Effects of lipid composition and packing on the adsorption of apolipoprotein A-I to lipid monolayers

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Abstract
To better understand the factors controlling the binding of apolipoprotein molecules at the surfaces of serum lipoprotein particles, the adsorption of human apolipoprotein A-I to phospholipid monolayers has been studied. The influence of lipid packing was investigated by spreading the monolayers at various initial surface pressure (.pi.i) and by using various types of lipid. The adsorption of 14C-methylated apolipoprotein A-I was monitored by simultaneously following the surface radioactivity (which could be converted to the surface concentration of protein, .GAMMA.) and the change in surface pressure (.DELTA..pi.). In general, increasing the .pi.i of lipid monolayers reduces the adsorption of apolipoprotein A-I; for expanded egg phosphatidylcholine (PC) monolayers at .pi.i .gtoreq. 32 dyn/cm, .GAMMA. and .DELTA..pi. are zero. The degree of adsorption of the apolipoprotein is also influenced by the physical state of the lipid monolayers. Thus, at a given .pi.i, apolipoprotein A-I adsorbs more to expanded monolayers than to condensed monolayers so that, at a given subphase concentration of protein, .GAMMA. of adpolipoprotein A-I with various phospholipid monolayer decreases in the order egg PC > egg sphingomyelin > distearoyl-PC. The plot of .GAMMA. against .pi.i for adsorption of apolipoprotein A-I to dipalmitoylphosphatidylcholine (DPPC) monolayers shows an inflection at .pi.i = 8 dyn/cm; at this .pi., the DPPC monolayer undergoes a phase transition from liquid (expanded) to solid (condensed) state. Addition of cholesterol generally decreases the adsorption of apolipoprotein A-I to egg PC monolayers. Analysis of the adsorption data suggests that the lateral compressibility of a lipid monolayer is a major determinant of the extent to which apolipoprotein A-I adsorbs. The protein penetrates into the interface to occupy space made available by the concomitant compression of phospholipid molecules so .GAMMA. is higher for relatively compressible lipid monolayers. Lipid-protein interactions appear to influence the degree of adsorption to only a minor degree.