"Oxidative stress" response in submerged cultures of a recombinant Aspergillus niger (B1-D)

A recombinant strain of Aspergillus niger (B1-D), engineered to produce the marker protein hen egg white lysozyme, was investigated with regard to its susceptibility to "oxidative stress" in submerged culture in bioreactor sy stems. The culture response to oxidative stress, produced either by additio n of exogenous hydrogen peroxide or by high-dissolved oxygen tensions, was examined in terms of the activities of two key defensive enzymes: catalase (CAT) and superoxide dismutase (SOD). Batch cultures in the bioreactor were generally found to have maximum specific activities of CAT and SOD (Umg (. ) protein(-1)) in the stationary/early-decline phase. Continuous addition o f H2O2 (16 mmole L-1 h(-1)), starting in the early exponential phase, induc ed CAT but did not increase SOD significantly. Gassing an early exponential -phase culture with O-2 enriched (25 vol%) air resulted in increased activi ties of both SOD and CAT relative to control processes gassed continuously with air, while gassing the culture with 25 vol% O-2 enriched air throughou t the experiment, although inducing a higher base level of enzyme activitie s, did not increase the maximum SOD activity obtained relative to control p rocesses gassed continuously with air. The profile of the specific activity of SOD (U mg CDW-1) appeared to correlate with dissolved oxygen levels in processes where no H2O2 addition occurred. These findings indicate that it is unsound to use the term "oxidative stress" to encompass a stress respons e produced by addition of a chemical (H2O2) or by elevated dissolved oxygen levels because the response to each might be quite different.(C) 2000 John Wiley & Sons, Inc.