1,N6-Ethenoadenine and 3,N4-ethenocytosine are excised by separate human DNA glycosylases

Abstract

We previously reported our finding that human cells contain glycosylase activity toward all four etheno bases formed in DNA by chloroacetaldehyde and related bi-functional aldehydes. By enzyme purification, including FPLC, we isolated two separate glycosylase activities for 1,N6-ethenoadenine (ΣA) and for 3,N4-ethenocytosine (ΣC) respectively, from crude HeLa cell-free extracts, which also contained a number of well-described glycosylases. When Mono-S FPLC purified proteins were assayed against defined oligomers containing either ΣA or ΣC, it was found that ΣA and ΣC glycosylases were completely separated. It could also be demonstrated that each enzyme bound to and cut only ΣA- or ΣC-containing oligomers respectively. There was no overlap in specificity for these two substrates. Several other human glycosylase substrates were also tested and none were cleaved by ΣC glycosylase. The ΣC glycosylase activity identified in the present study apparently represents a previously unknown glycosylase. This work also suggests that enzyme recognition of closely related DNA adducts may depend upon subtle changes in local conformation.