Bile acid production in human subjects: rate of oxidation of [24,25-3H]cholesterol compared to fecal bile acid excretion

The use of stable and radioactive sterol tracers as a tool to investigate cholesterol degradation to bile acids in humans in vivo

Alterations of cholesterol homeostasis represent important risk factors for atherosclerosis and cardiovascular disease. Different clinical-experimental approaches have been devised to study the metabolism of cholesterol and particularly the synthesis of bile acids, its main catabolic products. Most evidence in humans has derived from studies utilizing the administration of labeled sterols; these have several advantages over in vitro assay of enzyme activity and expression, requiring an invasive procedure such as a liver biopsy, or the determination of fecal sterols, which is cumbersome and not commonly available. Pioneering evidence with administration of radioactive sterol derivatives has allowed to characterize the alterations of cholesterol metabolism and degradation in different situations, including spontaneous disease conditions, aging, and drug treatment. Along with the classical isotope dilution methodology, other approaches were proposed, among which isotope release following radioactive substrate administration. More recently, stable isotope studies have allowed to overcome radioactivity exposure. Isotope enrichment studies during tracer infusion has allowed to characterize changes in the degradation of cholesterol via the "classical" and the "alternative" pathways of bile acid synthesis. Evidence brought by tracer studies in vivo, summarized here, provides an exceptional tool for the investigation of sterol metabolism, and integrate the studies in vitro on human tissue.

Simulation of the metabolism and enterohepatic circulation of endogenous deoxycholic acid in humans using a physiologic pharmacokinetic model for bile acid metabolism

The metabolism and enterohepatic circulation of deoxycholic acid (DCA), a major secondary bile acid in humans, was simulated using a linear multicompartmental physiologic pharmacokinetic model. The model was similar to that previously reported and used to simulate the metabolism of cholic acid and chenodeoxycholic acid, but differed in two respects: (a) the input of newly formed DCA molecules originated from colonic absorption rather than from de novo hepatic biosynthesis and (b) a new type of transfer coefficient was proposed to describe the movement of DCA molecules from an insoluble, bound compartment to a soluble compartment. Simulations were performed to define the effect of varying fractional colonic absorption (from 0.1 to 0.6) as well as varying fractional formation of DCA from cholic acid (from 0.3 to 1). The simulations indicated that the exchangeable total DCA pool expanded up to 12-fold as fractional colonic absorption was increased from 0.1 to 0.6. The fractional turnover rate of the DCA pool showed a corresponding decrease. Increased conversion of cholic acid to DCA had an effect on DCA pool size that was similar to that resulting from increased colonic fractional absorption. So long as ileal absorption was efficient, the "soluble" colonic pool of DCA remained small relative to other organ pools, and the absorption of unconjugated DCA from the colon was less than 10% of the total DCA absorption from the ileum. It is proposed that the relatively large proportion of DCA in the biliary bile acids of white adults in the Western world as compared with that of most other mammals is attributable to (a) a high fractional absorption of DCA because of a diet relatively low in fiber, (b) the absence of hepatic 7-hydroxylation of DCA, and (c) effective competition by DCA conjugates for active transport by the terminal ileum.