COMPETITIVE-INHIBITION OF HEPATIC STEROL 27-HYDROXYLASE BY SITOSTEROL- DECREASED ACTIVITY IN SITOSTEROLEMIA

We investigated the effect of sitosterol on hepatic sterol 27-hydroxyl ase activities in subjects with sitosterolemia, a recessive inherited disease associated with accelerated atherosclerosis and increased leve ls of sitosterol and other plant sterols and stanols in tissues. Hepat ic activities of mitochondrial sterol 27-hydroxylase, which catalyzes the first step in the conversion of cholesterol to bile acids via the acidic bile acid synthetic pathway, were measured in liver tissues and related to hepatic microsomal cholesterol 7 alpha-hydroxylase, which controls the rate of bile acid synthesis via the neutral synthetic pat hway, These measurements of cholesterol catabolism were correlated to sterol concentrations and composition in plasma and liver. Sterol 27-h ydroxylase activities in liver mitochondria of three homozygous sitost erolemic subjects were 68% lower than in 10 control subjects (p < .05) and were associated with increased levels of plant sterols in both pl asma and liver (13% and 16% of total sterols, respectively, compared t o trace amounts in controls). Analysis of Lineweaver-Burk double recip rocal plots of sterol 27-hydroxylase activities in control human liver specimens (where mitochondrial sterol 27-hydroxylase activities were measured with increasing concentrations of the cholesterol substrate, in the absence and presence of 100 mu M and 300 mu M sitosterol) revea led that sitosterol inhibited mitochondrial sterol 27-hydroxylase acti vity up to 50% by a competitive mechanism. In sitosterolemic subjects, competitive inhibition of hepatic sterol 27-hydroxylase activity by s itosterol was associated with competitively inhibited microsomal chole sterol 7 alpha-hydroxylase activity (averages from 4 sitosterolemic ho mozygotes and 14 controls were 12.4 +/- 1.9 and 23.6 +/- 2.5 pmol/mg/m in, respectively). Furthermore, decreased cholesterol catabolism in si tosterolemic subjects was associated with significantly elevated plasm a cholesterol concentrations (232 +/- 17 mg/dl, as compared to 180 +/- 13 mg/dl in controls) but with no change in hepatic cholesterol conce ntrations. In an animal model (rats infused intravenously with sitoste rol-containing liposomes that increased sitosterol in the liver and pl asma to levels similar to those found in sitosterolemic subjects), hep atic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7 alpha-hydroxylase activities also decreased significantly and were cou pled to markedly elevated plasma sterol concentrations (120.7 +/- 12.5 mg/dl, as compared to 59.2 +/- 6.3 mg/dl in control animals; p < .05) but to no change in hepatic cholesterol concentrations. Thus, decreas ed cholesterol catabolism due to competitive inhibition of both micros omal cholesterol 7 alpha-hydroxylase and mitochondrial sterol 27-hydro xylase by elevated hepatic sitosterol concentrations contributes to hy percholesterolemia and increased risk of atherosclerosis in sitosterol emia.