58] Hydroperoxide effects on redox state of pyridine nucleotides and Ca2+ retention by mitochondria

Abstract

Publisher Summary Most of the molecular oxygen consumed during mitochondrial respiration is reduced with four electrons directly to water at the level of cytochrome oxidase while some 1–5% of oxygen is reduced univalently to superoxide. Superoxide radicals are precursors of mitochondrial hydrogen peroxide. Most, if not all, mitochondrial hydrogen peroxide arises from the dismutation of superoxide radicals. While the metabolic significance of intramitochondrial superoxide and hydrogen peroxide remained unknown until recently, the presence of glutathione peroxidase in mitochondria suggested an enzymic reduction of hydrogen peroxide and organic peroxides. Experimental evidence for this was first obtained by Oshino and Chance, who showed coupling of the intramitochondrial pyridine nucleotide oxidation to the reduction of hydroperoxides. A possible relationship between the redox state of pyridine nucleotides and the transport of Ca 2+ across the inner mitochondrial membrane was later suggested by Lehninger. This chapter presents experiments that show a specific oxidation of both mitochondrial nicotinamide adenine dinucleotide phosphate (NADPH) and NADH by small amounts of hydroperoxides through the action of glutathione peroxidase, glutathione reductase, and energy-linked mitochondrial transhydrogenase. In the presence of succinate, the enzymic pyridine nucleotide oxidation is reversible. If mitochondria are loaded with Ca 2+ before the addition of hydroperoxides, the oxidation of pyridine nucleotides is irreversible, and Ca 2+ is released from mitochondria. The release is accompanied by an intramitochondrial hydrolysis of pyridine nucleotides at the N-glycosidic bond yielding ADPribose and nicotinamide.