The sensitivity of the microaerophilic protozoan to oxygen and products of its reduction, and the antioxidant defences employed by this organism, were investigated. Studies revealed that this amitochondrial flagellate is sensitive to oxygen tensions above those experienced in the vagina (i.e. > 60 μM) and that metronidazole-resistant strains (CDC 85 and IR78) were more sensitive to elevated oxygen levels than a metronidazole-sensitive isolate (1910). In the presence of radical scavengers, inactivation of organisms at 60 μM oxygen was significantly lessened. Investigation of the antioxidant enzymes present in this organism revealed that activities of peroxide-reducing enzymes (e.g. catalase and general peroxidase) were not detectable, but that a cyanide-insensitive, azide-sensitive superoxide dismutase was present in cell extracts. Measurement of thiol-cycling enzymes indicated that NADPH could drive the reduction of oxidized glutathione (thiol reductase); however, the corresponding peroxidase activity was not detected. Analysis of thiols in whole cells of indicated that glutathione was absent, but high levels of other thiols, propanethiol, methanethiol and HS, were present. No significant differences were detected in thiol levels or antioxidant enzyme activities on comparison of metronidazole-sensitive and resistant strains. These results indicate that the sensitivity of to oxygen above physiological levels is due to the lack of adequate peroxide-reducing enzymes and radical-scavenging mechanisms.


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