The interaction of γ-globulin with lipids

Abstract

γ-globulin interacts with cardiolipin (CL), phosphatidic acid (PA) and phosphatidylserine (PS) to yield an insoluble complex. The bound PA and PS can be nearly all removed by extraction with organic solvents, but about 50 per cent of the bound CL is not extracted. Amino acid analysis shows that lysine, arginine, histidine, tyrosine, methionine and cysteine are involved in the binding of CL to γ-globulin. The binding of CL to γ-glubulin exhibits a sharp pH profile in phosphate buffers with a maximum between 5.7 to 6.1. The interaction is influenced by ionic strength and shows a broad peak at 0.06 M NaCl. Freshly prepared cardiolipin (in water) shows maximal activity. The activity decreases with aging of the aqueous cardiolipin dispersion. Lecithin and cholesterol alone do not form an insoluble complex with γ-globulin and have either no influence or slightly inhibit the Cl-γ-globulin reaction. However, when lecithin and cholesterol are mixed they interact with γ-globulin to yield an insoluble complex. Moreover, the lecithin—cholesterol mixture greatly enhances the Cl—γ-globulin reaction. Sphingomyelin can not substitute for lecithin. The bound lecithin and cholesterol in the γ-globulin precipitate are completely extracted by organic solvents. A variety of γ-globulins were studied and all react with CL and with lecithin—cholesterol or lecithin—cholesterol mixtures but to varying degrees. The reaction of γ-globulin with CL is markedly inhibited by p-hydroxymercuribenzoate and iodoacetate. Bence—Jones protein reacts very weakly with CL but quite strongly with a mixture of CL—lecithin—cholesterol.