Hepatic aminopyrine N-demethylase system: further studies of assay procedure

Metabolic Fate of the Isocyanide Moiety: Are Isocyanides Pharmacophore Groups Neglected by Medicinal Chemists?

Despite the isolation of hundreds of bioactive isocyanides from terrestrial fungi and bacteria as well as marine organisms, the isocyanide functionality has so far received little attention from a medicinal chemistry standpoint. The widespread tenet that isocyanides are chemically and metabolically unstable has restricted bioactivity studies to their antifouling properties and technical applications. In order to confirm or refute this idea, the hepatic metabolism of six model isocyanides was investigated. Aromatic and primary isocyanides turned out to be unstable and metabolically labile, but secondary and tertiary isocyanides resisted metabolization, showing, in some cases, cytochrome P450 inhibitory properties. The potential therefore exists for the secondary and tertiary isocyanides to qualify them as pharmacophore groups, in particular as war-heads for metalloenzyme inhibition because of their potent metal-coordinating properties.

Cryopreserved Fetal Liver Cell Transplants Support the Chronic Failing Liver in Rats with CCl4-Induced Cirrhosis

Hepatocyte transplantation is a promising method for supporting hepatic function in a broad spectrum of liver diseases. The aim of this work was to test the efficacy of human fetal liver cells to support the chronic failing liver in an experimental model of carbon tetrachloride (CCl4)-induced cirrhosis in rats. Liver cirrhosis was induced by intraperitoneal administration of CCl4 at a dose of 0.2 ml (50% v/v solution)/100 g body weight, twice a week for 3 months in rats. Ten days after stopping CCl4 administration (experimental day 0), rats received intrasplenic injection of cryopreserved fetal liver cells (FLC, 1 × 107 cells in 0.3 ml medium). As a cirrhotic control group, CCl4-induced cirrhotic rats were used with intrasplenic injection of an equal volume of medium alone. Animals were sacrificed on experimental day 15. Human fetal liver cell transplantation almost completely prevented the death of cirrhotic animals during the 2 weeks after treatment, while high ongoing mortality was seen in the cirrhotic control group. Cell transplantation into the spleen normalized total bilirubin and TBARSs levels and increased albumin levels in blood serum, as well as restoring mitochondrial function and liver detoxification function (assessed by cytochrome P450 contents and activity) compared with the activities seen in the cirrhosis control group. In parallel with this restoration of biochemical and functional liver indices, morphological patterns of liver recovery or regeneration after liver cell transplantation were demonstrated in day 15 samples by light microscopy. These were absent in the group that had received only medium alone.

Functional hepatic recovery after xenotransplantation of cryopreserved fetal liver cells or soluble cell-factor administration in a cirrhotic rat model: are viable cells necessary?

BACKGROUND AND AIM

Chronic liver failure results in the decrease of the number of functioning hepatocytes. It dictates the necessity of using exogenous viable cells or/and agents that can stimulate hepatic regenerative processes. Fetal liver contains both hepatic and hematopoietic stem cells with high proliferative potential, which may replace damaged cells. Also, immature cells produce fetal-specific factors which may support the injured liver. Our aim was to test the ability of human fetal liver cells and cell-free fetal-specific factors of non-hepatic origin to stimulate recovery processes in an experimental model of carbon tetrachloride-induced cirrhosis in rats.

METHODS

Cirrhotic rats were intrasplenically injected with fetal liver cells (1 x 10(7) cells/0.3 mL medium) or cell-free fetal-specific factors (0.3 mL/1 mg protein). Control groups received medium alone. Serum indexes, hepatic functions, and morphology were evaluated for 15 days.

RESULT

Human fetal liver cell transplantation was shown to abrogate the mortality of cirrhotic animals, to improve serum markers, and to restore liver mitochondrial function and detoxification. Morphological patterns of liver recovery were observed by histology. In comparison, an injection of fetal-specific factors produced similar functional recovery, whilst a more limited liver regeneration was observed by histology.

CONCLUSIONS

The positive effects of fetal liver cell and cell-free fetal-specific factors in experimental cirrhosis may result from the presence of stage-specific factors activating hepatocellular repair.

