Bile acid synthesis is believed to be regulated by bile salts returning to the liver via the portal vein and suppressing cholesterol 7a-hydroxylase, the rate-limiting enzyme in the bile acid biosynthesis pathway. In order to characterize the relative effectiveness of bile salts in regulating bile acid synthesis, seven different bile acids were administered (1% wlw in chow) to rats over a 14day period. Biliary bile salt composition was determined from bile samples obtained prior to killing; in all cases, the fed bile acid became the predominant bile salt in bile. The specific activities of microsomal cholesterol 7a-hydroxylase, HMG-CoA reductase and acylcoenzyme A:cholesterol acyltransferase were determined after killing.
Hydrophilic bile salts (ursocholic, hyocholic, ursodeoxycholic and hyodeoxycholic) did not inhibit HMG-CoA reductase or cholesterol 7a-hydroxylase activities. By contrast, more hydrophobic bile salts (cholic, chenodeoxycholic and deoxycholic) inhibited the activities of these two enzymes in order of increasing hydrophobicity. Neither hydrophobic nor hydrophilic bile salts inhibited acylcoenzyme A:cholesterol acyltransferase activity. No consistent effect of bile acid feeding on total microsomal cholesterol was observed.
Based on the results of these studies, we propose that the hydrophilic-hydrophobic balance of the bile acid pool may play an important role in the regulation of bile acid synthesis. We postulate that the activities of cholesterol 7a-hydroxylase and HMG-CoA reductase may be regulated by hydrophobic bile acid-induced changes in the lipid composition and physicochemical properties (fluidity) of the microsomal membranes to which both of these rate-limiting enzymes are attached. Alternatively, hydrophobic bile salts by increasing biliary cholesterol secretion and suppressing HMG-CoA reductase activity may reduce the availability of cholesterol substrate for bile acid synthesis.
Bile acid synthesis is a principal route of cholesterol degradation, accounting for approximately 50% of daily