A study of the interaction of a series of substituted barbituric acids with the hepatic microsomal monooxygenase

Abstract

1. 1. A series of barbituric acid derivatives have been investigated in an attempt to correlate certain physicochemical parameters, such as lipid solubility and p K a, to their interaction as a substrate with the liver microsomal monooxygenase system. 2. 2. All of the barbiturates gave rise to a type I spectral change when added to suspensions of rat liver microsomes, indicating the formation of a cytochrome P-450-substrate complex. The maximal size of the type I spectral change, however, varied with the different barbiturates and was roughly correlated with their lipid solubility and p K a value. Furthermore, although the allyl-substituted barbiturates produced a type I spectral change at low concentrations, this turned into a modified type II spectral change upon increase in drug concentration. Maximal spectral changes were not additive when different barbiturates in combination were added to the microsomes. 3. 3. Binding affinity for cytochrome P-450, as judged from the spectrally determined dissociation constant ( K s ), varied among the different barbiturates studied, and only a rough correlation (correlation coefficient, r = 0.52) was observed between lipid solubility and binding affinity. There was also no good correlation between the rates at which the barbiturates were metabolized by the microsomal monooxygenase system and their lipid solubility ( r = 0.42). 4. 4. In contrast, the ability of the barbiturates to act on another membrane-bound enzyme system, the mitochondrial NADH oxidase, revealed a strong correlation ( r = 0.92) between lipid solubility and inhibitory efficiency. 5. 5. It is suggested that although lipid solubility is required for a substance to reach the microsomal cytochrome P-450, other properties of the molecule are of importance for determining the affinity with which it will interact with this cytochrome. In the mitochondria, however, lipid solubility alone appears to determine the efficiency with which the various barbiturates exert their inhibitory action on the respiratory chain.