Safety Evaluation and Drug Development based on Biological Fate of Drugs —Efforts Made to Overcome Drug Interaction in Drug Development—

1. Assay methods to detect drug interaction in toxicological samples were established by determining cytochrome P450 content and its activity in liver samples. The O-dealkylation reaction of 7-alkoxycoumarin was indicated to reflect changes in the molecular forms of P450s, and the enzyme induction or inhibition in the toxicological samples was easily detected by using the established methods. 2. During toxicological studies of 450191-S or the sleep inducer rilmazafone, a phenobarbital type-induction of hepatic drug metabolizing enzymes was observed in animals, and the doses required for the induction differed markedly between rats and dogs. Enzyme induction was caused by some specific metabolites of 450191-S, and the plasma concentrations of these metabolites were comparable when the enzyme induction was developed in both animals. 3. A nonsteroidal antiinflammatory compound 480156-S showed a slight or no effect on microsomal drug metabolizing activity in rats. On the other hand, repeated administration of this compound to humans resulted in a marked decrease in the oxidative metabolism of 480156-S, followed by a marked increase in the plasma concentrations of the compound. When volunteers were given 480156-S followed by several drugs, such as tolubutamide, the plasma clearance was delayed remarkably, indicating a severe drug interaction. 4. Cytochrome P450 belonging to the CYP2C family was indicated to participate in the oxidative metabolism of 480156-S in both rat and human liver microsomes. The preincubation of microsomes with 480156-S caused a concentration-dependent inhibition of CYP2C-dependent tolubutamide hydroxylation reaction in both rats and humans. There was a marked species difference in the susceptibility to the inhibitory effect of 480156-S, and the concentration required to inhibit rat CYP2C was almost 10 times higher than that required in humans. 5. The cephem antibiotics having N-methyltetrazolethiol (NMTT) at the 3'-position substituent were demonstrated to inhibit mitochondrial low K(m) aldehyde dehydrogenase (ALDH), and produced disulfiram-like (Antabuse) reaction during alcohol metabolism. Pharmacokinetic studies indicated that NMTT released from the antibiotics in bile duct or intestine cause the inhibitory action followed by the development of disulfiram-like reaction. 6. Attempts had been made to develop new cephem antibiotics lacking the disulfiram-like reaction by changing the chemical structure of 3'-position substituents, and a hydroxyethyltetrazolethiol was found not to inhibit the enzyme. Based on this result, together with the antibacterial activity, we have developed a new oxacephem antibiotic flomoxef (6315-S). Flomoxef showed no disulfiram-like reaction both in rats and human.

Depression of constitutive murine cytochromes P450 by staphylococcal enterotoxin B

Most in vivo studies demonstrating decreased activities of hepatic cytochromes P450 with inflammation have used Gram-negative bacterial lipopolysaccharide (LPS) as the inflammatory stimulant. But products of Gram-positive bacteria, such as staphylococcal enterotoxin B (SEB), also stimulate inflammatory mediators, albeit with a different pattern than LPS. Therefore, effects of SEB on the regulation of murine constitutive P450s were determined in this study and compared with those of LPS. LPS-responsive C3H/HeN and LPS-unresponsive C3H/HeJ mice were injected with either LPS (0.5 mg/kg) or SEB (0.66 to 6.6 mg/kg), and hepatic cytochromes P450 and serum tumor necrosis factor-alpha, interleukin-6, nitrate/nitrite, and serum amyloid A concentrations were determined up to 24 hr. HeJ mice were generally less responsive than HeN mice to both stimuli, with lower cytokine, nitrate/nitrite, and serum amyloid A responses. However, in both mouse strains SEB caused more prolonged cytokine, higher nitrate/nitrite, and lower serum amyloid A concentrations than LPS. Despite these differences, in HeN mice, after both SEB and LPS administration, total P450 concentrations were equally depressed by 40%. Both SEB and LPS depressed CYP1A1 and 1A2 microsomal protein concentrations by 45 and 30%, respectively; CYP2E1 by 64%; and CYP3A by 70%. There was comparable inhibition of enzymatic activities. In HeJ mice, SEB was only slightly more effective in depressing P450s than LPS, as might be expected. These data showed that the Gram-positive bacterial inflammatory stimulant SEB caused effects on murine hepatic cytochromes P450 similar to those of LPS, even though the pattern of inflammatory mediators induced after SEB exposure was different.

