Catalase Takes Part in Rat Liver Mitochondria Oxidative Stress Defense

Abstract

Highly purified rat liver mitochondria (RLM) when exposed to tert-butylhydroperoxide undergo matrix swelling, membrane potential collapse, and oxidation of glutathione and pyridine nucleotides, all events attributable to the induction of mitochondrial permeability transition. Instead, RLM, if treated with the same or higher amounts of H 2 O 2 or tyramine, are insensitive or only partially sensitive, respectively, to mitochondrial permeability transition. In addition, the block of respiration by antimycin A added to RLM respiring in state 4 conditions, or the addition of H 2 O 2 , results in O 2 generation, which is blocked by the catalase inhibitors aminotriazole or KCN. In this regard, H 2 O 2 decomposition yields molecular oxygen in a 2:1 stoichiometry, consistent with a catalatic mechanism with a rate constant of 0.0346 s ؊1 . The rate of H 2 O 2 consumption is not influenced by respiratory substrates, succinate or glutamate-malate, nor by N-ethylmaleimide, suggesting that cytochrome c oxidase and the glutathione-glutathione peroxidase system are not significantly involved in this process. Instead, H 2 O 2 consumption is considerably inhibited by KCN or aminotriazole, indicating activity by a hemoprotein. All these observations are compatible with the presence of endogenous heme-containing catalase with an activity of 825 ؎ 15 units, which contributes to mitochondrial protection against endogenous or exogenous H 2 O 2 . Mitochondrial catalase in liver most probably represents regulatory control of bioenergetic metabolism, but it may also be proposed for new therapeutic strategies against liver diseases. The constitutive presence of catalase inside mitochondria is demonstrated by several methodological approaches as follows: biochemical fractionating, proteinase K sensitivity, and immunogold electron microscopy on isolated RLM and whole rat liver tissue.Many human diseases, including cancer and other pathologies associated with aging, such as arteriosclerosis and cataracts, are related to mitochondrial dysfunctions provoked by reactive oxygen species (ROS) 2 (1). In this regard, the so-called free radical theory of aging has been proposed (2). ROS are highly reactive and may be extremely toxic in biological systems, as they attack a variety of molecules, including proteins, polyunsaturated lipids, and nucleic acid (3), causing the cell to die by apoptosis or necrosis. In physiological conditions, 1-2% of molecular oxygen consumption during mitochondrial respiration undergoes incomplete reduction by single electrons to form superoxide anion (O 2 . ) at the level of NADH-ubiquinone reductase (complex I) and ubiquinol-cytochrome c reductase (complex III). These two segments of the respiratory chain generate the superoxide radical by autoxidation of reduced flavin and by transferring an electron from reduced ubisemiquinone to molecular oxygen, respectively (4). Superoxide is rapidly converted to hydrogen peroxide by mitochondrial superoxide dismutase, which then produces the highly reactive hydroxyl radical (OH ⅐ ...

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