The reactions of adenosine with malonaldehyde and glyoxal, and with malonaldehyde and methylglyoxal resulted in the formation of one malonaldehyde-glyoxal and one malonaldehyde-methylglyoxal conjugate adduct, respectively. These adducts were isolated and purified by reversed-phase liquid chromatography, and structurally characterized by UV, (1)H- and (13)C-NMR spectroscopy, and mass spectrometry. The malonaldehyde-glyoxal adduct was identified as 8-(diformylmethyl)-3-(beta-D-ribofuranosyl)imidazo[2,1-i]purine (M(1)Gx-A), while the malonaldehyde-methylglyoxal one as 8-(diformylmethyl)-7-methyl-3-(beta-D-ribofuranosyl)imidazo[2,1-i]purine (M(1)MGx-A). Both adducts were also observed in calf thymus DNA when incubated in the respective aldehydes under physiological pH and temperature. Moreover, in the reaction of methylglyoxal and malonaldehyde with adenosine, an additional adduct was formed. This adduct was found to consist of one unit derived from methylglyoxal and one unit from formaldehyde. The adduct was identified as N(6)-(2,3-dihydroxy-2-methylpropanoyl)-9-(beta-D-ribofuranosyl)purine (MGxFA-A). Formaldehyde was found to originate from the commercial methylglyoxal in which it was present as an impurity.