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 expos ed to controlled concentrations of dissolved O-2 >8 muM, they gradually los e their ability to scavenge O-2. In a washed cell suspension stirred under 10 % air in N-2 (equivalent to 25 muM O-2), inactivation of the O-2-consumi ng system was complete after 3.5 h; during this period accumulation of H2O2 (3 mu mol per 10(6) organisms) and oxidation of cellular thiols to 16% of their initial level occurred. Under 20% air (50 muM O-2), respiratory inact ivation was complete after 1.5 h, and under air (258 muM O-2), after 50 min . Loss of O-2-consuming capacity was accompanied by loss of motility. Use o f the fluorogen 2,7-dichlorodihydrofluorescein acetate indicated that intra cellular H2O2 is produced at extranuclear sites. Flow cytometric estimation of the plasma membrane electrochemical potentials using bis(1,3-dibutylbar bituric acid) trimethine oxonol, DiBAC(4)(3), showed that values declined f rom -134 mV to -20 mV after 4.5 h aeration, Incubation of organisms with 60 muM H2O2 for 10 min gave partial collapse of plasma membrane potential and complete loss of O-2 uptake capacity; motility and viability as assessed b y 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 b lue.