Mixture Of Phosphatidylserine Research Articles

Interactions of Basic Proteins with Phospholipid Membranes: BINDING AND CHANGES IN THE SODIUM PERMEABILITY OF PHOSPHATIDYLSERINE VESICLES

Abstract The interactions of four positively charged proteins, lysozyme, cytochrome c, ribonuclease, and poly-l-lysine, with negatively charged phosphatidylserine vesicles and vesicles consisting of a mixture of phosphatidylserine and neutral phosphatidylcholine were studied. Protein binding, as measured by spectrophotometric techniques for cytochrome c and microelectrophoresis, was correlated with effects on 22Na+ permeability. Lysozyme, cytochrome c, and poly-l-lysine markedly increased the 22Na+ permeability of sonically and mechanically dispersed vesicles, lysozyme being the most effective (up to 400-fold increase). Ribonuclease, although effective in decreasing electrophoretic mobility, had very little effect on permeability. When mixtures of 10% phosphatidylserine in phosphatidylcholine were used the amount of protein bound and the permeability changes were both decreased. Lysozyme and cytochrome c had no effect on the permeability of pure phosphatidylcholine vesicles. Both protein binding and permeability changes were also affected by alterations in ionic strength. The 22Na+ permeability increase induced by cytochrome c is not accompanied by a change in the activation energy for this process, which was found to be 27 to 29 kcal per mole. The same increases in permeability and high activation energy were observed when cytochrome c was added only outside or both inside and outside the vesicles. It is considered that the data presented are best explained by initial electrostatic interactions between protein and phospholipid, followed, to varying degrees, by other interactions leading to the observed changes in permeability.

The interaction between cytochrome c and purified phospholipids

Isooctane-soluble complexes formed between cytochrome c and various mixtures of phosphatidylserine and phosphatidylcholine have been studied, with the following results. 1. 1. For a given phospholipid mixture there is a unique total lipid to protein ratio for maximum complex extraction. 2. 2. At maximum extraction, each cytochrome c molecule binds 9 molecules of phosphatidylserine and 5–60 molecules of phosphatidylcholine according to the composition of the original phospholipid mixture. When the phosphatidylserine to cytochrome c ratio exceeds 9:1, decreased extraction of both lipid and protein is observed. 3. 3. Other saturated paraffins exhibit similar bahaviour to isooctane, but unsaturated, or halogenated hydrocarbons, do not extract complexes under similar conditions. 4. 4. In 100 mM NaCl, each cytochrome c molecule is associated with 70–100 phosphatidylserine molecules, and a maximum of 400–500 phosphatidylcholine molecules. The results suggest an electrostatic interaction between the phosphatidylserine and protein. The mode of binding of the phosphatidylcholine is not clear.

Some lipid-protein interactions involved in prothrombin activation

1. 1. Bovine plasma prothrombin forms a macromolecular complex with an equimolar mixture of phosphatidylserine and phosphatidylcholine in aqueous dispersion. Complex formation, as detected by gel filtration on columns of Sephadex G-200, occurs only in the presence of an optimal concentration of Ca 2+. 2. 2. Thrombin, bovine plasma albumin and an α 1-acid glycoprotein are not adsorbed by the lipid particles either in the presence or absence of Ca 2+. 3. 3. At—1°, prothrombin becomes attached to the particulate prothrombin activator complex consisting of activated factor X, factor V, phosphatidylserine/phosphatidylcholine mixture and Ca 2+. At 37° in this system, prothrombin is converted to thrombin which itself is not adsorbed by the lipoprotein complex. 4. 4. The kinetics of prothrombin activation are consistent with a model involving catalysis of the reaction at the lipid-water interface and subsequent release of thrombin into the aqueous phase. Such a mechanism would favour the forward reaction by removing the reaction products from the site of their formation, at the same time creating sites for the renewed adsorption of prothrombin.