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Plasmodium berghei Resists Killing by Reactive Oxygen Species

La Jolla Bioengineering Institute, La Jolla, California 92037

Received 13 September 2004/ Returned for modification 19 January 2005/ Accepted 4 July 2005

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 µM) or with peroxynitrite (220 µM), 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 µM; the hemoglobin was mainly oxyhemoglobin (oxyHb) (96%), which was converted to methemoglobin (>95%) after treatment with 150 µM NO. The concentrations of 150 µM of NO and 220 µM of peroxynitrite were far in excess of the hemoglobin concentration (8 µM), 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.

Editor: J. F. Urban, Jr.

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