FREE-RADICAL DETOXIFICATION IN GIARDIA-DUODENALIS

Non-denaturing polyacrylamide gels were used to analyse superoxide dis mutase (SOD), catalase, peroxidase, NADH oxidase and NADH peroxidase i n the microaerophilic protozoan parasite Giardia duodenalis. A cytosol ic H2O-producing NADH oxidase and membrane-associated NADH peroxidase were readily detected from G. duodenalis. In all Giardia strains inves tigated the NADH oxidase was present in high levels (1.2-2 U (mg prote in)(-1)). Using the same technique, NADH oxidase activity was also det ected in the microaerophilic protozoan parasites Tritrichomonas foetus , Trichomonas vaginalis and Entamoeba histolytica and in the bacterium Escherichia coli. The conventional enzymes of oxidative stress manage ment (superoxide dismutase, catalase and peroxidase) were not detected in particulate or cytosolic extracts from recent and established stra ins of Giardia assayed in situ. Spectrophotometric assays also yielded negative results. The same methodology readily detected one or more o f these enzyme activities in T. foetus, T. vaginalis and E. coli. Supe roxide dismutase activity was not detected in lines of Giardia resista nt to high levels of metronidazole or furazolidone. Furthermore, the a gents 1,10 phenanthroline, diamide, MnCl2 and KNO3, which induce SOD i n anaerobically cultured E. coli, did not induce SOD in Giardia. 1,10 phenanthroline has also been shown to induce iron-containing (Fe-) SOD in Entamoeba. Neither peroxidase nor catalase activities were detecte d in a peroxide-resistant line of Giardia. Viable trophozoites from pa rent lines were able to decompose H2O2 at a significant rate. It appea rs that the conventional SOD, catalase and peroxidase utilised in aero bic metabolism have been substituted in Giardia by NADH oxidase and NA DH peroxidase, similar to anaerobic bacteria. The O-2-scavenging NADH oxidase explains the previously observed futile 'respiration' in Giard ia.