Alternative 2-keto acid oxidoreductase activities in Trichomonas vaginalis

We have induced high levels of resistance to metronidazole (1 mM or 170 microg ml(-1)) in two different strains of Trichomonas vaginalis (BRIS/92/STDL/F1623 and BRIS/92/STDL/B7708) and have used one strain to identify two alternative T. vaginalis 2-keto acid oxidoreductases (KOR) both of which are distinct from the already characterised pyruvate:ferredoxin oxidoreductase (PFOR). Unlike the characterised PFOR which is severely down-regulated in metronidazole-resistant parasites, both of the alternative KORs are fully active in metronidazole-resistant T. vaginalis. The first, KORI, localized in all membrane fractions but predominantly in the hydrogenosome fraction, is soluble in Triton X-100 and the second, KOR2, is extractable in 1 M acetate from membrane fractions of metronidazole-resistant parasites. PFOR and both KORI and KOR2 use a broad range of 2-keto acids as substrates (pyruvate, alpha-ketobutyrate, alpha-ketomalonate), including the deaminated forms of aromatic amino acids (indolepyruvate and phenylpyruvate). However, unlike PFOR neither KORI or KOR2 was able to use oz-ketoglutarate. Deaminated forms of branched chain amino acids (alpha-ketoisovalerate) were not substrates for T. vaginalis KORs. Since KOR I and KOR2 do not apparently donate electrons to ferredoxin, and are not down-regulated in metronidazole-resistant parasites, we propose that KORI and KOR2 provide metronidazole-resistant parasites with an alternative energy production pathway(s) which circumvents metronidazole activation.