Free spermidine evokes superoxide radicals that manifest toxicity.

Spermidine and other polyamines alleviate oxidative stress, yet excess spermidine seems toxic to unless it is neutralized by SpeG, an enzyme for the spermidine -acetyl transferase function. Thus, wild-type can tolerate applied exogenous spermidine stress, but Δ strain of fails to do that. Here, using different reactive oxygen species (ROS) probes and performing electron paramagnetic resonance spectroscopy, we provide evidence that although spermidine mitigates oxidative stress by lowering overall ROS levels, excess of it simultaneously triggers the production of superoxide radicals, thereby causing toxicity in the Δ strain. Furthermore, performing microarray experiment and other biochemical assays, we show that the spermidine-induced superoxide anions affected redox balance and iron homeostasis. Finally, we demonstrate that while RNA-bound spermidine inhibits iron oxidation, free spermidine interacts and oxidizes the iron to evoke superoxide radicals directly. Therefore, we propose that the spermidine-induced superoxide generation is one of the major causes of spermidine toxicity in .