ADSORPTION OF COMPLEMENT PROTEINS C3 AND C1Q

Surface localization plays a key but ill defined role in activation of the serum complement system with or without related ''opsonic'' prote ins. The adsorption of key complement components C3 and C1q and variou s opsonins, e.g., IgG, were therefore studied on different surfaces us ing in situ ellipsometry. The affinities of C3 and C1q for silica, met hylated silica, and various phospholipid surfaces were shown to be lar gely reciprocal. While C3 adsorbed more extensively at (hydrophilic an d negatively charged) silica than at (hydrophobic) methylated silica ( 3.1 versus 0.4 mg/m(2), respectively) the opposite trend was observed for C1q (1.9 versus 2.6 mg/m(2)). C3 and C1q adsorbed in 10- to 15-nm thick layers on both silica and methylated silica. Each protein appear ed to adsorb with consistent conformation and orientation on either su rface. Adsorbed layer formation involves increased protein packing den sity, and molecular extension normal to the surface. Phospholipid head group properties strongly affect the adsorption of C3 and C1q at phos pholipid coated surfaces. The saturation adsorption of C3 at phosphati dic acid was almost as significant as at silica, whereas the amount ad sorbed at phosphatidylcholine was three times lower. C3 adsorption at phosphatidylinositol and various poly(ethylene glycol) modified surfac es was virtually absent, as was the adsorption of various opsonins. C1 q adsorption was relatively low at all phospholipid and poly(ethylene glycol) coated surfaces investigated, more in the manner of IgG than C 3. Preadsorption of IgG increased C1q deposition at phospholipid surfa ces strongly. C3 and human serum albumin, but not C1q, showed apprecia ble hydrophobic affinity for a poly(ethylene glycol)-fatty acid ester of oleic acid. These results are discussed in relation to complement i nteraction with various surfaces and colloidal drug carriers. (C) 1996 Academic Press, Inc.