Abstract
Sulfate oxidation in the isolated mitochondrial fraction of rat liver proceeds exclusively through the respiratory chain in a sequence of electron flow from sulfite oxidase to cytochrome c and then to cytochrome oxidase. Thus, the SO 3 2− O ratio observed is almost unity and the ATP O ratio with sulfite is half that obtained with succinate as the respiratory substrate. Direct reduction of molecular oxygen to H 2O 2 by sulfite oxidase occurs only when the respiratory chain is inhibited by cyanide. These two reactions differ with respect to the effects of the O 2 concentration and the sulfite concentration on the activity and on the SO 3 2− O ratio. In the perfused liver, infusion of sulfite causes increased uptake of O 2, with concomitant reduction of the mitochondrial components, such as pyridine nucleotides, flavoproteins, cytochrome c and cytochrome oxidase. The SO 3 2− O ratio observed is 1.6−1.4 and no increase in H 2O 2 production is detected during sulfite oxidation, indicating that sulfite oxidase is located between the outer and inner membranes of mitochondrion and in contact with cytochrome c, thus providing reducing equivalents to the respiratory chain. In the presence of excess cyanide, O 2 consumption increases two-fold during sulfite oxidation but the rate of sulfite oxidation does not change significantly. Under these conditions the SO 3 2− O ratio is 0.8−0.6, indicating direct reduction of O 2 to H 2O 2 by sulfite oxidase in the perfused liver. The production of H 2O 2 is accompanied by a remarkable oxidation of pyridine nucleotides which is attributed to the stimulation of the glutathione-peroxidase reaction. The maximal rate of sulfite oxidation is more than 1.2 μmol/min/g wet wt in the control liver, and decreases to 0.6 μmol/min/g wet wt in the liver of the tungsten-pretreated rat which possesses 23% of the sulfite oxidase activity. As the rate of sulfite oxidation reaches its maximum, progressive inhibition of some dehydrogenases appears to occur in the perfused liver.
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