CHOLESTEROL EFFECT ON THE PHYSICAL STATE OF LIPID MULTIBILAYERS FROM THE PLATELET PLASMA-MEMBRANE BY TIME-RESOLVED FLUORESCENCE

There are indications that the plasma membrane lipid composition and, in particular, the cholesterol/phospholipid (C/PL) ratio, affects plat elet function. As a first approximation to the molecular characterizat ion of the effect of cholesterol on the order, fluidity and lateral he terogeneity of the platelet plasma membrane, the steady-state and time -resolved fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH) and tran s-parinaric acid (tPnA) has been studied in multibilayer vesicles of p hospholipids extracted from human platelet plasma membrane with differ ent cholesterol/phospholipid molar ratios modified in vitro from 0.07 to 0.9. The DPH studies show that the increased presence of cholestero l has a stronger effect on the order than on the fluidity of the bilay er, as has been previously observed in other lipid membranes. On the o ther hand, from the analysis of the fluorescence kinetics of tPnA we c onclude that a higher cholesterol content gives rise to an increase of the heterogeneity of the bilayer, due to a larger fraction of solid-l ike lipid domains. These domains contain a cholesterol concentration m uch higher than the macroscopic average value.