The microaerophilic flagellate Giardia intestinalis: Oxygen and its reaction products collapse membrane potential and cause cytotoxicity: Oxygen and its reaction products collapse membrane potential and cause cytotoxicity

Trophozoites of the microaerophilic flagellate parasitic protozoon Giardia intestinalis have only a limited capacity to detoxify O2. Thus, when exposed to controlled concentrations of dissolved O2>8 μM, they gradually lose their ability to scavenge O2. In a washed cell suspension stirred under 10% air in N2 (equivalent to 25 μM O2), inactivation of the O2-consuming system was complete after 3·5 h; during this period accumulation of H2O2 (3μmol per 106 organisms) and oxidation of cellular thiols to 16% of their initial level occurred. Under 20% air (50 μM O2), respiratory inactivation was complete after 1·5 h, and under air (258 μM O2), after 50 min. Loss of O2-consuming capacity was accompanied by loss of motility. Use of the fluorogen 2,7-dichlorodihydrofluorescein acetate indicated that intracellular H2O2 is produced at extranuclear sites. Flow cytometric estimation of the plasma membrane electrochemical potentials using bis(1,3-dibutylbarbituric acid) trimethine oxonol, DiBAC4(3), showed that values declined from -134 mV to -20 mV after 4·5 h aeration. Incubation of organisms with 60 μM H2O2 for 10 min gave partial collapse of plasma membrane potential and complete loss of O2 uptake capacity; motility and viability as assessed by DiBAC4(3) exclusion were completely lost after 1 h. Inactivation of the O2-consuming system and loss of viability were also observed on exposure to singlet oxygen photochemically generated from rose bengal or toluidine blue.