Influence of cholesterol feeding on liver microsomal metabolism of steroids and bile acids in conventional and germ-free rats.

The present investigation has aimed at defining the factor responsible for the differences in microsomal metabolism of steroids between germ-free and conventional rats. Cholesterol, cholic acid, taurocholic acid, and chenodeoxycholic acid were fed to conventional and germ-free male rats and the effects on liver microsomal metabolism of 4-[4-14C]androstene-3,17-dione, 5alpha-[4-14C]androstane-3alpha,17beta-diol, [4-14C]-cholesterol, 7alpha-hydroxy-4-[6beta-3H]cholesten-3-one, and [24-14C]lithocholic acid were studied. The most consistent effects were found with dietary cholesterol that stimulated the activities of several of the hydroxylases active on 4-androstene-3,17-dione and 5alpha-androstane-3alpha,17beta-diol and that decreased the 5alpha reduction of 4-androstene-3,17-dione, increased the 7alpha hydroxylation of cholesterol, decreased the 12alpha hydroxylation of 7alpha-hydroxy-4-cholesten-3-one, and increased by 6beta hydroxylation of lithocholic acid. These effects of cholesterol feeding on the microsomal metabolism of steroids in conventional rats made the pattern of microsomal enzyme activities resemble that characteristic of germ-free rats. Cholesterol feeding led to a pronounced increase in the intestinal concentration of beta-muricholic acid in conventional rats. Furthermore, cholesterol feeding to conventional animals led to an intestinal ratio of chenodeoxycholic acid (including its metabolites alpha- and beta-muricholic acid and hyodeoxycholic acid) to cholic acid (including deoxycholic acid) that was almost identical to that in germ-free rats. Conventionalization of germ-free rats for a period of up to 56 days led only to a partial normalization of the liver microsomal metabolism of 5alpha-[4-14C]androstane-3alpha, 17beta-diol and 7alpha-hydroxy-4-[6beta-3H]cholesten-3-one and of the liver microsomal concentration of cytochrome P-450. The concentration of cholesterol was higher in both total liver homogenate and liver microsomal fraction of germ-free rats than in corresponding preparations from conventional rats. In conclusion, it is suggested that cholesterol is one of the factors responsible for the different microsomal metabolism of steroids in germ-free and conventional rats. It is also suggested that cholesterol may play a role as regulator of microsomal enzyme activities.