Bile salt-binding polypeptides in brush-border membrane vesicles from rat small intestine revealed by photoaffinity labeling.

Photoaffinity labeling of small intestinal brush-border membrane vesicles with photolabile bile salt derivatives was performed to identify bile salt-binding polypeptides in these membranes. The derivatives used in this study were the sodium salts of 7,7-azo-3 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oic acid, 3 beta-azido-7 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oic acid, their respective taurine conjugates, and (11 xi-azido-12-oxo-3 alpha, 7 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-aminoethanesulfonic acid. With ileal brush-border membrane vesicles, photoaffinity labeling resulted in the identification of 5 polypeptides with apparent molecular weights of 125,000, 99,000, 83,000, 67,000, and 43,000. The extent of labeling depended on the photolabile derivative employed. In jejunal brush-border membrane vesicles, polypeptides with apparent molecular weights of 125,000, 94,000, 83,000, 67,000, and 43,000 were labeled. The results indicate that the binding polypeptides involved in bile salt transport in ileal brush-border membrane vesicles are 1) similar with one exception to those concerned with bile salt transport in jejunal brush-border membranes, and 2) markedly different from those previously shown to be concerned with bile salt transport in plasma membranes of hepatocytes.