Superoxide dismutase-rich bacteria. Paradoxical increase in oxidant toxicity.

M.D Scott,S.R Meshnick,J.W Eaton

doi:10.1016/s0021-9258(18)61401-2

Abstract

Superoxide dismutase is considered important in protection of aerobes against oxidant damage, and increased tolerance to oxidant stress is associated with induction of this enzyme. However, the importance of superoxide dismutase in this tolerance is not clear because conditions which promote the synthesis of superoxide dismutase likewise affect other antioxidant enzymes and substances. To clarify the role of superoxide dismutase per se in organismal defense against oxidant-generating drugs, we employed Escherichia coli transformed with multiple copies of the gene for bacterial iron superoxide dismutase. These bacteria have greater than ten times the superoxide dismutase activity of wild-type E. coli but, importantly, are normal in other oxidant defense parameters including catalase, peroxidases, glutathione, and glutathione reductase. High superoxide dismutase and control bacteria were exposed to the O2- -generating drug paraquat and to elevated pO2. We find; high superoxide dismutase E. coli are more readily killed by paraquat under aerobic, but not anaerobic, conditions. During exposure to paraquat, high superoxide dismutase E. coli accumulate more H2O2. Coincidentally, the reduced glutathione content of high superoxide dismutase E. coli declines more than in control E. coli. E. coli with high superoxide dismutase activity are also more readily killed by hyperoxia. Interestingly, the susceptibility of the parental and high superoxide dismutase E. coli to killing by exogenous H2O2 is not significantly different. Thus, under these experimental conditions, greatly enhanced superoxide dismutase activity accelerates H2O2 formation. The increased H2O2 probably accounts for the exaggerated sensitivity of high superoxide dismutase bacteria to oxidant-generating drugs. These results support the concept that the product of superoxide dismutase, H2O2, is at least as hazardous as the substrate, O2-. We conclude that effective organismal defense against reactive oxygen species may require balanced increments in antioxidant enzymes and cannot necessarily be improved by increases in the activity of single enzymes.

Highlights

Superoxide dismutase is considered important in The superoxide anion radical, 0 2, the one-electron reducprotection of aerobes against oxidantdamage, and in- tion product of oxygen, is generated in aerobic organisms both creased tolerance to oxidant stress is associated with spontaneously and as aresult of orderly metabolic processes
Inspection of stained electrophoretograms revealed that the manganese superoxide dismutase activity in LE392(1.4) is normal despite the 11-fold increase in iron superoxide dismutase
This is in accord with an earlier report that manganese superoxide dismutase activity is unaffected by changes in iron superoxide dismutase [31]

Summary

Introduction

Superoxide dismutase is considered important in The superoxide anion radical, 0 2 , the one-electron reducprotection of aerobes against oxidantdamage, and in- tion product of oxygen, is generated in aerobic organisms both creased tolerance to oxidant stress is associated with spontaneously and as aresult of orderly metabolic processes. Some studies suggest 0; per se is not dant-generating drugs,we employed Escherichia coli reactive while its dismutationproduct, H202is,a transformed with multiple copies of the gene for bacterial iron superoxide dismutase. These bacteria have greater thanten times the superoxide dismutase activity of wild-type E. coli but, importantly,are normal in other oxidant defense parameters including catalase, peroxidases, glutathione,andglutathione reductase. 5 ) Interestingly, the susceptibility of the parental anhdigh superoxide dismutase E. coli to killing by exogenous HzOzis not significantly different Under these experimental conditions, greatly potent oxidant which readily reacts with organic material [1, 4]. In prokaryotes such as E. coli, iron superoxide dismutase is synthesized constitutively, whereas manganese superoxide dismutase is inducible and its concentration is directly related to the oxygen metabolism of the organism [10, 11].Both prokaryotic superoxide dismutases are cytoplasmic [12] and have similar second-order rate constants of catalysis [13]

Methods

Results

Conclusion