Alterations in the physicochemical characteristics of low and high density lipoproteins after lipolysis with phospholipase A(2). A spin-label study

Human low and high density lipoproteins (LDL and HDL, respectively) were treated with porcine pancreatic phospholipase A(2) (PLA(2)) in the presence of albumin resulting in hydrolysis of 40-84% of the lipoproteins phospholipids. The resulting PLA(2)-treated LDL and HDL and concurrent control lipoproteins incubated without PLA(2) were reisolated by ultracentrifugation and labelled with 5-doxyl- and 16-doxyl-stearic acid, and with a spin-labelled analogue of maleimide. Analysis of ESR spectra showed that phospholipid hydrolysis both of LDL and HDL resulted in an increase in order, micro-viscosity and polarity of lipid regions in the surface monolayer of the particles. in the temperature range from 3 degrees C to 50-60 degrees C, Arrhenius plots of a spectral parameter of LDL and HDL labelled with 5-doxyl-stearate exhibited alterations which suggest an increase in free cholesterol content near the surface of the lipoproteins after PLA(2)-treatment. ESR spectra of the maleimide analogue bound covalently to the protein moiety of the lipoproteins have demonstrated that, following phospholipid hydrolysis, the conformation of the apoproteins became more condensed, with more masked domains. The possible implications of the revealed alterations for enhanced delivery of LDL and HDL cholesterol to cells after phospholipolysis of the lipoproteins are discussed.