Catalytic Properties of Human Manganese Superoxide Dismutase

• Published in: The Journal of biological chemistry 1996-07, Vol.271 (30), p.17687-17691
• Main Authors: Hsu, J L, Hsieh, Y, Tu, C, O'Connor, D, Nick, H S, Silverman, D N
• Format: Article
• Language: English
• Subjects: Base Sequence; Catalysis; Enzyme Inhibitors; Escherichia coli - genetics; Humans; Hydrogen-Ion Concentration; Kinetics; Models, Chemical; Molecular Sequence Data; Pulse Radiolysis; Recombinant Proteins - metabolism; Spectrophotometry; Superoxide Dismutase - antagonists & inhibitors; Superoxide Dismutase - genetics; Superoxide Dismutase - metabolism; Superoxides - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.271.30.17687

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°C was k cat = 4 × 10 4 s −1 and k cat / K m = 8 × 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 (ϵ 480 = 525 M −1 cm −1 ) showed a strong dependence on pH that could be described by an ionization of p K a 9.4 ± 0.1 with a maximum at low pH.