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Approximately 70% of the initial ferriprotoporphyrin IX polymerizing activity in cell-free preparations of erythrocytes infected with Plasmodium berghei was recovered in a chloroform extract. No polymerizing activity remained in the residue. In studies to identify substances that promote FP polymerization, arachidonic, linoleic, oleic, and palmitoleic acids, 1-mono- and di-oleoylglycerol, and the detergents, SDS, Tween 80, and n-octyl-glucopyranoside, were active. Tri-oleoylglycerol, cholesterol, di-oleoylphosphatidylethanolamine, and stearic and palmitic acids were inactive. The model lipid, mono-oleoylglycerol (250 nmol), co-precipitated with FP from a 0.09 M acetate medium at pH 5 and promoted the polymerization of 215 nmol (61%) of the ferriprotoporphyrin IX in the precipitate during a 24-h incubation at 37 degrees C. Polymerization was maximal at pH 5, it was approximately linear for 2 h, and it continued at a decreasing rate for 24 h. The polymer contained exclusively ferriprotoporphyrin IX (97+/-1.3%, mean+/-S.E., n=4) and exhibited the solubility and the electronic absorption and infrared spectral characteristics of the sequestered ferriprotoporphyrin IX of hemozoin. Detergents presumably promote polymerization in an acid medium by helping to dissolve monomeric FP. We suggest that unsaturated lipids co-precipitate with FP in the parasite's acidic food vacuole and also dissolve sufficient monomeric FP to allow polymerization.

Erythrocytes infected with chloroquine-sensitive Plasmodium falciparum bind chloroquine with an apparent intrinsic association constant of 1.5 x 10(7) liters per mole. Such high-affinity binding of chloroquine is absent or deficient in uninfected erythrocytes and in erythrocytes infected with chloroquine-resistant Plasmodium falciparum.

A B S T R A C T Incubation of a 0.5% suspension of washed, normal mouse erythrocytes with ferriprotoporphyrin IX (FP) at 37°C and pH 7.4 caused potassium loss, swelling, increased susceptibility to hypotonic lysis, and finally hemolysis. Hemolysis was not inhibited by incubation in the dark, malonyldialdehyde was not produced, and various free radical scavengers had no effect on the hemolysis. Only the sulfhydryl compounds, cysteine, dithiothreitol, and mercaptoethanol protected erythrocytes from FP. Potassium loss reached 90% within 30 min of exposure to 5 uM FP. This amount of FP caused >50% hemolysis within 2.5 h. Sucrose (0.1 M) completely prevented hemolysis but had no effect on potassium loss. Likewise, reducing the temperature from 37 to 25°C greatly retarded hemolysis but had no effect on potassium loss. These observations indicate that FP impairs the erythrocyte's ability to maintain cation gradients and induces hemolysis by a colloid-osmotic mechanism.