The microaerophilic flagellate Giardia intestinalis: 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 O(2). Thus, when exposed to controlled concentrations of dissolved O(2) >8 microM, they gradually lose their ability to scavenge O(2). In a washed cell suspension stirred under 10% air in N(2) (equivalent to 25 microM O(2)), inactivation of the O(2)-consuming system was complete after 3.5 h; during this period accumulation of H(2)O(2) (3 micromol per 10(6) organisms) and oxidation of cellular thiols to 16% of their initial level occurred. Under 20% air (50 microM O(2)), respiratory inactivation was complete after 1.5 h, and under air (258 microM O(2)), after 50 min. Loss of O(2)-consuming capacity was accompanied by loss of motility. Use of the fluorogen 2, 7-dichlorodihydrofluorescein acetate indicated that intracellular H(2)O(2) is produced at extranuclear sites. Flow cytometric estimation of the plasma membrane electrochemical potentials using bis(1,3-dibutylbarbituric acid) trimethine oxonol, DiBAC(4)(3), showed that values declined from -134 mV to -20 mV after 4.5 h aeration. Incubation of organisms with 60 microM H(2)O(2) for 10 min gave partial collapse of plasma membrane potential and complete loss of O(2) uptake capacity; motility and viability as assessed by DiBAC(4)(3) exclusion were completely lost after 1 h. Inactivation of the O(2)-consuming system and loss of viability were also observed on exposure to singlet oxygen photochemically generated from rose bengal or toluidine blue.