AlkA Glycosylase and AlkB Dioxygenase Constitute an Effective Protective System for Endogenously Arising Acrolein

Abstract

Acrolein (ACR) is a ubiquitous environmental pollutant but also formed endogenously as a metabolite in oxidative stress conditions. Its adduct to adenine 1,N6-α-hydroxypropanoadenine (HPA) is a mutagenic lesion effectively repaired by the AlkB dioxygenase. Here, we provide in vivo, in vitro, and in silico evidence that it is also the substrate for the AlkA glycosylase. We studied the role of AlkA and AlkB in E. coli cells under conditions of induced adaptive response. Both alkA and alkB defective strains were not more sensitive to exogenous ACR than the wild type was. To simulate endogenously arising adducts, we used acrolein-modified plasmids, allowing monitoring of all kinds of substitutions originating from the acrolein modification of adenine. Both the AlkA and AlkB proteins were engaged in alleviating HPA-induced mutagenesis. Moreover, HPA was effectively repaired by AlkA and AlkB in vivo, even without induction of adaptive response. These findings suggest that the main contribution to acrolein mutagenicity comes from its endogenous sources, whereas AlkA and AlkB can play an additional role in controlling the level of DNA adducts of endogenous origin. Acrolein does not induce the adaptive response. HPA contains an asymmetric carbon atom in the hydroxypropano ring and exists as two stereoisomers. AlkA excises both of them in vitro. Molecular modelling demonstrated how dsDNA carrying both HPA stereoisomers could be properly bound at the AlkA catalytic centre. So, in contrast to the reaction catalyzed by AlkB, the HPA repair by AlkA is not expected to be stereoselective.