Fragmentation of dimyristoylphosphatidylcholine vesicles by apomyoglobin

Abstract

It is found that apomyoglobin induces the micellization of phosphatidylcholine vesicles at low pH. Detailed studies on the effect of pH, lipid to protein (L/P) ratio, temperature, and the size of vesicles have been performed. The pH dependent study indicated that the micellization occurs when the protein assumes either the molten globular structure of random coil. The micellization occurs below L/P 1000 at pH 4. Maximal micellization occurs at the transition temperature of dimyristoylphosphatidylcholine (23℃). Also, the size of vesicles changes the physical stress of vesicles. The smaller the size of vesicle is, the better the micellization occurs. The micellar complex formed at L/P 40 is larger than micellar complexes formed by other proteins (apolipoprotein A-I, α-lactalbumin, and glucagon). Only 30-40% of apomyoglobin polypeptide is involved in the formation of micellar complex. The lipid bilayer is still maintained in the micellar complex according to the NMR study. Time-dependent hydrophobic labeling by 3-(trifluoromethyl)-3-(m-[$^125$I]iodophenyl)diazirine showed that there is an initial increase in contact between the protein and hydrophobic acyl chain of lipid followed by a decrease in the interaction. This may be explained as the initial stage of vesicle aggregation which is subsequently superseded by the fragmentation. The same samples were cleaved by cyanogen bromide and this experiment showed that all of three fragments cleaved by cyanogen bromide are involved in the formation of micellar complex from the start.