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The correlation between hemolytic activities of various phospholipids and their incorporation into membranes was studied. Short chain phosphatidylcholines can be spontaneously transferred between liposome-liposome and between liposome-erythrocyte membrane. The order of the rate of transfer is as follows: C10:0PC greater than C12:0PC much greater than C14:0PC. These findings indicate that the transfer process may be favored by the short fatty acyl chain of phosphatidylcholines. The transfer of C12:0PC was observed in the direction from egg yolk PC liposome to egg yolk PC liposome, from C12:0PC liposome to erythrocyte membrane, from erythrocyte membrane to egg yolk PC liposome, and from erythrocyte membrane to erythrocyte membrane, but not in the direction from egg yolk PC liposome to erythrocyte membrane or from erythrocyte membrane to C16:0PC liposome. The accumulation of a certain amount of C8:0PC, C10:0PC, or C12:0PC on the erythrocyte membrane caused hemolysis. The order of rate of hemolysis is C8:0PC greater than C10:0PC greater than C12:0PC. C8:0PC induced rapid hemolysis only when the concentration was relatively high; 100-200 microM C8:0PC and 5-10 microM C10:0PC, C11:0PC, or C12:0PC were required for 50% hemolysis. The distribution coefficient of C8:0PC between membranes and buffer may be small as compared to those of C10:0PC and C12:0PC. The hemolytic activity of PC may depend both on the rate of transfer and on the distribution coefficient of the molecule between membrane and buffer. Hemolytic activity of dilauroylphospholipids was also affected by head group modification. The order of hemolytic activity is as follows; dilauroylglycerophospho-choline, -serine greater than -dimethylethanolamine much greater than -ethanolamine. The weak hemolytic activity of dilauroylglycerophospho-dimethylethanolamine and -ethanolamine may be due to poor transfer of these lipids to erythrocytes.

The incorporation and accumulation of a certain amount of short-chain phosphatidylcholine or lysophosphatidylcholine into lipid bilayers of erythrocyte membranes is the first step causing membrane perturbation in the process of hemolysis. Accumulation of dilauroylglycerophosphocholine into membranes makes human erythrocytes "permeable cells"; Ions such as Na+ or K+ can permeate through the membrane, though large molecules such as hemoglobin can not. The "pore" formation was partially reproduced in liposomes prepared from lipids extracted from human erythrocyte membranes; C12:0PC induced the release of glucose from liposomes but did not significantly induce the release of dextran. It was suggested that the phase boundary between dilauroylglycerophosphocholine and the host membrane bilayer or dilauroylglycerophosphocholine rich domain itself behaves as "pores." Erythrocytes could expand to 1.5 times the original cell volume without any appreciable hemolysis when incubated with C12:0PC at 37 degrees C. The capacity of the erythrocytes to expand was temperature dependent. The capacity may play an important role in the resistance of the cells against lysis. The "permeable cell" stage could be hardly observed when erythrocytes were treated with didecanoylglycerophosphocholine and lysophosphatidylcholine. Perturbation induced by accumulation of didecanoylglycerophosphocholine or lysophosphatidylcholine may cause non specific destruction of membranes rather than formation of a kind of "pore."