Partially purified aldehyde oxidase (EC 1.2.3.1) has been prepared from human, rabbit, guinea pig, and baboon liver by heat treatment and precipitation with ammonium sulfate. The interaction of 35 substituted quinazolines and phthalazines with human liver enzyme has been studied using a spectrophotometric assay. Fifteen quinazoline and 14 phthalazine derivatives were found to be substrates for human liver aldehyde oxidase with Km values ranging from 5 to 500 microM. The substrate specificity of the quinazolines toward rabbit, guinea pig, and baboon liver aldehyde oxidase has also been investigated; the reaction of substituted phthalazines with mammalian liver enzyme has been reported previously (Beedham et al., 1990, Biochem. Pharmacol. 39, 1213-1221). Oxidation products of 2-substituted (4-substituted) quinazolines with rabbit liver aldehyde oxidase were identified by MS as 4-oxo (2-oxo)-quinazolines, respectively. In all cases, unsubstituted compounds gave the highest oxidation rates and the presence of lipophilic substituents presumably facilitated hydrophobic binding to the enzymes. However, there were marked differences in substrate specificity between human liver aldehyde oxidase and hepatic enzyme from rabbit, guinea pig, and baboon with the size of substrate being the differentiating factor. The molecular sizes of the substrates, estimated using calculated molar refractivities, ranked the size of the binding site of aldehyde oxidase in the order rabbit < guinea pig < baboon < man. Isoelectric points of the different aldehyde oxidase isozymes ranged from pH 5.10 for rabbit to 6.40 for the human liver isozyme. These results indicate that rabbit liver aldehyde oxidase shows marked differences from the human liver enzyme in its handling of quinazoline and phthalazine substrates.