Abstract
Interaction of aminopyrine with microsomal membrane-bound cytochrome P-450 was studied spectrophotometrically at various pH. Aminopyrine-induced type I spectral change in untreated rat microsomes was observed in neutral and alkaline media, and the absorption magnitude between peak and trough in the spectra increased markedly by increasing pH. On the other hand, an anomalous spectral change (λmax, 425 nm; λmin, 410 nm) was obtained in acid medium, and the absorption magnitude of the anomalous spectral change was enhanced by decreasing pH. The spectral dissociation constant for the anomalous aminopyrine-binding reaction at pH 6.32 was about one order of magnitude greater than that for the type I binding reaction at pH 8.22. The type of aminopyrine-induced spectral change differed depending upon the age and pretreatment of animals. Neonatal microsomes elicited only the anomalous spectral change in all pH media. Liver microsomes from 3-methylcholanthrene-pretreated rats showed a reverse type I spectral change. Antipyrine produced only a reverse type I spectral change in all microsomes tested, and the absorption magnitude was enhanced by decreasing the pH. In the presence of a saturated concentration of a reverse type I compound, i.e., ethanol or antipyrine, aminopyrine induced the type I spectral change, even in acid medium. The binding mechanism of cytochrome P-450 with aminopyrine is discussed on the basis of these results.