Cytochrome P450-dependent metabolism of trichloroethylene: interindividual differences in humans

Trichloroethylene (TRI) is an industrial solvent with a history of use in anesthesia, and is a common groundwater contaminant. Cytochrome P450 (CYP)-dependent metabolism of TRI produces chloral hydrate (CH) and is rate limiting in the ultimate production of trichloro- and/or dichloroacetic acid from TRI. Exposure of rodents to TRI results in lung and liver tumors (mice) and nephrotoxicity (rats). The toxicity is exacerbated by pretreatment of mice with CYP inducers. We report significant variability in TRI metabolism in a sample of 23 human hepatic microsomal samples and demonstrate the dependence of TRI metabolism on CYP2E1. K(m) values in this limited sample population are not normally distributed. We have correlated microsomal CH formation with the activity toward routine CYP2E1 substrates and with immunologically detectable CYP2E1 protein. Further, TRI metabolism in microsomes from lymphoblastoid cell lines expressing CYP2E1, CYP1A1, CYP1A2, or CYP3A4 indicated minimal involvement of the latter forms, with CYP2E1 catalyzing more than 60% of total microsomal TRI metabolism. These results indicate that humans are not uniform in their capacity for CYP-dependent metabolism of TRI and increased CYP2E1 activity may increase susceptibility to TRI-induced toxicity in the human.

Metabolism of gemcitabine in rat and dog

1. The metabolism of 14C-gemcitabine in the male rat has been studied after intravenous administration of a single dose (10 mg/kg) or five doses (1 mg/kg/day) of 14C-gemcitabine. The metabolism in male dog has been studied after only single dosing. The effects of gemcitabine on hepatic drug-metabolizing enzymes in rat has also been studied. 2. The concentration of gemcitabine in the plasma was 11.84 micrograms/ml at 5 min, and then rapidly decreased after a single administration to rat. A deaminated uracil analogue of gemcitabine progressively increased with time. Gemcitabine and the uracil metabolite accounted for 80.0 and 11.8% of the radioactive dose in the 0-24-h urine samples respectively. Gemcitabine was the major component identified in lung, liver and kidney at 5 min after administration. 3. After repeated administration to rat, metabolites in the plasma and tissues were not remarkably different from those found after a single administration. 4. After a single administration to dog, the plasma concentration of gemcitabine was 12.39 micrograms/ml at 5 min. Gemcitabine and the uracil metabolite accounted for 8.3 and 71.8% of the dose in the 0-24-h urine samples respectively. 5. No differences were observed in enzymatic activities per whole liver between the gemcitabine-treated and control rat.

The effect of propylene glycol on the P450-dependent metabolism of acetaminophen and other chemicals in subcellular fractions of mouse liver

Propylene glycol (PG) decreases the hepatotoxicity of acetaminophen (APAP). To elucidate the mechanism for this response, we measured the effect of PG on the in vitro metabolism of APAP by subcellular liver fractions from 6-10 week-old male B6C3F1 mice. The fractions were assayed for their ability to bioactivate APAP to N-acetyl-p-benzoquinone imine, which was trapped as APAP-glutathione conjugates or APAP-protein adducts, and for dimethyl-nitrosamine-N-demethylase (DMN), 4-nitrophenol hydroxylase (4-NPOH), and phenacetin-O-deethylase (PAD) activities. Activity in the crude mitochondrial-rich (10,000 x g pellet) fraction was low and PG had no effect. PG inhibited DMN and 4-NPOH, indicators of IIE1-dependent activity, and the formation of APAP-glutathione conjugates and APAP-protein adducts in both heavy (15,000 x g pellet) and light (100,000 x g pellet) microsomes. PAD, a measure of IA2-dependent activity, was not inhibited. These data demonstrate that PG selectively inhibits IIE1 activity, including the bioactivation of APAP, and implicates this as the mechanism for PG-mediated protection of APAP hepatotoxicity in mice.

Cytochrome P-450 catalytic activity and isoform composition following liver denervation in old male rats

A study was made concerning the role of adrenergic and cholinergic neural regulation in the functional activity of enzymes of microsomal oxidation in the liver. Experiments were performed on adult and old rats using surgical denervation of liver (vagotomy and sympathotomy). The results obtained showed changes that occurred in the monooxygenase activity (aminopyrine demethylase and aniline hydroxylase), as well as in the isoform composition and inductive synthesis of cytochrome P-450. The neural control over liver detoxication function was found to weaken in old age.

