Superoxide Dismutase

Abstract

Abstract Fresh chicken liver contains two types of superoxide dismutase, one of which is localized in the mitochondria while the other is found in the cytosol. The mitochondrial superoxide dismutase was inactivated by treatment with mixtures of chloroform and ethanol whereas the cytosol superoxide dismutase was not. The cytosol superoxide dismutase was inhibited by cyanide, whereas the mitochondrial enzyme was not. The cytosol superoxide dismutase was purified to homogeneity and was found to contain copper and zinc and to be similar to the other cupro-zinc superoxide dismutases which have been isolated from diverse eukaryotes. In contrast, the superoxide dismutase present in chicken liver mitochondria, when isolated, was found to a manganoprotein which has a molecular weight of 80,000. It is composed of four subunits of equal size, which are not covalently joined. It contains 2.3 atoms of manganese per molecule and is strikingly similar to the superoxide dismutase previously isolated from bacteria (Keele, B. B., Jr., McCord, J. M., and Fridovich, I. (1970) J. Biol. Chem. 245, 6176–6181; Vance, P. G., Keele, B. B., Jr., and Rajagopalan, K. V. (1972) J. Biol. Chem. 247, 4782–4786). Avimanganin which was isolated from chicken liver mitochondria (Scrutton, M. C. (1971) Biochemistry 10, 3897–3905) appears to be an inactive form of mitochondrial superoxide dismutase. The similarity between the mitochondrial superoxide dismutase and the corresponding enzymes from Escherichia coli and from Streptococcus mutans and the marked dissimilarity to the superoxide dismutase found in the cytosols of eukaryotes lends support to the theory that mitochondria have evolved from aerobic prokaryotes, which entered into an endocellular symbiosis with a protoeukaryote. Chicken liver was not unique in possessing distinct superoxide dismutases in cytosol and in mitochondrial compartments, since the same situation was shown to occur in pig heart. The chicken liver cytosol superoxide dismutase exhibited two peculiar properties. Thus when stored in deep, concentrated solutions it underwent autoreduction to a yellow cuprous form which retained enzymatic activity, and it polymerized to forms which likewise retained activity. Both of these changes were reversible and appeared to be due to reactive sulfhydryl groups which could act as the endogenous reductant and could form intermolecular disulfide bridges. The cytosol superoxide dismutase consisted of a family of electrophoretically distinct forms which could readily be separated on a preparative scale by isoelectric focusing. These multiple forms were indistinguishable in size and in specific activity. The chemical basis of these differences in charge was not explored.