Molecular Properties of Superoxide Dismutases

Cu2Zn2Superoxide dismutase is an enzyme of intriguingly high stability in vitro. It can be heated to 100 °C for one minute without any detectable loss of activity and it survives pH-ranges from 2-12 Moreover, the enzymic activity survives the presence of ten molar urea and/or four per cent sodium dodecylsulphate. In a solution of six per cent guanidinium hydrochloride where most of the other proteins are deteriorated the denaturation of superoxide dismutase is reversible 

The Cu2Zu2 enzymes can be obtained from many sources. Apart from two exceptionsthey are exclusively found in eucaryotic organisms in nearly all tissues Only the enzymes from higher species have a blocked N-terminal group. These enzymes are generally more stable. Whether this phenomenon can be ascribed to the acetylation or not is still open to discussion. Iron SOD s are exclusively prokaryotic, whereas manganese superoxide dismutases are present in procaryotic cells as well as in mitochrondria and the serum of vertebrates Their relative molecular 

The Cu2Zn2Superoxide dismutase of bovine erythrocytes is a homodimer of 31,300 daltons and contains one g atom of both, copper and zinc per subunit. SOD s from fungi, plants and vertebrates have remarkable homologies in the amino acid sequence . The sequence of the bovine erythrocyte enzyme is summarized in Table 3 

The sequence of human erythrocyte superoxide dismutase has been elucidated some years laterBovine superoxide dismutase contains no tryptophane and very few aromatic and heteroaromatic amino acid residues, respectively. Eight histidines, one tyrosine and one phenylalanine are detected. The main portion of the protein is formed by neutral amino acids. Four sulphur containing amino acids (three cysteins and one methionine) are also present. Cysteine residues 55 and 144 are disulphide bridged whereas one cysteine sulphhydryl group remains unbound. These data are contrasted by the properties of the human enzyme where 153 amino 

The spectral properties of Cu Zn superoxide dismutases are known since many years. Apart from the data obtained by classical methods and which are summarized in Fig. 3 there are many investigations employing more sophisticated techniques. 

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