STUDIES ON DNA STRAND BREAK AND PROMOTION OF ITS REPAIR

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The effect of niacin on NAD content and on DNA repair in human lymphocytes was investigated. Lymphocytes were incubated in culture medium supplemented with various concentrations of nicotinic acid or nicotinamide. Incubation of lymphocytes with nicotinic acid at 5 muM or nicotinamide at 10 mM resulted 2-3 fold increase in NAD content. Under these conditions lymphocytes were treated with N-methyl-N-nitro-N-nitrosoguanidine (MNNG) to investigate the effect of niacin on DNA repair synthesis. Interestingly, the rejoining of DNA strand breaks was promoted by nicotinic acid but nicotinamide inhibited the rejoining. The dramatic degradation of NAD was observed when lymphocytes were treated with MNNG after elevating NAD level in the cells with nicotinic acid but not with nicotinamide. These results suggest the possible role of poly (ADP-ribose) polymerase to DNA repair.