EFFECTS OF PHOTODYNAMIC-ACTION ON RESPIRATION IN NONPHOSPHORYLATING MITOCHONDRIA

We have studied the effects of singlet oxygen produced by photodynamic action on respiration in nonphosphorylating mitochondria (state 4). I solated rat liver mitochondria were incubated with 3 mu M hematoporphy rin and irradiated at 365 nm with a fluence rate of 25 W/m(2). After s hort durations of irradiation, state 4 respiration with beta-hydroxybu tyrate as substrate increases while respiration with succinate is negl igibly affected. When mitochondria have been uncoupled with carbonylcy anide-p-trifluoromethoxyphenyl hydrazone before irradiation, no change occurs in beta-hydroxybutyrate-driven respiration, while succinate-dr iven respiration strongly decreases. Stimulation of state 4 NADH respi ration cannot be explained by slippage of the NADH ubiquinone oxidored uctase because the stoichiometry of the redox pump was found insensiti ve to photodynamic action. In the light of the metabolite theory for l inear enzymatic chains applied to state 4 respiration (Brand et at, Bi ochem. J. 255, 535-539, 1988), these results suggest that stimulation of NADH respiration is simply due to an increase of membrane leaks whi ch occurs after irradiation. In the case of succinate-driven respirati on, a strong inhibition of succinate dehydrogenase activity has been d emonstrated after irradiation. It can be suggested that this inhibitio n introduces a negative control coefficient over state 4 respiration, counterbalancing the effects due to leakage. (C) 1998 Academic Press.