CATALYTIC PROPERTIES OF HUMAN MANGANESE SUPEROXIDE-DISMUTASE

The depletion of superoxide catalyzed by human manganese superoxide di smutase (MnSOD) was observed spectrophotometrically by measuring the a bsorbance of superoxide at 250-280 nm following pulse radiolysis and b y stopped-flow spectrophotometry. Catalysis showed an initial burst of activity lasting approximately 1 ms followed by the rapid emergence o f a greatly inhibited catalysis of zero-order rate. These catalytic pr operties of human MnSOD are qualitatively similar to those reported fo r MnSOD from Thermus thermophilus (Bull, C,, Niederhoffer, E, C,, Yosh ida, T,, and Fee, J, A. (1991) J, Am, Chem, Sec. 113, 4069-4076), Howe ver, 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 k (cat) = 4 x 10(4) s(-1) and k(cat)/K-m 8 x 10(8) M(-1) s(-1), determin ed by a simulated fit of the model of Bull ct al, (1991) to the pulse radiolysis data, We also report that the maximum of the visible absorp tion spectrum of human MnSOD (epsilon(480) = 525 M(-1) cm(-1)) showed a strong dependence on pH that could be described by an ionization of pK(a) 9.4 +/- 0.1 with a maximum at low pH.