SUBSTRATE-SPECIFICITY OF HUMAN LIVER ALDEHYDE OXIDASE TOWARD SUBSTITUTED QUINAZOLINES AND PHTHALAZINES - A COMPARISON WITH HEPATIC ENZYME FROM GUINEA-PIG, RABBIT, AND BABOON
C. Beedham et al., SUBSTRATE-SPECIFICITY OF HUMAN LIVER ALDEHYDE OXIDASE TOWARD SUBSTITUTED QUINAZOLINES AND PHTHALAZINES - A COMPARISON WITH HEPATIC ENZYME FROM GUINEA-PIG, RABBIT, AND BABOON, Archives of biochemistry and biophysics, 319(2), 1995, pp. 481-490
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.