Although the phospholipid composition of the erythrocyte membrane has been studied extensively, it remains an enigma as to how the observed composition arises and is maintained. We show here that the phospholipid composition of the human erythrocyte membrane as a whole, as well as the composition of its individual leaf lets, is closely predicted by a model proposing that phospholipid head groups tend to adopt regular, superlattice-like lateral distributions. The phospholipid composition of the erythrocyte membrane from most other mammalian species, as well as of the platelet plasma membrane, also agrees closely with the predictions of the superlattice model. Statistical analyses indicate that the agreement between the observed and predicted compositions is highly significant, thus suggesting that head group superlattices may indeed play a central role in the maintenance of the phospholipid composition of the erythrocyte membrane.

We have synthesized 1-palmitoyl-2-pyrenedecanoyl-sn-glycero derivatives of 3-phosphatidylcholine, 3-phosphatidylethanolamine, 3-phosphatidylserine, 3-phosphatidylglycerol, 3-phosphatidylinositol, and 3-phosphatidic acid and investigated their behavior in monolayers and in neat and mixed bilayers. Fluorescence spectroscopy of neat pyrene phospholipid dispersions revealed a well-defined thermotropic transition at 13.5-19 degrees C depending on the polar head group. An endotherm coinciding with this transition was observed with differential scanning calorimetry, indicating it to be due to the melting of the lipid acyl chains. For pyrenephosphatidylethanolamine, the endotherm was observed at a much higher temperature (70 degrees C). Compression isotherms obtained at an argon/water interface revealed that the pyrene moiety somewhat increases the mean molecular area of a phospholipid molecule but does not prevent the expression of head-group-dependent packing behavior. Partition of the pyrene lipids between coexisting fluid and solid phases was investigated with fluorometry and calorimetry. Both techniques indicate that these lipids prefer the fluid phase and that this preference is independent of the head group. The rates and apparent activation energies of lateral diffusion in fluid bilayers were found to be similar for most pyrene lipids, suggesting that the lateral movement of phospholipids is not critically dependent on interactions at the head-group level. Lateral distribution of the pyrene lipids in gel and fluid phosphatidylcholine bilayers was studied with the excimer technique and calorimetry. In gel-state dipalmitoylphosphatidylcholine bilayers, the pyrene lipids form clusters. These clusters, however, do not consist of pure pyrene lipid but of aggregates (compounds) of the labeled and unlabeled lipid.(ABSTRACT TRUNCATED AT 250 WORDS)

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