Escherichia coli, Saccharomyces cerevisiae, rat and human 3-methyladenine DNA glycosylases repair 1,N6-ethenoadenine when present in DNA

Abstract

The human carcinogen vinyl chloride is metabolized in the liver to reactive intermediates which generate various ethenobases in DNA. It has been reported that 1,N6-ethenoadenine (epsilon A) is excised by a DNA glycosylase present in human cell extracts, whereas protein extracts from Escherichia coli and yeast were devoid of such an activity. We confirm that the human 3-methyladenine-DNA glycosylase (ANPG protein) excises epsilon A residues. This finding was extended to the rat (ADPG protein). We show, at variance with the previous report, that pure E.coli 3-methyladenine-DNA glycosylase II (AlkA protein) as well as its yeast counterpart, the MAG protein, excise epsilon A from double stranded oligodeoxynucleotides that contain a single epsilon A. Both enzymes act as DNA glycosylases. The full length and the truncated human (ANPG 70 and 40 proteins, respectively) and the rat (ADPG protein) 3-methyladenine-DNA glycosylases activities towards epsilon A are 2-3 orders of magnitude more efficient than the E.coli or yeast enzyme for the removal of epsilon A. The Km of the various proteins were measured. They are 24, 200 and 800 nM for the ANPG, MAG and AlkA proteins respectively. These three proteins efficiently cleave duplex oligonucleotides containing epsilon A positioned opposite T, G, C or epsilon A. However the MAG protein excises A opposite cytosine much faster than opposite thymine, guanine or adenine.