A study of the adsorption of bile salts onto model lecithin membranes

Abstract

Bile salts play a central role in the promotion of cytotoxicity or cytoprotection. In this study, we examined the interaction of different bile salts with egg lecithin vesicles using 31P NMR spectroscopy. The effects of taurochenodeoxycholate (TCDC or 3α,7α,-dihydroxy-5β-cholanoyl taurine, of tauroursodeoxycholate (TUDC) or 3α,7β,-dihydroxy-5β-cholanoyl taurine) and of tauroβmuricholate (TβMC or 3α,6β,7β,-trihydroxy-5β-cholanoyl taurine), at various bile salt/lecithin ratios, were evaluated. From the percent 31P present in vesicles, the micellar capacity of bile salts to dissolve lecithin was determined. TCDC was incorporated into vesicles for bile salt/lecithin molar ratios lower than 0.62 while for TUDC and TβMC, the critical ratios were 0.94 and 1.1, respectively. The 31P chemical shift change was markedly larger with TCDC than that found with TUDC and TβMC. In order to specify the low interactions observed between hydrophilic bile salts and lecithin, we determined the intermixed micellar/vesicular bile salt concentrations (IMVC) of bile salt/lecithin solutions using rapid ultrafiltration–centrifugation for TUDC and lecithin solubility measurements for TUDC, TβMC and TCDC. The low IMVC obtained indicate that even hydrophilic bile salts were bound mostly to the mixed aggregates. In conclusion, the low disturbance in the arrangement of lecithin induced by TUDC and TβMC appears to be due to the interfacial location of these bile salts. TCDC (7α OH) penetrates more deeply in the membrane than the 7β hydroxylated bile salts that may partly explain the distinct damaging effects of these bile salts.