Use of 1H-NMR to determine the distribution of lecithin between the micellar and vesicular phases in model bile.

Biliary cholesterol/phospholipid vesicles play an important role in the pathogenesis of gallstone disease. A prerequisite for the study of the lipid composition and stability of these vesicles is a reliable method to quantify the amount of vesicular lipid. In the present report we show that NMR can be used to determine the distribution of biliary lecithin between the micellar and vesicular phases. The relatively large size of the vesicles leads to such a broadening of the lipid resonances that they are no longer visible in high resolution 1H-NMR spectra. Since micelles are much smaller, lipid present in the micellar phase does give rise to sharp peaks in 1H-NMR spectra. Micellar lecithin can easily be quantified in these spectra. The resonances of cholesterol are masked by the closely related bile acid that is present in a much higher concentration. By determining the difference between chemically and NMR estimated lecithin, the distribution of this phospholipid between the micellar phase and vesicular phase can be assessed. We have compared the results of NMR with gel permeation and density gradient ultracentrifugation. Using standard fractionation conditions, both gel permeation and density gradient ultracentrifugation lead to an underestimation of vesicular lecithin, the difference being minor at relatively high total lipid concentrations (10 g/dl) but large in diluted model bile. We conclude that 1H-NMR can be used to determine the distribution of lecithin in model bile.(ABSTRACT TRUNCATED AT 250 WORDS)