Effects of primaquine on hepatic microsomal haemoproteins and drug oxidation

Abstract

Administration of the antimalarial agent primaquine to male rats (50 mg/kg i.p. daily for 4 days) resulted in a 30% decrease in hepatic microsomal cytochrome P-450 ( P-450) content; levels of other microsomal haemoproteins were unaltered. Kinetic analysis of 2 mixed-function oxidase activities (aminopyrine N-demethylase and aniline p-hydroxylase) in primaquine-pretreated rat liver microsomes revealed a significant and similar decrease in the maximal reaction velocities of these enzymes ( V max), but the apparent Michaelis constants ( K m) were not changed. The activity of mitochondrial δ-amonolaevulinic acid synthetase (the rate-liniting step in haem biosynthesis) was normal in primaquine-pretreated rat liver but haem oxygenase activity (the rate-limiting step in haem degradation) was elevated approximately 2-fold. Haem availability for haemoprotein assembly (determined as the haem-saturaiton ratio of the cytosolic haemoprotein tryptophan pyroolase) was also normal although the absolute activity of tryptophan pyrrolase was decreased after primaquine pretreatment. order to cacilitate an analysis of the P-450 isozyme profile in control and primaquine-treated rat live,, total microsomal P-450 was isolated by hydrophobic affinity chromatography on n-octylamino-Sepharose 4B of stained polyacrylamide gels following electrophoresis of these partially-purified P-450 fractions indicated that primaquine exposure did not selectively decrease any of the 3 protein bands in the P-450 molecular weight region (48–56 kD). These observations, when considered together, suggest that primaquine may affect P-450 and mixed-function oxidase activity by inhibition of protein synthesis. The characteristic rapid turnover rates of P-450 isozymes may predispose these haemoproteins to the toxic effects of primaquine whereas those haemoproteins that turn over less rapidly, such as cytochrome b 5, appear to be less susceptible. Microsomal haem oxygenase activity may be elevated after primaquine administration since lowered haemoprotein requirements for haem could result in excess haem levels within the hepatocyte.