Dependence of protective functions of Escherichia coli polyamines on strength of stress caused by superoxide radicals

Mechanisms of antioxidant effect of polyamines were studied in dependence on the strength of superoxide stress. Under conditions of weak stress, polyamines from Escherichia coli cultures were shown to function mainly as a scavenger of free superoxide radicals, whereas under conditions of strong stress they mainly acted as positive modulators of antioxidant genes. Spectrofluorimetry was used to show that both polyamine-dependent mutants and wild type cells treated with inhibitors of polyamine synthesis contained an elevated amount of free oxygen radicals, which could be decreased to the normal level by addition of exogenous polyamines. Under conditions of strong stress, polyamines positively influenced expression of the soxRS regulon genes of antioxidant defense, which was accompanied by an increase in the quantity (activity) of their gene products, such as glucose-6-phosphate dehydrogenase (Zwf) and fumarase (FumC). These effects led to an increase in the number of live cells in the cultures subjected to superoxide stress.