No known deaminase acts on adenine in DNA, however, an adenosine deaminase (ADA) converts free dA to dI which can be further metabolized to Hx [7]. Hx can be salvaged to deoxyinosine monophosphate (dIMP) and subsequently converted to deoxyinosine triphosphate (dITP) by a currently poorly understood pathway [8]. dITP may also be produced by spontaneous deamination of dATP. DNA polymerases can use dITP as a substrate

during DNA replication and will most often insert dITP opposite a C (Figure 2a) [9 and 10]. Normally the intracellular dITP concentration is kept learn more low compared to the canonical deoxynucleotide triphosphates (dNTPs) [8]. The steady state level of inosine in DNA is 0.5–1 per 106 nucleotides in different mouse tissue, E. coli and S. cerevisiae [ 11, 12• and 13•]

comparable to the level of the more studied oxidation product of dG, 8-oxo-7,8-dihydro-2′-dG. In addition to its premutagenic properties, dI may lead to altered recognition sites for DNA binding proteins with consequences for example gene expression. To avoid such treats, cells harbor two main pathways for inosine repair: base excision repair (BER) and Endonuclease V. BER is the major pathway for repair of damaged DNA bases and proceeds through multiple steps requiring several enzymes [14]. The first step is initiated by DNA glycosylases recognizing and removing SD-208 solubility dmso damaged DNA bases such as alkylated, oxidized and deaminated bases. In most pro- and eukaryotic species the alkyl adenine DNA glycosylases (Escherichia coli AlkA; Saccharomyces cerevisiae MAG; mammalian Aag), remove Hx from genomic DNA ( Figure 2a) [ 15 and 16]. In Schizosaccharomyces pombe, thymine DNA glycosylase (Thp1)

rather than Mag1 appears to be the inosine-specific DNA glycosylase [ 17 and 18]. An alternative excision repair pathway for the removal of deaminated purine bases has been proposed, in which Endonuclease V (EndoV) initiates repair by cleavage of the second phosphodiester bond 3′ to inosine ( Figure 2 and Figure 3) [ 19]. Further, a small patch of DNA containing the lesion is removed by exonucleases or endonucleases and finally, DNA polymerase and DNA ligase completes repair by gap filling and ligation. Although this alternative C59 nmr excision repair mechanism has been reconstituted in vitro with recombinant proteins [ 20], in vivo data supporting this pathway is lacking. The molecular basis for recognition and cleavage of inosine-containing DNA by prokaryotic EndoV has been elucidated through structure determination of EndoV-DNA complexes [21]. EndoV has an αβα fold with a central 8-stranded β-sheet flanked on either side by α-helices (Figure 3b) — including a ribonuclease H-like motif shared with nucleases such as RNaseH [22 and 23], RuvC [24] and UvrC [25], as well as Piwi protein [26 and 27] and the Piwi subdomain of Argonaute [28 and 29], being part of the RNA-induced silencing complex (RISC).