Carnosine (␤-alanyl-L-histidine) and homocarnosine (␥-aminobutyric acid-L-histidine) are two naturally occurring dipeptides with potential neuroprotective and neurotransmitter functions in the brain. Peptidase activities degrading both carnosine and homocarnosine have been described previously, but the genes linked to these activities were unknown. Here we present the identification of two novel cDNAs named CN1 and CN2 coding for two proteins of 56.8 and 52.7 kDa and their classification as members of the M20 metalloprotease family. Whereas human CN1 mRNA and protein are brain-specific, CN2 codes for a ubiquitous protein. In contrast, expression of the mouse and rat CN1 orthologues was detectable only in kidney. The recombinant CN1 and CN2 proteins were expressed in Chinese hamster ovary cells and purified to homogeneity. CN1 was identified as a homodimeric dipeptidase with a narrow substrate specificity for Xaa-His dipeptides including those with Xaa ‫؍‬ ␤Ala (carnosine, K m 1.2 mM), N-methyl ␤Ala, Ala, Gly, and ␥-aminobutyric acid (homocarnosine, K m 200 M), an isoelectric point of pH 4.5, and maximal activity at pH 8.5. CN2 protein is a dipeptidase not limited to Xaa-His dipeptides, requires Mn 2؉ for full activity, and is sensitive to inhibition by bestatin (IC 50 7 nM). This enzyme does not degrade homocarnosine and hydrolyzes carnosine only at alkaline pH with an optimum at pH 9.5. Based on their substrate specificity and biophysical and biochemical properties CN1 was identified as human carnosinase (EC 3.4.13.20), whereas CN2 corresponds to the cytosolic nonspecific dipeptidase (EC 3.4.13.18).

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BackgroundCell free DNA (cfDNA) circulates throughout the bloodstream of both healthy people and patients with various diseases and acts upon the cells. Response to cfDNA depends on concentrations and levels of the damage within cfDNA. Oxidized extracellular DNA acts as a stress signal and elicits an adaptive response.Principal FindingsHere we show that oxidized extracellular DNA stimulates the survival of MCF-7 tumor cells. Importantly, in cells exposed to oxidized DNA, the suppression of cell death is accompanied by an increase in the markers of genome instability. Short-term exposure to oxidized DNA results in both single- and double strand DNA breaks. Longer treatments evoke a compensatory response that leads to a decrease in the levels of chromatin fragmentations across cell populations. Exposure to oxidized DNA leads to a decrease in the activity of NRF2 and an increase in the activity of NF-kB and STAT3. A model that describes the role of oxidized DNA released from apoptotic cells in tumor biology is proposed.Conclusions/SignificanceSurvival of cells with an unstable genome may substantially augment progression of malignancy. Further studies of the effects of extracellular DNA on malignant and normal cells are warranted.

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