Incorporation of bile acid of low concentration into model and biological membranes studied by H and ³¹P NMR¹

We have analyzed the manner of incorporation of bile acid into lipid bilayers and resultant perturbation of the bilayer structure with lower bile acid/lipid ratios relevant to the physiological conditions (̃1 mM) by ²H and ³¹P NMR methods, as an aid to understanding the possible role as an endogenous tumor promoter in colon cancer besides the primary physiological function of solubilizing lipids. On the basis of the ²H quadrupole splittings of [6,6,7,7,8-²H₅]deoxycholate and [11,11,12,12-²H₄]chenodeoxycholate in the presence of lamellar multibilayers of egg yolk lecithin, these bile acids were found to be incorporated in such a manner that the B-D rings lie parallel with the normal of the bilayers when the ratio of the bile acid to lipid is low (<0.11). When the ratio is increased, these bile acid molecules are not dispersed entirely in the bilayer but aggregate to form micelles with lipids. Further, we studied the resultant perturbation of the multibilayers of egg yolk lecithin analyzed by using the ²H quadrupole splitting of [l8,18,18-²H₃]stearic acid as a probe and by ³¹P chemical shift anisotropy. We found that the bilayer structure is retained even at the bile acid-to-lipid ratio of 0.25, although a small amount of an isotropic phase appeared such as small vesicles and micelles. The molecular ordering of fatty acyl chains was rather enhanced by the presence of 1 mM deoxycholate in erythrocyte ghosts as seen from the ²H quadrupole splitting of [16,16,16-²H₃]- palmitic acid, although deoxycholate caused hemolysis in this condition. The former observation can be explained by the way the lipid-protein interaction is modified by deoxycholate located in the interface between the lipids and proteins.