The rate-limiting step in the synthesis of bile acids is catalyzed by:

The rate-limiting step in the formation of bile acids is the 7a-hydroxylation of cholesterol, catalyzed by a microsomal NADPH-dependent enzyme 7a-hydroxylase. 12a-Hydroxylase is also involved in the bile acid synthesis, but is not regulated. HMG CoA reductase and pyro diphosphomevalonate decarboxylase are both cholesterol-synthesizing enzymes; 25-hydroxycholecalciferol 1-hydroxylase is a kidney enzyme, which catalyzes the formation of an active vitamin D3 metabolite, 1,25-di-OH D3. Ref: Lippincott's Biochemistry, 5th Ed, page 241