Transbilayer movement of fully ionized taurine-conjugated bile salts depends upon bile salt concentration, hydrophobicity, and membrane cholesterol content.

Abstract

Taurine-conjugated bile salts mediate rapid transmembrane flux of divalent cations, irrespective of whether bile salts and divalent cations are initially on the same or opposite side of the membrane. We therefore hypothesized that ionized bile salts can equilibrate between membrane hemileaflets. We quantitated bile salt binding to large unilamellar egg yolk phosphatidylcholine (EYPC) +/- cholesterol (Ch) vesicles under conditions in which one or both hemileaflets were initially exposed to bile salts. At unbound taurodeoxycholate (TDC) concentrations >0.2 mM, the dependence of binding on TDC concentration after 30 min was indistinguishable for vesicles prepared by either method and did not change from 30 minutes to 24 h. At unbound TDC concentrations <0.1 mM, the ratio of bound/free TDC to EYPC vesicles doubled over a single exponential time course. Equilibration times were greater for the more hydrophilic bile salts taurocholate and tauroursodeoxycholate, for EYPC/Ch vesicles, and at lower temperatures. For glycine-conjugated bile salts, time-dependent changes in binding did not occur, consistent with more rapid equilibration of the small fraction of the protonated form. We conclude that fully ionized conjugated bile salts translocate between lipid bilayer hemileaflets, in contrast to previous observations that equilibration of fully ionized unconjugated bile salts occurs at a negligible rate in small unilamellar vesicles. The rate of "flip-flop" increases with increases in intramembrane bile salt concentration and hydrophobicity but decreases with cholesterol content and lower temperature. We speculate that physiologically, even in the absence of a specific membrane transporter, bile salts can gain access to intracellular compartments and mediate increases in divalent cation flux that may underlie cytotoxicity.