SUMMARYThe action of purified phospholipases on monomolecular films of various interfacial pressures is compared with the action on erythrocyte membranes. The phospholipases which cannot hydrolyse phospholipids of the intact erythrocyte membrane, phospholipase C from Bacillus cereus, phospholipase A 2 from pig pancreas and Crotalus adamanteus and phospholipase D from cabbage, can hydrolyse phospholipid monolayers at pressure below 31 dynes/cm only.The phospholipases which can hydrolyse phospholipids of the intact erythrocyte membrane, phospholipase C from Clostridium welchii phospholipase A2 from Naja naja and bee venom and sphingomyelinase from Staphylococcus aureus, can hydrolyse phospholipid monolayers at pressure above 31 dynes/cm. It is concluded that the lipid packing in the outer monolayer of the erythrocyte membrane is comparable with a lateral surface pressure between 31 and 34.8 dynes/cm.

SUMMARYI. The force-area characteristics of monolayers of synthetic lecithins with one to six double bonds in one acyl chain have been studied.2. The area per molecule increases stepwise. The most significant increase is observed after the introduction of the first double bond. The subsequent introduction of two, three or four double bonds or polyunsaturated chains at both ester positions produces some further increase.3. The interaction with cholesterol depends on the unsaturation and the distribution of the double bonds between the acyl chains.4. A condensing effect with cholesterol was evident for (i-stearoyl-2-oleoyl)-3-lecithin, (I-palmitoyl-2-1ineoyl)-3-1ecithin, (I-palmitoyl-2-1inolenoyl)-3-1ecithin, (I-palmitoyl-2-arachidonoyl)-3-1ecithin at 22 °. No effect is observed for (I,2-dilinoleoyl)-3-1ecithin and (I,2-dilinolenoyl)-3-1ecithin. (I-palmitoyl-2-docosahexaenoyl)-3-lecithin shows a limited effect at 22 °, but no effect at 37 °.5. No significant differences in behavior are found for the two structural isomers with a mono-or disaturated chain at the i-or 2-position.6. The permeability of liposomes, derived from the above mentioned lecithins, to glucose, erythritol and glycerol increases in the same order as the area per molecule at the air-water interface.7. The presence of cholesterol reduced the permeability to glucose, erythritol, glycerol only for the lecithins which showed a condensation effect.8. The unsaturation and the distribution of the double bonds appear to be of critical importance for the barrier properties of lecithins and for the interaction with cholesterol.

SUMMARYI. The interaction of the five polyene antibiotics filipin, etruscomycin, pimaricin, nystatin and amphotericin B with sterol, primarily free, liposomal and membrane bound cholesterol has been examined. Each of these antibiotics has a characteristic ultraviolet absorption spectrum in aqueous or organic solvents with three or four ultraviolet absorption maxima.Addition of free cholesterol to aqueous solutions of these antibiotics results in a change of the ratio of the ultraviolet absorbance maxima. The order of effectiveness of interaction with cholesterol as judged by this criterion was filipin, amphotericin B, etruscomycin and pimaricin.2. The same alteration in ultraviolet absorption spectra of filipin, etruscomycin and amphotericln B observed with addition of free cholesterol to aqueous solutions of these antibiotics also occurs upon addition of these antibiotics to liposomal, erythrocyte or Acholeplasma membrane bound cholesterol. No spectral change was found in membranes devoid of cholesterol. This spectral alteration of the antibiotic was accompanied by a binding of the antibiotic to the membrane. Both nystatin and pimaricin showed little change in spectrum with sterol in these systems, either the artificial or natural membranes which contained cholesterol.3. The structural requirements of the sterol for the spectral change with filipin, etruscomycin and amphotericin B include a planar sterol nucleus, an intact side chain at C-I 7 and a 3r-hydroxyl group. The spectral change was not affected by the pH except in the case of amphotericin B.4. Measurements by differential scanning calorimetry of the effect of these polyene antibiotics on the phase transition of lecithin and lecithin-cholesterol showed that all polyenes can reduce the lecithin-cholesterol interaction.

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