Differential effects of glycero- and sphingo-phospholipolysis on human high-density lipoprotein fluidity

This study investigates the effect of enzymatic modifications of the HDL(3) surface lipid composition on their physical properties. Human HDL(3) (d: 1.125-1.21 g/ml) was treated either by an exogenous phospholipase A(2) from Crotalus adamanteus or by a sphingomyelinase from Staphylococcus aureus in the presence of albumin for various periods of time in order to obtain several degrees of hydrolysis. Glycerophospholipid hydrolysis ranged from 13 to 81% and sphingomyelinase action led to a 31-92% sphingophospholipid degradation. Physical properties of the surface of HDL(3) were examined by two spectroscopic methods: fluorescence polarisation and electron spin resonance. Glycerophospholipolysis treatment of HDL(3) enhanced the fluorescence anisotropy values (6-18%) and both relaxation correlation time (30-100%) and degree of order. All these results indicated a more rigid environment, a decreased mobility and an increased order of the surface lipids. Conversely, treatment of the HDL(3) with sphingophospholipase induced a progressive fluidization: fluorescence polarisation and degree of order decreasing down to 10% and relaxation correlation time down to 35% compared to native HDL(3). Taken together, all these observations suggest the relative importance of the two major phospholipids to modulate the fluidity and order of the surface of HDL(3) and could account for several recent physiological observations.