Cysts and trophozoites of the parasitic protozoon both showed respiratory activity but respiration in cysts was only 10 to 20% that of trophozoites. The O dependence of respiration in cysts and trophozoites showed O maxima above which respiration decreased. The O concentration at which the respiration rate was greatest was higher for cysts than trophozoites. The effects of various inhibitors on cyst and trophozoite respiration suggested that flavoproteins and quinones play some role in respiration. The substrate specificities and the effects of inhibitors on trophozoites were similar to those observed for . Metronidazole, the drug most commonly used in the treatment of giardiasis completely inhibited respiration and motility in trophozoites; however, it had no effect on either respiration or viability in cysts. Menadione, a redox cycling naphthoquinone, stimulated then completely inhibited respiration in cysts and trophozoites; a complete loss of cyst viability or trophozoite motility was also observed. The effects of menadione on may indicate that redox cycling compounds have potential as chemotherapeutic agents for the treatment of giardiasis.


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