Effect of sitosterol on the rate-limiting enzymes in cholesterol synthesis and degradation

Attempts were made to develop an animal model for phytosterolemia. Infusion of Intralipid containing 0.2% sitosterol in rats gave circulating levels of sitosterol of about 2.5 mmol/l, which is similar to or higher than those present in patients with untreated phytosterolemia. In addition, the infusions gave serum levels of cholesterol nearly twice those obtained in rats infused with Intralipid alone or Intralipid containing 0.2% cholesterol. The hepatic HMG-CoA reductase activity was unaffected or slightly increased by the sitosterol infusions (not statistically significant). The cholesterol 7α-hydroxylase activity was slightly depressed (ca. 30%). In the case of 7α-hydroxylation of endogenous cholesterol, the depression reached statistical significance (p<0.05). The microsomal content of sitosterol in the sitosterol-infused rats was about 30% of that of microsomal cholesterol. The effect of sitosterol on 7α-hydroxylation of cholesterol was investigated by incubations of acetone powder of rat liver microsomes with mixtures of cholesterol and sitosterol. Sitosterol mixed with cholesterol to a composition similar to that found in the above microsomal fraction had a depressing effect on 7α-hydroxylation of cholesterol. This degree of depression was of the same magnitude as that found in the sitosterol infusion experiments. The possibility is discussed that the hypercholesterolemia obtained in the β-sitosterol-infused rats is due to the inhibitory effect of sitosterol on the cholesterol 7α-hydroxylase.