We have investigated the effect of single amino acid substitutions of conserved arginines on the catalytic activities of the human Ogg1 protein (alpha-hOgg1-Ser(326)) (wild-type alpha-hOgg1). Mutant forms of hOgg1 with mutations Arg(46)-->Gln (alpha-hOgg1-Gln(46)) and Arg(154)-->His (alpha-hOgg1-His(154)) have previously been identified in human tumors. The mutant proteins alpha-hOgg1-Gln(46) and alpha-hOgg1-His(154) were expressed in Escherichia coli and purified to homogeneity. The substrate specificities of these proteins and wild-type alpha-hOgg1 were investigated using gamma-irradiated DNA and the technique of gas chromatography/isotope-dilution mass spectrometry. All three enzymes excised 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua) and 8-hydroxyguanine (8-OH-Gua) from gamma-irradiated DNA containing a multiplicity of base lesions. Michaelis-Menten kinetics of excision were measured. Significant differences between excision kinetics of these three enzymes were observed. Excision of FapyGua and 8-OH-Gua by wild-type alpha-hOgg1 was greater than that by alpha-hOgg1-Gln(46) and alpha-hOgg1-His(154). The latter mutant protein was less active than the former. The diminished activity of the mutant proteins was more pronounced for 8-OH-Gua than for FapyGua. Cleavage assays were also performed using (32)P-labeled 34mer oligonucleotide duplexes containing a single 8-OH-Gua paired to each of the four DNA bases. The results obtained with the oligonucleotide containing the 8-OH-Gua/Cyt pair were in good agreement with those observed with gamma-irradiated DNA. Wild-type alpha-hOgg1 and its mutants repaired the three mismatches less efficiently than the 8-OH-Gua/Cyt pair. The substitution of Arg(154), in addition to diminishing the activity on 8-OH-Gua, relaxes the selectivity found in the wild-type alpha-hOgg1 for the base opposite 8-OH-Gua. Taken together the results show that the mutant forms alpha-hOgg1-Gln(46) and alpha-hOgg1-His(154) found in human tumors are defective in their catalytic capacities.