Measurement of total fasting-state serum bile acids: comparison of a solid-phase bioluminescence enzymatic assay, a homogeneous fluorescence enzymatic assay and isotope dilution gas chromatography-mass spectrometry

Key discoveries in bile acid chemistry and biology and their clinical applications: history of the last eight decades

During the last 80 years there have been extraordinary advances in our knowledge of the chemistry and biology of bile acids. We present here a brief history of the major achievements as we perceive them. Bernal, a physicist, determined the X-ray structure of cholesterol crystals, and his data together with the vast chemical studies of Wieland and Windaus enabled the correct structure of the steroid nucleus to be deduced. Today, C24 and C27 bile acids together with C27 bile alcohols constitute most of the bile acid "family". Patterns of bile acid hydroxylation and conjugation are summarized. Bile acid measurement encompasses the techniques of GC, HPLC, and MS, as well as enzymatic, bioluminescent, and competitive binding methods. The enterohepatic circulation of bile acids results from vectorial transport of bile acids by the ileal enterocyte and hepatocyte; the key transporters have been cloned. Bile acids are amphipathic, self-associate in solution, and form mixed micelles with polar lipids, phosphatidylcholine in bile, and fatty acids in intestinal content during triglyceride digestion. The rise and decline of dissolution of cholesterol gallstones by the ingestion of 3,7-dihydroxy bile acids is chronicled. Scientists from throughout the world have contributed to these achievements.

Bile acid separation

A review of the methods available for the separation of bile acids is presented, highlighting the most recent developments. The major chromatographic techniques (TLC, GC, HPLC) and combined detection systems for the determination of bile acids are critically evaluated and their advantages and disadvantages discussed. Moreover, future directions in which progress might occur are also indicated. Capillary GC-MS is the more established method since it provides higher efficiency combined with greater sensitivity and specificity and has proven crucial in identifying unusual bile acids. However, it requires deconjugation and derivatization and hence the conjugated species must be inferred from the initial isolation procedure. HPLC is directly amenable to the different forms of bile acids, but it suffers from insufficient resolving power which can be enhanced by exploiting the mobile-phase selectivity. The development of HPLC detection systems with higher sensitivity and specificity than conventional HPLC-UV is reported. In particular, methods for the direct coupling of HPLC to MS are examined with special emphasis on soft ionization processes (thermospray, fast atom bombardment, ion spray). Finally, the analytical potential for bile acid assays of more recent techniques including supercritical fluid chromatography and capillary electrophoresis is evaluated.

Pharmacology of ursodeoxycholic acid, an enterohepatic drug

The pharmacokinetics, metabolism, as well as the pharmacodynamic actions of ursodeoxycholic acid are reviewed and related to its physicochemical properties. Ursodeoxycholic acid is absorbed incompletely because of its low aqueous solubility. After absorption, it is conjugated with glycine or taurine and circulates with the endogenous bile acids. At usual doses (8-10 mg/kg/day), the pool of ursodeoxycholyl conjugates constitutes 30-60% of circulating bile acids. Ursodeoxycholic acid is metabolized by intestinal bacteriae to lithocholic acid which does not accumulate in the circulating bile acids because of efficient hepatic sulfation. Administration of ursodeoxycholic acid causes decreased cholesterol absorption, increased bile acid biosynthesis, and decreased biliary cholesterol secretion. Ursodeoxycholic acid is a choleretic agent, as all bile acids, but differs from other dihydroxy-bile acids in being non-cytotoxic because it has less affinity for membranes, and when present at micellar concentrations does not solubilize membranes. Chronic administration of ursodeoxycholic acid appears to increase canalicular transport.