ESCHERICHIA-COLI, SACCHAROMYCES-CEREVISIAE, RAT AND HUMAN 3-METHYLADENINE DNA GLYCOSYLASES REPAIR 1,N-6-ETHENOADENINE WHEN PRESENT IN DNA

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,N-G-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 oligodeoxynucieotides 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 K-m 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.