Aminopyrine demethylase--multiplicity shown by dieldrin and DDT inhibition

Aminopyrine metabolism by multiple forms of cytochrome P-450 from rat liver microsomes: simultaneous quantitation of four aminopyrine metabolites by high-performance liquid chromatography

Four aminopyrine metabolites generated by hepatic microsomes were simultaneously assayed by high-performance liquid chromatography. The metabolites were 4-monomethylaminoantipyrine (MAA), 4-aminoantipyrine (AA), 3-hydroxymethyl-2-methyl-4-dimethylamino-1-phenyl-3-pyrazoline-5-one (AM-OH), and one unidentified metabolite. MAA was the major metabolite generated by the microsomes; its formation was induced by phenobarbital but not by 3-methylcholanthrene. Female rats had lower N-demethylation activity of aminopyrine than male rats. The production of AA by microsomes was low. The formation of AM-OH was strongly induced by phenobarbital, but treatment with 3-methylcholanthrene reduced its formation. These differences in the microsomal aminopyrine monooxygenase activity are dependent on the relative amounts of the individual cytochrome P-450 isozymes. Therefore, we examined aminopyrine metabolism in a reconstituted system with purified cytochrome P-450s. P-450 UT-2 (P-450h) had high aminopyrine N-demethylation and hydroxylation activities, but P-450 F-2 (P-450i) had low N-demethylation activity and no hydroxylation activities, but P-450 F-2 (P-450i) had low N-demethylation activity and no hydroxylation activity. P-450 PB-4 (P-450b) and P-450 PB-5 (P-450e) had high aminopyrine hydroxylation activity and their N-demethylation activity also was high. The 3-methylcholanthrene-inducible forms P-450 MC-1 (P-450d) and MC-5 (P-450c) had aminopyrine N-demethylation activity but no hydroxylation activity. P-450 UT-4 (RLM2) is a unique form that produced a large amount of the unknown metabolite. P-450 UT-7 had the highest N-demethylation activity. Addition of cytochrome b5 to the reconstituted system enhanced the aminopyrine hydroxylation activities of P-450s UT-1, UT-2, PB-2, and PB-5. Also, the N-demethylation activities of P-450s UT-1, PB-1, PB-2, and MC-1 were increased by cytochrome b5. Metyrapone inhibited the catalytic activities of P-450s PB-4, PB-5, MC-1, and MC-5, and especially those of P-450s UT-4, and UT-7. The kinetics of the four major cytochrome P-450s (P-450 UT-2, UT-4, PB-4, and MC-5) for aminopyrine N-demethylation and hydroxylation activities were studied. P-450s PB-4 and UT-2 had similar Km values (0.50 and 0.62 mM, respectively) in aminopyrine N-demethylation activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Aminopyrine N-demethylation by rats with liver cirrhosis. Evidence for the intact cell hypothesis. A morphometric-functional study

The intact cell hypothesis states that a reduced number of intrinsically normal hepatocytes, together with hemodynamic alterations, explains decreased drug metabolism in cirrhosis. We explored this hypothesis by comparing results of the aminopyrine breath test with in vitro measurements of aminopyrine N-demethylation and morphometrically determined liver cell volume in a rat model of cirrhosis. Aminopyrine N-demethylation in vivo (ABT-k) was 0.98 +/- 0.10/h (mean +/- SD) in controls. The cirrhotic rats were separated into those with normal (NCR) and those with abnormal ABT-k (PCR). Microsomal aminopyrine N-demethylase averaged 2.08 +/- 0.77 and 2.09 +/- 0.54 mumol/min in controls and NCRs, respectively; it was reduced to 1.00 +/- 0.81 mumol/min (p less than 0.02) in PCRs. Morphometrically determined hepatocellular volume was 18.8 +/- 2.8, 17.1 +/- 1.9, and 11.6 +/- 6.1 ml in controls, NCRs, and PCRs, respectively, PCRs being lower than controls (p less than 0.01) and NCRs (p less than 0.05). When N-demethylase and cytochrome P450 were related to hepatocellular volume (in milliliters), no significant difference between the three groups was apparent. We conclude that reduced aminopyrine N-demethylation in progressed cirrhosis is mainly due to a loss of liver cell volume. The function per liver cell volume remains constant, however, thus favoring the intact cell hypothesis for the handling of slowly metabolized compounds such as aminopyrine.

