The human liver-specific homolog of very long-chain acyl-CoA synthetase is cholate: CoA ligase

Unconjugated bile acids must be activated to their CoA thioesters before conjugation to taurine or glycine can occur. A human homolog of very long- chain acyl-CoA synthetase, hVLCS-H2, has two requisite properties of a bile acid:CoA ligase, liver specificity and an endoplasmic reticulum subcellular localization. We investigated the ability of this enzyme to activate the primary bile acid, cholic acid, to its CoA derivative. When expressed in COS- 1 cells, hVLCS-H2 exhibited cholate:CoA ligase (choloyl-CoA synthetase) activity with both nonisotopic and radioactive assays. Other long- and very long-chain acyl-CoA synthetases were incapable of activating cholate. Endogenous choloyl-CoA synthetase activity was also detected in liver-derived HepG2 cells but not in kidney-derived COS-1 cells. Our results are consistent with a role for hVLCS-H2 in the re-activation and re-conjugation of bile acids entering liver from the enterohepatic circulation rather than in de novo bile acid synthesis.