The effect of ferriprotoporphyrin IX and chloroquine on phospholipid monolayers and the possible implications to antimalarial activity

Ferriprotoporphyrin IX intercalates into phospholipid membranes, as evidenced from its effect on the surface pressure of monolayers composed of different phospholipids. Ferriprotoporphyrin intercalation is enhanced by membrane hydrophobicity and decreased by negative surface potential. Chloroquine enhances the effect of ferriprotoporphyrin in relatively hydrophobic membranes but reduces it in monolayers composed of highly unsaturated phospholipids. These results are consistent with the differential effect of chloroquine on ferriprotoporphyrin-induced lysis of erythrocytes and of malarial parasites, thus supporting the membrane-lesion hypothesis of antimalarial action.     

Interactions of hemin, antimalarial drugs and hemin-antimalarial complexes with phospholipid monolayers

Hemin, antimalarial drugs and complexes formed between them, have demonstrable effects on biological membranes. Using the phospholipid monolayer model, we show that hemin intercalates into the membrane and increases its surface pressure, depending on the lipid composition and the initial surface pressure: negative surface charges and particularly looser compaction of the phospholipids reduce the effect of hemin. With increasing surface pressure hemin tends to intercalate as a monomer, and the half-saturation concentration of its effect increases exponentially. The antimalarial monovalent drugs quinine and mefloquine, but not chloroquine, also penetrate into the membrane and expand it. All three drugs markedly increase the effect of hemin, but chloroquine reduces the effect in monolayers composed of unsaturated phospholipids. The drugs' effect is mostly due to an increase in the maximal surface pressure and suggests a complexation of hemin and drug within the membrane phase. Preformed hemin-drug complexes decrease the half-saturation concentration of the effect and suggest that the complexes adsorb to the membrane, releasing the hemin through an apolar continuum into the phospholipid phase. The implications of the results to the membrane toxicity mechanism proposed for the molecular mode of action of antimalarial drugs are discussed.     

Interaction of plasma apolipoproteins with lipid monolayers

The monolayer technique has been used to study the interaction of lipids with plasma apolipoproteins. Apolipoprotein C-II and C-III from human very low density lipoproteins, apolipoprotein A-I from human high density lipoproteins and arginine-rich protein from swine very low density lipoproteins were studied. The injection of each apoprotein underneath a monolayer of egg phosphatidyl[¹⁴C]choline at 20 mN/m caused an increase in surface pressure to approximately 30 mN/m. With apolipoprotein C-II and apolipoprotein C-III there was a decrease in surface radioactivity indicating that the apoproteins were removing phospholipid from the interface; the removal of phospholipid was specific for apolipoprotein C-II and apolipoprotein C-III. Although there was a removal of phospholipid from the monolayer, the surface pressure remained constant and was due to the accumulation of apoprotein at the interface. The rate of surface radioactivity decrease was a function of protein concentration, required lipid in a fluid state and, of the lipids tested, was specific for phosphatidylcholine. Cholesterol and phosphatidylinositol were not removed from the interface. The addition of 33 mol% cholesterol to the phosphatidylcholine monolayer did not affect the removal of phospholipid by apolipoprotein C-III.The addition of phospholipid liposomes to the subphase greatly facilitated the apolipoprotein C-II-mediated removal of phospholipid from the interface.     

The protein-mediated net transfer of phosphatidylinositol in model systems

The phospholipid monolayer technique has been used to study the transfer activity of the phospholipid exchange protein from beef brain. In measuring the transfer between a monolayer consisting of equimolar amounts of phosphatidylcholine and phosphatidylinositol and liposomes consisting of 98 mol% phosphatidylcholine and 2 mol% phosphatidylinositol, the beef brain protein demonstrates an 8-fold higher transfer activity for phosphatidylinositol than for phosphatidylcholine. Under similar conditions the phosphatidylcholine exchange protein from beef liver showed a great preference for phosphatidylcholine. Phosphatidylcholine liposomes devoid of phosphatidylinositol still functioned as receptors of phosphatidylinositol when the beef brain exchange protein was present. This indicates that this protein can catalyse a net transfer of phosphatidylinopsitol. Binding of both phosphatidylinositol and phosphatidylcholine to the beef brain protein was shown.     

Lipid interaction of diphtheria toxin and mutants. A study with phospholipid and protein monolayers

To study the structural change of diphtheria toxin (DT) induced by low pH and its influence on the interaction with membrane lipids, protein and lipid monolayers were formed and characterized. DT at neutral and acidic pH forms stable monolayers, whose surface-pressure-increase curves allow an estimation of the apparent molecular area of 29.5 nm2/molecule at pH 7.4 (corresponding to a radius of 3.06 nm) and 34.5 nm2/molecule at pH 5.0 (corresponding to a radius of 3.32 nm). DT at pH 7.4 does not insert into phospholipid monolayers, while at pH 5.0 it penetrates into the lipid layer with a portion of apparent molecular area of 21.0 nm2/molecule (corresponding to a radius of 2.6 nm). The low-pH driven lipid interaction of the toxin is favoured by the presence of acidic phospholipids, without an apparent requirement for a particular class of negative lipids. The DT mutants crm 45 and crm 197 are capable of hydrophobic interaction already at neutral pH and cause an increase of surface pressure with a further increase upon acidification.