Catalytic properties of human manganese superoxide dismutase

The depletion of superoxide catalyzed by human manganese superoxide dismutase (MnSOD) was observed spectrophotometrically by measuring the absorbance of superoxide at 250-280 nm following pulse radiolysis and by stopped-flow spectrophotometry. Catalysis showed an initial burst of activity lasting approximately 1 ms followed by the rapid emergence of a greatly inhibited catalysis of zero-order rate. These catalytic properties of human MnSOD are qualitatively similar to those reported for MnSOD from Thermus thermophilus (Bull, C., Niederhoffer, E. C., Yoshida, T., and Fee, J. A.(1991) J. Am. Chem. Soc. 113, 4069-4076). However, there are significant quantitative differences; the emergence of the inhibited form is approximately 30-fold more rapid for human MnSOD. The turnover number for human MnSOD at pH 9.4 and 20 degrees C was kcat = 4 x 10(4) s-1 and kcat/Km = 8 x 10(8) M-1 s-1, determined by a simulated fit of the model of Bull et al. (1991) to the pulse radiolysis data. We also report that the maximum of the visible absorption spectrum of human MnSOD (epsilon480 = 525 M-1 cm-1) showed a strong dependence on pH that could be described by an ionization of pKa 9.4 +/- 0.1 with a maximum at low pH.