In VitroMetabolism ofp-Xylene by Rabbit Lung and Liver

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Abstract
1. Optimum conditions for metabolism in vitro of p-xylene by rabbit liver or lung microsomal enzymes have been studied. Reactions were linear with time for at least 30 min at a microsomal protein concentration of 1 mg/ml. Addition of cytosol fraction to incubation mixtures of microsomes, NADPH, and substrate increased enzyme activity but the increase was independent of amount of cytosol added (over range of 0.1 ml to 0.5 ml). The pH optimum for the lung and liver microsomal system was 7.3 and 7.8, respectively, using Hepes buffer. The apparent Km and Vmax for the liver and lung systems were determined. The NAD+ and NADP+ requirements were studied. 2. The major metabolite of p-xylene in vitro, as determined by t.l.c., of liver and lung microsomal incubation mixtures was p-toluic acid. 3. Phenobarbital pre-treatment of rabbits (multiple doses, intraperitoneal or intravenous) induced the liver microsomal xylene-metabolizing system approximately 3-fold, whereas there was no change in activity of the lung enzyme. Maximum induction of liver microsomal metabolism after a single intraperitoneal dose of phenobarbital (100 mg/kg) was reached in 2–3 days and declined slowly thereafter; during this same period lung xylene-metabolizing activity remained unchanged or increased only slightly. Chlorpromazine pre-treatment, intraperitoneal (but not intravenous), of rabbits stimulated hepatic microsomal xylene-metabolizing activity. Administration of 1,2,3,4-dibenzanthracene or 3-methylcholanthrene to rabbits resuited in decreased xylene metabolism in vitro by liver and lung microsomes.