DNA DAMAGE AND MUTATION IN HUMAN-CELLS EXPOSED TO NITRIC-OXIDE INVITRO

Nitric oxide (NO.) is a physiological messenger formed by several cell types. Reaction with O2 forms oxides that nitrosate amines at pH valu es near 7. We now report experiments in which NO. was added to intact human cells and to aerobic solutions of DNA, RNA, guanine, or adenine. TK6 human lymphoblastoid cells were mutated 15- to 18-fold above back ground levels at both the HPRT and TK gene loci. Xanthine and hypoxant hine, from deamination of guanine and adenine, respectively, were form ed in all cases. NO. induced dose-responsive DNA strand breakage. Yiel ds of xanthine ranged from nearly equal to about 80-fold higher than t hose of hypoxanthine. Yields of xanthine and hypoxanthine from nucleic acids were higher than those from free guanine and adenine. This was most pronounced for xanthine; 0.3 nmol/mg was formed from free guanine vs. 550 nmol/mg from calf thymus RNA. Nitric oxide added to TK6 cells produced a 40- to 50-fold increase in hypoxanthine and xanthine in ce llular DNA. We believe that these results, plus the expected deaminati ons of cytosine to uracil and 5-methylcytosine to thymine, account for the mutagenicity of nitric oxide toward bacteria and mammalian cells.