Plasmodium berghei resists killing by reactive oxygen species

Reactive oxygen species (ROS) are widely believed to kill malarial parasites. C57BL/6 mice injected with P. berghei inocula incubated with supraphysiological doses of NO (<= 150 mu M) or with peroxynitrite (220 mu M), however, exhibited parasitemia similar to that seen with those given control inocula, and there was no difference in disease development. Only treatment of inocula with NO doses nearing saturation (>= 1.2 mM) resulted in no detectable parasitemia in the recipients; flow cytometric analysis with a vital dye (hydroethidine) indicated that 1.5 mM NO lysed the erythrocytes rather than killing the parasites. The hemoglobin level in the inocula was about 8 mu M; the hemoglobin was mainly oxyhemoglobin (oxyHb) (96%), which was converted to methemoglobin (>95%) after treatment with 150 mu M NO. The concentrations of 150 mu M of NO and 220 mu M of peroxynitrite were far in excess of the hemoglobin concentration (similar to 8 mu M), and yet no parasite killing was detected. We therefore conclude that hemoglobin protects Plasmodium parasites from ROS, but the parasite likely possesses intrinsic defense mechanisms against ROS.