRNA were detected in the livers of both control and sitostero- lemic subjects. Bile acid malabsorption after ileal bypass surgery stimulated cholesterol 7a-hydroxylase activity 78% in sitostero- lemic whole liver microsomes and reduced plasma cholesterol, sitosterol, and cholestanol levels 6l%, 55% and 91%, respec- tively, producing a pronounced decrease in the cholestanolkho- lesterol ratio without changing the sitosterolkholesterol ratio. These results demonstrate that increased cholestanol is synthe- sized from 4-cholesten-3-one and not 7a-hydroxycholesterol in sitosterolemia. Enhanced pools and plasma concentrations are related inversely to hepatic cholesterol 7a-hydroxylase activity. Competitive inhibition of cholesterol 7a-hydroxylase by the large microsomal plant sterol pool diverts cholesterol into cholestanol. Alternatively, stimulating cholesterol 7a-hydroxylase activity after ileal bypass surgery markedly diminished plasma cholestanol levels. We propose that increased cholestanol is syn- thesized in sitosterolemic subjects because cholesterol 7a- hydroxylase is competitively inhibited by the large microsomal sitosterol pool so that cholesterol that cannot be transformed to bile acids is available as substrate for cholestanol formation. --den, G., A. K. Batta, G. S. Tint, S. Shefer, and G. C. Ness. Inverse relationship between plasma cholestanol concen- trations and bile acid synthesis in sitosterolemia. J. Lipid Res. 1994. 35: 1a78-1887.