Chemical synthesis of N-acetylcysteine conjugates of bile acids and in vivo formation in cholestatic rats as shown by liquid chromatography/electrospray ionization-linear ion trap mass spectrometry

N-Acetylcysteine (NAC) conjugates of the five major bile acids occurring in man were synthesized in order to investigate the possible formation in vivo of these conjugates. Upon collision-induced dissociation, structurally informative daughter ions were observed. The transformation of cholyl-adenylate and cholyl-CoA thioester into a N-acetyl-S-(cholyl)cysteine by rat hepatic glutathione S-transferase was confirmed by liquid chromatography/electrospray ionization-linear ion trap mass spectrometry (LC/ESI-MS2). Lithocholic acid was administered orally to bile duct-ligated rats that also received NAC intraperitoneally. The NAC conjugate of lithocholic acid was identified in urine by means of LC/ESI-MS2. Rapid hydrolysis of the BA-NAC conjugates by rabbit liver carboxylesterase was found, demonstrating the possible labile nature of the NAC conjugates formed in the liver.