SUBSTRATE-SPECIFICITY OF HUMAN LIVER ALDEHYDE OXIDASE TOWARD SUBSTITUTED QUINAZOLINES AND PHTHALAZINES - A COMPARISON WITH HEPATIC ENZYME FROM GUINEA-PIG, RABBIT, AND BABOON

Partially purified aldehyde oxidase (EC 1.2.3.1) has been prepared fro m human, rabbit, guinea pig, and baboon liver by heat treatment and pr ecipitation with ammonium sulfate. The interaction of 35 substituted q uinazolines and phthalazines with human liver enzyme has been studied using a spectrophotometric assay. Fifteen quinazoline and 14 phthalazi ne derivatives were found to be substrates for human liver aldehyde ox idase with K-m values ranging from 5 to 500 mu M. The substrate specif icity of the quinazolines toward rabbit, guinea pig, and baboon liver aldehyde oxidase has also been investigated; the reaction of substitut ed phthalazines with mammalian liver enzyme has been reported previous ly (Beedham et al., 1990, Biochem. Pharmacol. 39, 1213-1221), Oxidatio n products of 2-substituted (4-substituted) quinazolines with rabbit L iver aldehyde oxidase were identified by MS as 4-oxo (2-oxo)-quinazoli nes, respectively. In all cases, unsubstituted compounds gave the high est oxidation rates and the presence of lipophilic substituents presum ably facilitated hydrophobic binding to the enzymes. However, there we re marked differences in substrate specificity between human liver ald ehyde oxidase and hepatic enzyme from rabbit, guinea pig, and baboon w ith the size of substrate being the differentiating factor, The molecu lar sizes of the substrates, estimated using calculated molar refracti vities, ranked the size of the binding site of aldehyde oxidase in the order rabbit < guinea pig < baboon < man, Isoelectric points of the d ifferent aldehyde oxidase isozymes ranged from pH 5.10 for rabbit to 6 .40 for the human liver isozyme. These results indicate that rabbit li ver aldehyde oxidase shows marked differences from the human liver enz yme in its handling of quinazoline and phthalazine substrates. (C) 199 5 Academic Press, Inc.