Peroxisomal cholesterol synthesis in vivo: accumulation of 4-methyl intermediate sterols after aminotriazole inhibition of cholesterol synthesis.

To clarify the importance and pathway of peroxisomal cholesterol synthesis in vivo, we have examined whether or not 4,4-dimethyl-5 alpha-cholest-8-en-3 beta-ol and 4 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol are accumulated in hepatic peroxisomes of aminotriazole-treated rats (we have shown that these intermediate steroids accumulate in rat liver when cholesterol synthesis is inhibited by aminotriazole: Hashimoto, F. and Hayashi, H. (1991) Biochim. Biophys. Acta 1086, 115). Differential centrifugation and Nycodenz gradient centrifugation showed that these intermediate steroids were localized in peroxisomes and microsomes. Cholestyramine (3-hydroxy-3-methylglutaryl-CoA reductase activator) pretreatment of aminotriazole-treated rats increased the contents of the intermediate steroids in both peroxisomes and microsomes. In peroxisomes, both 4 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol and 4,4-dimethyl-5 alpha-cholest-8-en-3 beta-ol were increased to about 3 times the control (aminotriazole-treated rat), and they were predominantly (about 70%) recovered in the membrane fraction after treatment with 0.05% deoxycholate or 100 mM Na2CO3. Gemfibrozil (peroxisomal proliferator) pretreatment enhanced the contents of 4 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol and 4,4-dimethyl-5 alpha-cholest-8-en-3 beta-ol of peroxisomes to 4.5 times and 37 times the control, respectively. The effects of aminotriazole, cholestyramine and gemfibrozil on the intermediate contents were different between peroxisomes and microsomes. We suggest that peroxisomes in addition to microsomes participate in cholesterol synthesis in vivo, and the biosynthetic pathway includes 4 alpha-methyl-5 alpha-cholest-7-en-3 beta-ol and 4,4-dimethyl-5 alpha-cholest-8-en-3 beta-ol.