Quaternary Structure and Half-Site Reactivity

SOD occurs as a homodimer in solution about 33,000 , twice the value 

The rapid-freezing EPR data obtained in pulse radiolysis experiments had been interpreted by half-site reactivity In the meantime preparations have, however, been obtained with a definitely higher specific activity (second-order rate constant 

3 X 10 against 2.4x 10 M s for the preparations used in the pulse radiolysis experiments) so that the half-site reactivity has been seriously questioned by the presence of inactive enzyme in the earlier experiments Large differences in specific activity (up to a factor of two) and in the ability to protect mouse bone marrow stem cells against X-irradiation have been reported for five SOD preparations 

New arguments in favor of an interaction between the Cu sites in the dimer have nevertheless been advanced. The binding of Cu to a first site of BESOD seems to facilitate the binding to the second site On reconstitution of SOD by the addition of Cu to a Cu-free preparation, while the recovered spectral properties were a linear function of the Cu content, the recovery of the activity was interpreted by a specific activity of the dimer with one Cu which was twice that of the dimer with two Cu . 

The SOD of the swordfish Xiphias gladius remained fully active in 8 M urea. It was concluded from gel chromatography that active monomers were present But the value in 8 M urea of the SOD, presumably very stable in this medium, was obtained with standard proteins, considered unstable under these conditions. Bovine and yeast SOD showed also an apparent value of 16,000 in 8 M urea by gel chromatography after an abrupt decrease between 6 and 7 M urea. BESOD retained its full enzymatic activity, while yeast SOD was reversibly inactivated 

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