Bile acid inhibition of taurocholate uptake by rat hepatocytes: role of OH groups.
To define further the structural specificity of the taurocholate uptake site, we studied the ability of a variety of taurine-conjugated bile acids with differing hydroxyl substituents on the sterol moiety to inhibit [14C]taurocholate uptake. Rat hepatocytes isolated by collagenase perfusion were incubated in a tris(hydroxymethyl)aminomethane-phosphate buffer containing [14C]taurocholate (2.5-100 microM) in the presence or absence of inhibitor bile acid. Stronger inhibitors were studied at a fixed concentration of 5 microM, weaker ones at 25 microM. Initial uptake velocity was measured by sedimenting an aliquot of cells through silicone oil into 3 N KOH every 15 s for 1 min. Uptake velocity (nmol X mg protein-1 X min-1) could then be related to taurocholate concentration and a Vmax and Km could be determined by applying a nonlinear least squares fit to the data obtained with or without inhibitor. The kinetic parameters allowed the determination of the type of inhibition and of inhibition constants (Ki) of the various test bile acids. The data indicate that bile acids containing a 6- or 7-OH group exhibit competitive inhibition, whereas bile acids with no 6- or 7-OH group exhibit noncompetitive inhibition. Of the compounds exhibiting competitive inhibition, Ki varied with the number of hydroxyl groups on the sterol moiety. We conclude that the presence or absence of a 6- or 7-OH group dictates the mechanism of inhibition; the number of hydroxyl substituents determines the potency of competitive inhibition.
D000818:Animals; D001647:Bile Acids and Salts / Q000494:pharmacology*; D001692:Biological Transport / Q000187:drug effects; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D051381:Rats; D012964:Sodium / Q000378:metabolism; D013261:Sterols / Q000494:pharmacology; D013329:Structure-Activity Relationship; D013656:Taurocholic Acid / Q000378:metabolism*
D001647:Bile Acids and Salts; D013261:Sterols; D013656:Taurocholic Acid; D012964:Sodium