In the body, 27-hydroxycholesterol is synthesized in multiple tissues, exported to the liver, and converted there to bile acids and bile salts. This pathway is only a minor source of bile acids and bile salts, but may play a significant role particularly in the mobilization of cholesterol from lung phagocytes (Bjorkhem et al. 1994; Babiker et al. 1999; Javitt 2002).
In the liver, conversion of 27-hydroxycholesterol to bile acids and bile salts is initiated with hydroxylation and oxidoreductase reactions to form 4-cholesten-7alpha,27-diol-3-one. The pathway then branches: hydroxylation of 4-cholesten-7alpha,27-diol-3-one to 4-cholesten-7alpha,12alpha,27-triol-3-one leads ultimately to the formation of cholate, while its reduction to 5beta-cholestan-7alpha,27-diol-3-one leads to chenodeoxycholate formation. In both branches, reactions in the cytosol, the mitochondrial matrix, and the peroxisomal matrix result in modifications to the ring structure, shortening and oxidation of the side chain, conversion to a Coenzyme A derivative, and conjugation with the amino acids glycine or taurine (Russell 2003). These reactions are outlined in the figure below. The final nine reactions are identical to ones of bile salt synthesis initiated by 7alpha-hydroxylation and are shown as arrows with no substrates.