Synthesis of bile acids and bile salts via 27-hydroxycholesterol

Stable Identifier
Homo sapiens
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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.

Literature References
PubMed ID Title Journal Year
10428977 Elimination of cholesterol as cholestenoic acid in human lung by sterol 27-hydroxylase: evidence that most of this steroid in the circulation is of pulmonary origin

Babiker, A, Andersson, O, Lindblom, D, van der Linden, J, Wiklund, B, Lutjohann, D, Diczfalusy, U, Bjorkhem, I

J Lipid Res 1999
8078928 Atherosclerosis and sterol 27-hydroxylase: evidence for a role of this enzyme in elimination of cholesterol from human macrophages

Bjorkhem, I, Andersson, O, Diczfalusy, U, Sevastik, B, Xiu, RJ, Duan, C, Lund, E

Proc Natl Acad Sci U S A 1994
12543708 The enzymes, regulation, and genetics of bile acid synthesis

Russell, DW

Annu Rev Biochem 2003
11969205 Cholesterol, hydroxycholesterols, and bile acids

Javitt, NB

Biochem Biophys Res Commun 2002
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