What are the sources of hydrogen peroxide production by heart mitochondria?

Abstract

Coupled rat heart mitochondria produce externally hydrogen peroxide at the rates which correspond to about 0.8 and 0.3% of the total oxygen consumption at State 4 with succinate and glutamate plus malate as the respiratory substrates, respectively. Stimulation of the respiratory activities by ADP (State 4-State 3 transition) decreases the succinate- and glutamate plus malate-supported H2O2 production 8- and 1.3-times, respectively. NH4+ strongly stimulates hydrogen peroxide formation with either substrate without any effect on State 4 and/or State 3 respiration. Rotenone-treated, alamethicin-permeabilized mitochondria catalyze NADH-supported H2O2 production at a rate about 10-fold higher than that seen in intact mitochondria under optimal (State 4 succinate-supported respiration in the presence of ammonium chloride) conditions. NADH-supported hydrogen peroxide production by the rotenone-treated mitochondria devoid of a permeability barrier for H2O2 diffusion by alamethicin treatment are only partially (approximately 50%) sensitive to the Complex I NADH binding site-specific inhibitor, NADH-OH. The residual activity is strongly (approximately 6-fold) stimulated by ammonium chloride. NAD+ inhibits both Complex I-mediated and ammonium-stimulated H2O2 production. In the absence of stimulatory ammonium about half of the total NADH-supported hydrogen peroxide production is catalyzed by Complex I. In the presence of ammonium about 90% of the total hydrogen peroxide production is catalyzed by matrix located, ammonium-dependent enzyme(s).

Fig. 1

Limited permeability of mitochondrial membranes for hydrogen peroxide. Trace 1, Mitochondria (Mito) (1.8 mg/ml) were added to the standard reaction mixture in a closed vessel with oxygen-sensitive electrode and oxygen consumption and production were followed. The additions were: G+M, glutamate and malate 5 mM each; ADP 2 mM; Rot, rotenone 5 μM; H₂O₂ 1 mM; Ala, alamethicin 40 μg/ml and MgCl₂ 2.5 mM. Trace 2, alamethicin and MgCl₂ were added just before hydrogen peroxide. Trace 3, control, no mitochondria were added. The numbers (in italics) at the traces show the rates of oxygen consumption or production in two-electron equivalents per min per mg protein.

Fig. 2

Distribution of the total NADH-supported hydrogen peroxide generating activities between the membrane-bound and matrix proteins in rat heart mitochondria. Mitochondria were treated, fractionated, and assayed as described in Table 3.