Quantitative aspects of the interaction of bile acids with human serum albumin.

A Roda,G Cappelleri,R Aldini,E Roda,L Barbara

doi:10.1016/s0022-2275(20)38147-5

Abstract

The interaction of human serum albumin with twelve bile acids (ba) has been studied by equilibrium dialysis technique using 3H- and 14C-labeled bile acids. The physiological bile acids studied were: cholic, chenodeoxycholic, deoxycholic, lithocholic, ursodeoxycholic, and 7-ketolithocholic acids, all in the free and conjugated (with glycine and taurine) forms. For each bile acid studied, the interaction was characterized by two classes of binding sites, the first consisting of 2--4 sites and the second of 8--30. K1 values (liter/mol) for the different bile acids were: cholic acid, 0.3 x 10(4); chenodeoxycholic acid, 5.5 x 10(4); deoxycholic acid, 4.0 x 10(4); ursodeoxycholic acid, 3.8 x 10(4); 7-ketolithocholic acid, 1.9 x 10(4); lithocholic acid, 20 x 10(4). The affinity constant of a bile acid for albumin decreases with an increase in the number of hydroxy groups and also with the replacement of 7-hydroxy by 7-keto groups. The affinity constant is similar for glycine and taurine conjugated bile acids, but is slightly higher for unconjugated than conjugated forms.

Highlights

T h e interaction of human serum albumin with twelve bile acids has been studied by equilibrium dialysis technique using 3H- and 14C-labeled bile acids
The interaction of bile acids (BA) with plasma proteins has been known since 1957 when Rudman and Kendall ( l ), using an equilibrium dialysis method, reported that among the plasma proteins, the human albumin (HSA) fraction exhibits the greatest binding activity towards BA and that the affinity is reduced by the introduction of polar groups into the steroid moiety of the BA
Burke, Panvelivalla, and Tabaqchali, [2], using a steady-state gel filtration method, measured the affinity constant for cholic and taurocholic acid and found that the interaction with albumin was characterized by two classes of BA binding sites and that cholic acid is more avidly bound than its taurine conjugate

Summary

MATERIALS AND METHODS

Unlabeled bile acids were supplied by Calbiochem, La Jolla, CA. They proved to be more than 98% pure by GLC. Labeled BA were supplied by the Radiochemical Centre, Amersham, United Kingdom, and by New England Nuclear Corp., Boston, MA. 3H-7-Ketolithocholic acid was generously supplied by Dr H. Fromm (Gastroenterology and Nutrition Unit, University of Pittsburgh, PA). BA were purified by thin-layer chromatography a few days before the experiment using propionic acid-isoamyl acetate-water-N-propanol 3:4:1:2 (v/v/v/v) as the solvent system. Crystalline HSA, purchased from Sigma, London (essentially fatty acid free: less than 0.005%),was used without further purification

Procedure

RESULTS

GCA 4 TCA f 0 c

DISCUSSION