Photoaffinity labeling studies of the rat renal sodium bile salt cotransport system

The uptake of a photolabile taurocholate derivative, (7,7-azo-3 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oyl)-2-aminoethanesulfonate, 7,7-azo-TC, into rat renal brush-border membrane vesicles was stimulated by Na+ and inhibited by taurocholate indicating an interaction with the Na+/bile salt cotransport system. Irradiation of membrane vesicles in the presence of 7,7-azo-TC inhibited Na+-dependent taurocholate uptake irreversibly. Photoaffinity labeling with [3H]7,7-azo-TC resulted in a predominant incorporation of radioactivity into a polypeptide with apparent molecular weight of 99,000. These results suggest that the proteins involved in Na+/bile salt cotransport are similar in renal and ileal brush-border membranes, but differ from those in hepatocytes.