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Role of guinea pig and rabbit hepatic aldehyde oxidase in oxidative in vitro metabolism of cinchona antimalarials.

Cinchona alkaloids (quinine, quinidine, cinchonine, and cinchonidine) were incubated with partially purified aldehyde oxidase from rabbit or guinea pig liver. Reversed-phase HPLC methods were developed to separate the oxidation products from the parent drugs, and the metabolites were identified on the basis of their infrared and mass spectral characteristics. All four alkaloids were oxidized at carbon 2 of the quinoline ring to give the corresponding lactams. In addition, the dihydro contaminants of the cinchona alkaloids were also metabolized by aldehyde oxidase to the 2-quinolone derivatives. Kinetic constants for the oxidation reactions were determined spectrophotometrically and showed that these substrates have a low affinity (KM values of around 10(-5) M) for hepatic aldehyde oxidase, coupled with a relatively low oxidation rate. However, the overall efficiency of the enzyme (Vmax/KM) toward this group of compounds indicates that in vivo biotransformation by aldehyde oxidase will be a significant pathway. Microsomal metabolites were also isolated from quinine and quinidine incubations with rabbit or guinea pig liver fractions. 3-Hydroxyquinine (quinidine) and O-desmethylquinine (quinidine) were identified in microsomal and 10,000g supernatant extracts from quinine and quinidine, respectively. Oxidation of quinine via aldehyde oxidase appeared to be the predominant pathway in rabbit 10,000g fractions, because 2'-quininone was the major metabolite under these conditions with lower concentrations of the microsomal metabolites produced along with a dioxygenated derivative thought to be 3-hydroxy-2'-quininone.