An ontogenically regulated 48-kDa protein is a component of the Na⁺-bile acid cotransporter of rat liver

Recent evidence suggests that the Na⁺-coupled carrier mechanism for bile acids on the hepatocyte basolateral plasma membrane is a polypeptide in the molecular weight range of 48,000-50,000. In this study we used a strategy for the identification and isolation of this transport protein based on the observation that Na⁺-dependent transport activity is abruptly expressed in fetal rat liver just before birth [Suchy et al. Am. J. Physiol. 251 (Gastrointest. Liver Physiol. 14): G665-G673, 1986]. Analysis of basolateral plasma membranes by SDS-PAGE revealed that a protein of apparent molecular weight 48,000 was absent from fetal rat liver on day 19 of gestation, barely detectable on day 20, and thereafter increased progressively with postnatal development. Monospecific, polyclonal antibodies raised against the 48-kDa protein but not preimmune antibodies significantly inhibited the initial rate of Na⁺-dependent taurocholate uptake by isolated rat hepatocytes. In contrast, Na⁺-independent taurocholate transport and uptake of another anion, ³⁵SO₄^2-, were not affected by antibody treatment. When an extract containing the total complement of basolateral proteins was incorporated into asolectin liposomes, Na⁺ gradient-dependent uptake of taurocholate was observed, including a 2- to 2.5-fold accumulation of substrate above its equilibrium concentration (overshoot). However, if the membrane extract was first selectively depleted of the 48-kDa protein by immunoprecipitation with the anti-48-kDa antibody before reconstitution, Na⁺-dependent stimulation of taurocholate transport was completely abolished. These studies indicate that an ontogenically regulated 48-kDa protein is a component of the basolateral Na⁺-dependent transport system for bile acids.