Applications of monolayer techniques to biological systems: Symptoms of specific lipid-protein interactions

Abstract

Four examples of specific lipid-protein interaction are described as they are revealed by monolayer techniques. One, interaction of a purified antilactoside antibody with a lecithin monolayer containing 20 mole % fN-acyl dihydrosphingosyl lactoside resulted in blocking of the ΔΠ- mechanism for the penetration of protein into the lipid film; addition of lactose to the subphase rextored the large ΔΠ value typical of the nonspecific free penetration of γ-globulin. Two, mitochondrial structural protein (MSP) at low concentration (2 αg/ml) does not penetrate monolayers of either phosphatidyl choline or mitochondrial phospholipid, most likely for the same reason that this protein does not form a film by surface denaturation; however, a dramatic rise in film pressure took place when mitochondrial phospholipid and mitochondrial structural protein were mixed together in the bulk phase in the lipid/protein ratio 1 4 by weight. Three, in contrast, a marked absence of film pressure was observed when the apoprotein was mixed in the bulk phase with the total lipid of the lipoprotein from rat serum in a lipid/protein ratio 1 1 ; the marked loss of pressure, as compared with the pressure produced by either the lipid alone or the lipid-protein mixtures at other ratios, is explained by a model in which the lipid character is lost probably because the lipid is removed by interaction with the protein at that critical ratio. The sharp pressure changes occurred at lipid/protein ratios of known significance, namely, 1 1 for rat lipoprotein and 1 4 for the lipid-MSP complex. Four, and finally, the accessibility of phosphatidyl choline of films of high-density lipoprotein to phospholipase A from Crotalus atrox snake venom varied with different animal species; the results were interpreted as reflecting differences in the molecular organization of the native lipoproteins and particularly in the specific interfacial conformations of the apoprotein. Accordingly, the accessibility of egg lecithin in lecithin-protein films was influenced markedly ( 1) by the nature of the protein, namely, specific vs. nonspecific, and ( 2) by the order of addition of lipid and protein at the interface. The data are consistent with variations in the relative orientation of lipid and protein, in a model in which a mosaic structure is prevailing with lipid packages surrounded by protein. Differences in lipase effect among animal species and among experimental conditions are related to variations in the degree of masking of the lecithin surface by the specific configurations of the apoproteins.