Kinetics of ethylmorphine N-demethylase at low substrate concentrations and its implications concerning the role of the stimulation of NADPH-cytochrome P-450 reductase activity in mixed-function oxidase activity

The H+, K(+)-ATPase inhibitor pantoprazole (BY1023/SK&F96022) interacts less with cytochrome P450 than omeprazole and lansoprazole

The gastric acid antisecretory compound omeprazole (5-methoxy-2-((4-methoxy- 3,5-dimethyl-2-pyridinylmethyl)-sulphinyl)-1H-benzimidazole), a member of the new class of H+, K(+)-ATPase inhibitors, is known to interact with the metabolism of other drugs in vitro and in vivo. In this study, two other substituted benzimidazoles, pantoprazole (5-difluoromethoxy-2-((3,4-di-methoxy-2-pyridinylmethyl)-s ulp hinyl)-1H- benzimidazole) and lansoprazole (2-((3-methyl-4-(2,2,2-trifluoroethoxy)-2-pyridinylmethyl)- sulphinyl)-1H-benzimidazole) are compared for their ability to inhibit cytochrome P450 dependent biotransformation in vitro with regard to three representative reactions: O-dealkylation of 7-ethoxycoumarin (EC), N-demethylation of ethylmorphine (EM) and hydroxylation of lonazolac (Lona). These reactions can be seen in microsomes from phenobarbital pretreated rats representing the cytochrome P450IIB1 subfamily. As shown in presence of known inhibitors of cytochrome P450, e.g. SK&F 525A, metyrapone, chlorpromazine and nitrendipine, different enzymes seem to be responsible for these three indicator reactions of the cytochrome P450IIB1 complex. These reactions are inhibited to a different extent by the three H+, K(+)-ATPase inhibitors. Pantoprazole shows the lowest inhibitory activity versus the three reactions (Ki, mumol/L): EC, 138; EM, 104; Lona, 128. A greater effect is observed with omeprazole: EC, 38; EM, 68; Lona, 20. Lansoprazole exceeds omeprazole in inhibiting the three cytochrome P450 dependent enzymes: EC, 17; EM, 34; Lona, 8. In microsomes from untreated rats with the predominant cytochrome P450IIA1 subfamily as well as in microsomes from isopropanol treated rats (induction of cytochrome P450IIE1) which catalyse only lonazolac hydroxylation to a detectable amount, the latter reaction was inhibited by pantoprazole with a somewhat lower Ki of 77 whereas the values for omeprazole and lansoprazole remained unchanged in comparison to those found in microsomes from phenobarbital pretreated rats. The biotransformation rate of the substituted benzimidazoles themselves in microsomes from control and induced rats is lowest for pantoprazole followed by lansoprazole and omeprazole.

Xenobiotic-metabolizing enzyme systems in test fish--II. The ethylmorphine N-demethylase activity of guppy (Poecilia reticulata) liver

The oxidative demethylation of the model substrate ethylmorphine has been characterized for the first time in the liver of a fish (Poecilia reticulata). The enzyme showed maximal activity at 35 degrees C and pH values higher than 8. The values of Km and Vmax for the reaction were 0.83 +/- 0.11 mM and 4.64 +/- 0.81 nmol HCHO/(mg microsomal protein) per min. The activity is attributed to the cytochrome P-450-dependent monoxygenase system, since it is inhibited by CO and requires NADPH; moreover it is inhibited competitively by alpha-naphthoflavone and non-competitively by metyrapone. The enzyme activity is induced by a two-week treatment of fish with phenobarbital and may be associated with a protein band of Mr 54,000.

Further studies on microsomal drug metabolizing enzyme activity and protein levels in animal models showing different susceptibilities to aflatoxicosis

1. The total protein content and the activities of aniline hydroxylase and p-aminopyrine N-demethylase were measured in the microsomal fraction of rat, mouse, duck, pigeon and turkey. The relative Km values of the enzymes were also determined. 2. The microsomal protein content and the enzyme activities in the birds were lower than that in the rodents. Km values for aniline hydroxylase and p-aminopyrine N-demethylase were higher in birds than in either rat or mouse. 3. The microsomal protein content of mouse was relatively lower than that in rat, although its aniline hydroxylase and p-aminopyrine N-demethylase activities were higher than those in the rat. Km values for aniline hydroxylase in mouse and rat were 0.98 and 1.25 mM, respectively, while the Km values for p-aminopyrine N-demethylase in mouse and rat were 0.55 and 0.62 mM, respectively. 4. These results are discussed in relation to the species differences in susceptibility of animals to toxic or carcinogenic substances.

Levels of microsomal drug-metabolizing enzymes in animals which are highly susceptible to aflatoxin carcinogenicity: the case of the duck

A comparative study of the microsomal fractions of the duck and rat has shown significant differences in the total protein contents in these fractions and also in the activities of 2 phase I drug-metabolizing enzymes, ethylmorphine N-demethylase and aniline hydroxylase. These results are discussed in relation to the high susceptibility of birds (especially ducks) to toxic or carcinogenic substances.