Characterization of house fly microsomal mixed function oxidases: inhibition by juvenile hormone i and piperonyl butoxide

The microsomal mixed function oxidase system of the house fly (Musca domestica [L.]) was characterized with respect to N-demethylation of p-chloromethylaniline, O-demethylation of methoxyresorufin, epoxidation of aldrin and the formation of a metabolite-cytochrome P-450 complex during oxidation of piperonyl butoxide (PB). The inhibition of these reactions by juvenile hormone I (E, E cis methyl 10,11-epoxy-7-ethyl-3, 11-dimethyl-2, 6-tridecadienoate, JH-I) was competitive for the N-demethylase, epoxidase and the formation of the PB metabolite-complex. Non-competitive inhibition was observed for the O-demethylase. The inhibition of these reactions by JH-I provides evidence that the mixed function oxidase system participates in the degradation of juvenile hormone, and that the juvenile hormone-like properties of PB and other methylenedioxyphenyl compounds are derived from their inhibition of this degradation. The PB metabolite-cytochrome P-450 complex has 2 absorbance maxima in the reduced form (427 and 455 nm), and a single absorbance maximum in the oxidized form (438 nm). pH affected the extinction of the 427 and 455 nm absorbance bands. The pH equilibrium point was 8.3 for the reduced PB metabolite complex, and 9.5 for ethylisocyanide. In addition, the PB metabolite complex could be generated in vivo.

The functional roles of cytochrome P-450 mediated systems: present knowledge and future areas of investigations

Research in recent years has considerably enhanced our understanding of the membrane-bound cytochrome P-450-catalyzed biotransformations in organisms. A remarkable similarity has become apparent between diversified animal groups with respect to the components and functional roles of the cytochrome P-450 systems. For example, the steroidal hormone metabolism role for this cytochrome in mammals was recently extended to insect species. In addition, this cytochrome functions importantly in mammalian species for the metabolism of a variety of other endogenous substrates such as bile acids, anthranilic acid, fatty acids, bilirubin and heme, indoles, amines, and purines. Regarding the exogenous role, the system has been known to convert toxic lipophilic substances such as drugs and insecticides to more hydrophilic metabolites which can be easily eliminated by the water-based excretory system in organisms. This detoxication ability apparently arose in organisms, particularly the herbivores, in response to the need to detoxify plant-defensive toxins such as tannins, phenols, quinones, alkaloids, steroids, lecithins, and lignins. The interface between plants and insects is enormous, and therefore it appears probable that cytochrome P-450 will one day be shown to be important in the conversion/utilization of other plant secondary substances, e.g., synthesis of cholesterol from phytosterols, and sex pheromones from olefins, terpenes, and alkaloids.

Enhancement in vivo of drug oxidations following administration of benzphetamine, acetone, metyrapone and dimethylsulfoxide

Effects of benzphetamine, acetone, metyrapone and dimethylsulfoxide administration to rats on the metabolism of drugs by liver 9,000 x g supernatant fraction were studied herein. Activities for aniline hydroxylation and phenacetin O-deethylation were increased while ethylmorphine and benzphetamine N-demethylations were unchanged by the single administration of acetone, metyrapone or dimethylsulfoxide. Increase in aniline hydroxylase activity by about 53.4% and in phenacetin O-deethylase activity by about 44.4% were observed at 30 min after the single administration of benzphetamine whereas ethylmorphine N-demethylase activity was slightly decreased. NADPH-cytochrome P-450 reductase activity and cytochrome P-450 content were unaltered until 12 hr after the single administration of benzphetamine. Aniline hydroxylation was increased by the addition of benzphetamine to the incubation mixture and the increase in aniline hydroxylation caused by benzphetamine could be reversed by washing the microsomes.

Inhibition by cyanide of drug oxidations in rat liver microsomes

Cyanide inhibited microsomal activities of aniline hydroxylation and aminopyrine, ethylmorphine and codeine demethylations and produced a modified type II difference spectrum of cytochrome P-450 to give two spectral dissociation constants, 0.21mM and 1.05 mM. The binding of cyanide to cytochrome P-450 resulted in innhibition of NADPH-cytochrome P-450 reductase activity. The cyanide inhibition of drug oxidations was partially avoided by increasing oxygen tension. A possible mechanism for the inhibition of drug oxidations by cyanide is discussed.