Induction of cytochrome P450 isozymes in human amnion FL cells

The human epithelial FL cell line contains low levels of constitutive ethoxyresorufin O-deethylase, ethoxycoumarin O-deethylase and aminopyrine N-demethylase activities, of which ethoxyresorufin O-deethylase could be induced by norepinephrine, beta-naphthoflavone and 3-methylcholanthrene, but ethoxy-couarin O-deethylase and aminopyrine N-demethylase activities could be induced by norepinephrine, beta-naphthoflavone and 3-methylcholanthrene as well as phenobarbital. Inducibility of ethoxyresorufin O-deethylase, ethoxycoumarin O-deethylase and aminopyrine N-demethylase activities was 3.1-6.7-, 1.8-3.1- and 1.4-2.0-fold respectively. Co-treatment with 3-methylcholanthrene and norepinephrine resulted in higher ethoxyresorufin O-deethylase and aminopyrine N-demethylase activities than treatment with 3-methylcholanthrene alone. The induced ethoxyresorufin O-deethylase and ethoxycoumarin O-deethylase activities remained at a high level for 24-36 h after removal of the inducer from the medium. The metabolizing ability of induced FL cells is stronger than that of non-induced ones in a given time following induction. The results further confirm that it is feasible to use the cytochrome P450 isozyme-induced FL cell as a biological indicator in short-term tests for screening promutagens/procarcinogens.

The effect of carbon monoxide on aminopyrine metabolism in the isolated perfused rabbit lung

Carbon monoxide (CO) is a ubiquitous environmental pollutant widely recognized for its ability to inhibit cytochrome P450-mediated metabolism of xenobiotics in vitro. In recent years, the importance of the lung in the metabolic disposition of certain airborne and systemically administered xenobiotics has been demonstrated. The purpose of this investigation was to establish a threshold for the CO-induced inhibition of cytochrome P450-mediated activity in the isolated perfused rabbit lung and to determine if hemoglobin would alter the carbon monoxide-cytochrome P450 interaction. On the basis of its half-life and the stoichiometry of its metabolism, aminopyrine was shown to be a good substrate for monitoring mixed function oxidase activity in the intact rabbit lung. First-order rate constants for aminopyrine metabolism were significantly lower in isolated rabbit lungs perfused with either artificial medium (39%) or whole blood (67%) and ventilated with a 7.5% CO/20% O2 mixture for 2.5 hr than in the respective control lungs ventilated with breathing air. The threshold level (7.5% CO) for this inhibition is the same in lungs perfused with artificial medium and in whole blood-perfused lungs and is well above environmentally relevant levels of exposure.

Effects of a testicotoxic dose of cadmium on the liver and drug metabolism in the rat

1. The effect of an acute testicotoxic dose of cadmium (CdCl2.H2O, 2.0 mg/kg i.p.) on liver morphology and drug-metabolizing enzyme activities were studied in adult male and female rats. 2. Cd treatment to female rats caused a slight and reversible decrease in hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and aminopyrine N-demethylase (APND) activities. 3. No significant changes were noted in the liver morphology, serum alanine aminotransferase activities, enzyme induction by phenobarbital and 3-methylcholanthrene, and glucuronosyl-transferase (GT) and glutathione S-transferase (GST) activities. 4. The same Cd treatment to male rats, however, resulted in a much more pronounced and prolonged reduction in AHH and APND activities, which was attributable to a Cd-induced testicular necrosis and, hence, impairment of androgen secretion. 5. Accordingly, Cd treatment to castrated male rats did not lower the enzyme activities any further, and full recovery of activities was obtained after the administration of testosterone. 6. Both GT and GST, the two sex-independent enzymes, were not significantly affected by either Cd or gonadectomy in the male rat. 7. The present data show that a low acute dose of Cd induces chemical castration without severely altering hepatic function.

