Antioxidant vitamins C and E affect the superoxide-mediated induction of the soxRS regulon of Escherichia coli

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Abstract
The mechanism of activation of Escherichia coli redox sensory protein SoxR still unclear: a [2Fe--25] cluster contained in a SoxR dimer is potentially redo sensitive, but the nature of the signal is unknown. Antioxidant vitamins C (ascorbate) and E (a-tocopherol) were used to explore the mechanism of activation of the SoxR protein in vivo. Treating E. coli cells with ascorbate o a-tocopherol increased their tolerance to paraquat (PQ, a redox-cycling compound), even in the absence of the soxRS locus, suggesting a radical-quenching activity. When using a soxS:: lacZ fusion, whose expression is governed by activated SoxR, ascorbate and a-tocopherol also prevented the expression of a-galactosidase after PQ treatment. A secondary activity was observed in cells carrying soxR101, a mutation resulting in the constitutive expression of the sox regulon, where the overexpression of soxS::lacZ was also reduced by ascorbate or a-tocopherol treatment. Additionally, different mechanisms of action were revealed as a-tocopherol was capable of preventing both PQ and menadione (MD) lethality, whilst ascorbate prevent PQ lethality but increased MD-mediated cell death. It is proposed that a-tocopherol, positioned in membranes, can prevent superoxide-dependent membrane damage; however, water-soluble ascorbate is unable to do so an can even increase the concentration of oxygen radicals reacting with release membrane-associated Fe(ll).