Specificity of an Na+ -dependent taurocholate transport site in isolated rat hepatocytes

The structural specificity of the taurocholate uptake mechanism was studied by determining the ability of 100 microM structural analogues of taurocholate to inhibit the uptake of 5 microM [14C]-taurocholate into freshly isolated rat hepatocytes. Taurocholate was a more potent inhibitor than cholate. The fewer the number of hydroxyl groups, the more potent the inhibitors. Neither hydroxyl group orientation (alpha or beta) nor esterification with formic acid strongly influenced inhibitory potency. Replacement of hydroxyl groups with keto groups, however, diminished inhibitory potency. These facts suggested that distortion of the sterol backbone may be an important factor in diminishing inhibitory potency. Charge on the nonsteroid moiety affected potency. Both neutral and negatively charged compounds inhibited more strongly than positively charged compounds. As important as charge, however, was length of the nonsteroidal moiety. Inhibitory potency increased progressively as length beyond C-17 increased up to 11 A. Inhibitory potency was reduced again, however, with an extremely long end group of 18 A. The inhibitors do not act as cytotoxins since cellular oxygen consumption was not diminished by any of them. Furthermore, inhibition was specific for the taurocholate uptake mechanism. The three most potent inhibitory compounds inhibited L-alanine uptake little or not at all. The kinetics of inhibition, determined for three selected compounds, were consistent with competitive inhibition. The structural specificity of the rat hepatocyte sodium-dependent taurocholate uptake site is similar to that of the guinea pig ileum uptake site.