Induction of rat hepatic cytochromes P-450 by environmental nitropolycyclic aromatic hydrocarbons

Nitrated polycyclic aromatic hydrocarbons (PAHs) are environmental contaminants that result from various incomplete combustion processes. We have examined the activity of hepatic microsomal enzymes in rats pretreated with a series of environmentally occurring nitrated PAHs including: 1- and 4-nitropyrene, 1,3-, 1,6- and 1,8-dinitropyrene, 6-nitrochrysene, 7-nitrobenz[a]anthracene, 3-nitrofluoranthene, and 1-, 3-, and 6-nitrobenzo[a]pyrene. None of the compounds increased the cytochrome P-450 content more than 2-fold. 1,8-Dinitropyrene, 6-nitrochrysene, and 1- and 3-nitrobenzo[a]pyrene significantly increased arylhydrocarbon hydroxylase activity 2- to 8-fold higher than solvent-treated controls. The induction of 7-ethoxycoumarin O-deethylase activity paralleled that found with arylhydrocarbon hydroxylase. The maximum induction of aminopyrine N-demethylase was only 1.5-fold, and none of the nitrated PAHs caused significant increases in epoxide hydrase or NADPH-cytochrome c reductase. 1-Nitropyrene reductase activity was induced by each of the compounds with the exception of 6-nitrobenzo[a]pyrene. The greatest increase was caused by 1-nitrobenzo[a]pyrene followed by 1,3-dinitropyrene, 3-nitrobenzo[a]pyrene and 6-nitrochrysene. These data suggest that nitrated PAHs may potentiate the effects of subsequent exposures to various chemical carcinogens.

Role of propiolaldehyde and other metabolites in the pargyline inhibition of rat liver aldehyde dehydrogenase

The metabolism of pargyline proceeds by way of three separate cytochrome P-450 catalyzed N-dealkylation reactions: N-depropargylation, N-demethylation and N-debenzylation. Propiolaldehyde, a product of N-depropargylation, is a potent inhibitor of aldehyde dehydrogenase (AlDH). The formation of pargyline-derived propiolaldehyde by isolated rat liver microsomes in vitro was confirmed using gas chromatographic/mass spectrometric techniques. The measured rates of propiolaldehyde formation for uninduced and phenobarbital-induced microsomes in vitro were 0.2 +/- 0.03 and 0.9 +/- 0.2 mumole/30 min/g wet weight liver respectively. However, these rates may have been artificially low due to competition between semicarbazide, the trapping agent, and microsomal proteins for the generated propiolaldehyde. CO significantly inhibited the microsome-catalyzed N-depropargylation reaction in vitro, whereas CoCl2 pretreatment of rats partially blocked the pargyline-induced rise in blood acetaldehyde after ethanol. Inhibition of the low Km liver mitochondrial AlDH by propiolaldehyde in vitro exhibited first-order kinetics, which is consistent with irreversible inhibition. Acetaldehyde did not attenuate the inhibition of AlDH by propiolaldehyde in vitro or by pargyline in vivo. Propargyl alcohol, a substance which is metabolized to propiolaldehyde by alcohol dehydrogenase, also inhibited AlDH in vivo and caused a quantitatively similar rise in blood acetaldehyde after ethanol as pargyline. Other putative metabolites of pargyline, namely benzylamine and propargylamine, inhibited AlDH in vivo, albeit to a lesser degree than pargyline, but neither of these amines inhibited AlDH directly. Monoamine oxidase was implicated in the conversion of benzylamine to an active inhibitory species, possibly an imine. From these studies, we conclude that propiolaldehyde was the primary metabolite responsible for the pargyline inhibition of AlDH in vivo; however, certain amine metabolites may have contributed to a lesser degree by conversion to yet unknown inhibitory forms.

The effect of Sudan III on drug metabolizing enzymes

We have examined the induction of drug metabolizing enzymes in rat liver microsomes by azo dye, 1-(p-phenylazophenylazo)-2-naphthol (Sudan III). Marked increases were observed in the levels of cytochrome P-448 as well as in p-nitroanisole O-demethylase (p-NAD), amaranth (AR) and neoprontosil reductases (NPR) and 7-ethoxycoumarin O-deethylase (ECD) activities. On the other hand, aminopyrene N-demethylase activity was not significantly increased. Further, induced ECD activity was inhibited 90% by a specific antibody against cytochrome P-448 while the inhibition observed with an antibody against cytochrome P-450 was less than 25%. Simultaneous administration of Sudan III and 3-methylcholanthene (3-MC) induced cytochrome P-448 up to a level brought about by either Sudan III or 3-MC treatment alone. In contrast, Sudan III did not induce cytochrome P-448 in the 3-MC insensitive DBA/2 mouse. Solubilized microsomes from Sudan III-treated rats showed an identical sodium dodecyl sulfate polyacrylamide gel electrophoretic (SDS-PAGE) pattern with those from 3-MC-treated animals. It is concluded that the cytochrome P-448 induced in liver by Sudan III is very similar to that induced by 3-MC. Sudan III also induced UDP-glucuronyltransferase activity towards 1-naphthol and estradiol. It did not induce NADPH-cytochrome c reductase, nor any of the enzymes which constitute the microsomal electron transport chain except for cytochrome P-448.

