Cytosolic bile acid binding protein in rat liver: radioimmunoassay, molecular forms, developmental characteristics...

Rat liver efficiently extracts bile acids from the portal blood and rapidly excretes them into bile. Little is known about the process by which bile acids traverse the liver cell from the sinusoidal to the canalicular membrane. In order to begin to define this process, we recently identified a pair of similar monomeric (33 kilodalton) cytosolic bile acid binding proteins (bile acid binders I and II, J. Biol. Chem. 1983; 258:3602-3607, Abstract). These bile acid binders have comparable binding affinities for bile acid as the YaYa and YaYc members of the family of glutathione S-transferases, the previously recognized cytoplasmic bile acid binding protein. We now report the establishment of a sensitive and specific radioimmunoassay which equally detects both bile acid binders I and II. Specificity of the antiserum was verified by the co-purification of bile acid binder immunoreactivity with the bile acid binders. Liver contained the greatest concentration of bile acid binder, where it constituted 0.33% of the total cytosolic proteins. Phenobarbital administration and lithocholate feeding had no significant effect on hepatic bile acid binder content. Examination of the ontogeny of bile acid binder revealed a rapid increase after birth to near adult levels by Day 14. In summary, we have established a sensitive radioimmunoassay for the bile acid binders. Its localization mainly in liver and its increase after birth parallel bile acid transport in the liver.