SURFACE-PROPERTIES OF NATIVE HUMAN PLASMA-LIPOPROTEINS AND LIPOPROTEIN MODELS

Plasma lipoprotein surface properties are important but poorly underst ood determinants of lipoprotein catabolism. To elucidate the relation between surface properties and surface reactivity, the physical proper ties of surface monolayers of native lipoproteins and lipoprotein mode ls were investigated by fluorescent probes of surface lipid fluidity, surface lateral diffusion, and interfacial polarity, and by their reac tivity to Naja melanoleuca phospholipase A(2) (PLA(2)). Native lipopro teins were human very low, low-, and subclass 3 high-density lipoprote ins (VLDL, LDL, and HDL3); models were 1-palmitoyl-2-oleoyl-sn-glycero -3-phosphocholine (POPC) or its ether analog in single-bilayer vesicle s, large and small microemulsions of POPC and triolein, and reassemble d HDL (apolipoprotein A-I plus phospholipid). Among lipoproteins, surf ace lipid fluidity increased in the order HDL3 < LDL < VLDL, varying i nversely with their (protein + cholesterol)/phospholipid ratios. Model s resembled VLDL in fluidity. Both lateral mobility in the surface mon olayer and polarity of the interfacial region were lower in native lip oproteins than in models. Among native lipoproteins and models, increa sed fluidity in the surface monolayer was associated with increased re activity to PLA(2). Addition of cholesterol (up to 20 mol%) to models had little effect on PLA(2) activity, whereas the addition of apolipop rotein C-lll stimulated it. Single-bilayer vesicles, phospholipid-trio lein microemulsions, and VLDL have surface monolayers that are quantit atively similar, and distinct from those of LDL and HDL3. Surface prop erty and enzymatic reactivity differences between lipoproteins and mod els were associated with differences in surface monolayer protein and cholesterol contents. Thus differences in the surface properties that regulate lipolytic reactivity are a predictable function of surface co mposition.