Influence of ozone and nitrogen dioxide on hepatic microsomal enzymes in mice

Since ambient concentrations of ozone and nitrogen dioxide increase drug-induced sleeping time in female mice, potential mechanisms were sought by investigating the effects of these gases on hepatic microsomal mixed-function oxidases in female CD-1 mice. A 3-h exposure to 9800 microgram O3/m3 (5 ppm) or 9400 microgram NO2/m3 (5 ppm) did not change the concentration of cytochrome P-450 significantly. Aniline hydroxylase, but not aminopyrine N-demethylase or p-nitroanisole O-demethylase, activities were increased following a 3-h exposure to 9400 microgram O3/m3 (5 ppm). Aniline hydroxylase activity was also increased after a 2-d (5 h/d) exposure to 1960 microgram O3/m3 (1 ppm). None of these enzyme activities were affected by a 3-h exposure to 9400 microgram NO2/m3 (5 ppm). In these studies, O3 sometimes increased wet liver weight, and thus additional experiments were conducted. A 5-h exposure to 1960 microgram O3/m3 (1 ppm) caused a lesser decrease in body weight than the decrease observed after a similar air exposure. Liver wet weights were elevated after O3 exposure. However, there were no significant changes in liver dry weight, liver dry-to-wet-weight ratio, or ratios of liver (wet or dry) weight to body weight. From these data, it is concluded that mechanisms other than those investigated are responsible for the effect of O3 and NO2 on drug-induced sleeping time. However, the activity of one mixed-function oxidase was slightly increased by O3, indicating a hitherto unrecognized systemic effect of O3 exposure.

Evaluation and comparison of colorimetric, radiometric and high performance liquid chromatographic assays for aminopyrine-N-demethylation by rat liver microsomes

Aminopyrine (DMAP) is metabolized by two successive N-demethylations to monomethyl-4-aminoantipyrine (MMAP) and 4-aminoantipyrine (AAP). Separation and quantification of DMAP and its metabolites in microsomal incubation mixtures by h.p.l.c. showed that other reactions also occur. In the radiometric method where [Me2-14C]DMAP is used as substrate, [14C]formaldehyde is formed during the N-demethylation. However since commercial [14C]DMAP, is not completely double labelled, and both MMAP and AAP are formed, it is impossible to calculate the formaldehyde formation accurately from the specific activity of [14C]DMAP. Moreover, it was shown that DMAP, AAP and particularly MMAP may all develop considerable colour intensity with the Nash reagent, which is used to determine formaldehyde. Despite this difficulty, DMAP may still be used as a model substrate in vitro, with the Nash assay being used to determine formaldehyde if low substrate concentrations and a short incubation time are used. Thus the interference of DMAP or its metabolites with the Nash assay is negligible. As the same Km and Vmax values were obtained from both the radiometric and the relatively precise colorimetric assay it is suggested that there is little wrong with either method, at least under our experimental conditions. The h.p.l.c. method however, underestimates formaldehyde formation, probably because metabolites other than MMAP and AAP are formed. The latter method however may be used to analyse the aminopyrine metabolism in more detail.

Hepatic aminopyrine N-demethylase system: effect of cyanide on microsomal N-demethylase activity

Cyanide, an inhibitor of many hemoproteins, was shown to affect a number of microsomal drug-metabolizing activities catalyzed by cytochrome P-450. The N-demethylation reaction of aminopyrine was inhibited noncompetitively by this inhibitor in microsomal preparations from rats. The binding reaction of aminopyrine with microsomal cytochrome P-450 was also modified by cyanide, and an abnormal aminopyrine-induced difference spectrum of microsomes by cyanide, and an abnormal aminopyrine-induced difference spectrum of microsomes appeared when cyanide was added to the reaction mixture. Partial dissociation of cytochrome P-450. Cyanide complex by aminopyrine was observed by spectrophotometrical and epr spectroscopic methods. These results suggest that aminopyrine and cyanide reciprocally affect binding with cytochrome P-450 and modification by cyanide of aminopyrine binding reaction with the hemoprotein produces an inhibition of N-demethylase activity.