Monolayer characteristics of saturated 1,2,-diacyl phosphatidylcholines (lecithins) and phosphatidylethanolamines at the air-water interface

Physical studies of the interactions of acetylcholine chloride with membrane constituents

The binding of acetylcholine to pure lipids, and lipids, proteins and lipoproteins extracted from synaptic membranes, was investigated by monolayer and n.m.r. techniques. No specific binding of acetylcholine could be detected at the concentration used, although its muscarinic and nicotinic antagonists [atropine and (+)-tubocurarine respectively] could be shown to interact with the membrane components. It is concluded that the binding of the nicotinic and muscarinic antagonists of acetylcholine is not necessarily indicative of the existence of a specific acetylcholine receptor. Measurements of the displacement of (45)Ca(2+) from monolayers of phosphatidylserine by acetylcholine and the variation of electrophoretic mobility of phosphatidylserine particles with concentration of acetylcholine indicated that in these systems acetylcholine was acting as a counterion at the negatively charged lipid interface. But studies of the salting-in and salting-out of negatively charged lipid aggregates showed that acetylcholine and other quaternary ammonium compounds did not here behave simply as counterions. Electrostrictively hydrated cations such as Na(+) and K(+) were found to salt out, whereas hydrophobically hydrated cations such as acetylcholine salted in such aggregates. The possible role of the hydration of acetylcholine in synaptic transmission is discussed.

The penetration of serum albumin into phospholipid monolayers of different fatty acid chain length and interfacial charge

1. The highest surface pressure of phosphatidylcholine monolayers allowing penetration of delipidated serum albumin decreased in the order dibehenoyl>distearoyl>dipalmitoyl=dimyristoyl. This pressure was not related to the area occupied or to the space available between the phospholipid molecules at the interface. 2. Penetration of albumin into yeast phosphatidylcholine monolayers was increased by adding a small percentage of long-chain anions (phosphatidic acid, dicetylphosphoric acid) to the film but only when the protein was below its isoelectric point (i.e. positively charged). 3. Stearylamine added to phosphatidylcholine monolayers had no effect on albumin penetration even when the protein was oppositely charged to that of the phospholipid/water interface. 4. The results are discussed in relation to the activation of certain phospholipases by anionic amphipathic substances.

Interactions of cytochrome c and [14C]

1. The interactions between cytochrome c (native and [(14)C]carboxymethylated) and monolayers of phosphatidylcholine, phosphatidic acid and cardiolipin at the air/water interface was investigated by measurements of surface radioactivity, pressure and potential. 2. On a subphase of 10mm-or m-sodium chloride, penetration of cytochrome c into egg phosphatidylcholine monolayers, as measured by an increase of surface pressure, and the number of molecules penetrating, as judged by surface radioactivity, were inversely proportional to the initial pressure of the monolayer and became zero at 20dynes/cm. The constant of proportionality was increased when the cytochrome c was carboxymethylated or decreased when the phospholipid was hydrogenated, but the cut-off point remained at 20dynes/cm. 3. Penetrated cytochrome c could be removed almost entirely by compression of the phosphatidylcholine monolayer above 20dynes/cm. 4. With phosphatidic acid and cardiolipin monolayers on 10mm-sodium chloride the binding of cytochrome c was much stronger and cytochrome c penetrated into films nearing the collapse pressure (>40dynes/cm.). The penetration was partly electrostatically facilitated, since it was decreased by carrying out the reaction on a subphase of m-sodium chloride, and the relationship between the surface pressure increment and the initial film pressure moved nearer to that observed with phosphatidylcholine. 5. Surface radioactivity determinations showed that [(14)C]carboxymethylated cytochrome c was still adsorbed on phosphatidic acid and cardiolipin monolayers after the cessation of penetration. This adsorption was primarily electrostatic in nature because it could be prevented and substantially reversed by adding m-sodium chloride to the subphase and there was no similar adsorption on phosphatidylcholine films. 6. The penetration into and adsorption on the three phospholipid monolayers was examined as a function of the pH of the subphase and compared with the state of ionization of both the phospholipid and the protein, and the area occupied by the latter at an air/water interface. 7. It is concluded that the binding of cytochrome c to phospholipids can only be partially understood by a consideration of the ionic interaction between the components and that subtle conformational changes in the protein must affect the magnitude and stability of the complex. 8. If cytochrome c is associated with a phospholipid in mitochondria then cardiolipin would fulfil the characteristics of the binding most adequately.