The effect of age on the oxidative metabolism of antipyrine by uninduced microsomal fractions of rat liver

The influence of ageing on the metabolism of antipyrine by different forms of cytochrome P-450 was studied in vitro, by measuring the formation of antipyrine metabolites by microsomes isolated from untreated rats, which were grouped into 5 different ages. Km, Vmax and Vmax/Km values were determined for the metabolites: 3-hydroxymethylantipyrine (HMA), norantipyrine (NORA) and 4-hydroxyantipyrine (OHA). Km values for all metabolites ranged from 2.0 to 5.0 mM and Vmax values from 1.84 to 3.66 nmol/min per mg. The Vmax/Km ratios varied from 0.65 to 1.73 microliters/min per mg. With respect to the Km values, a decrease for HMA from 12 to 24 months, no change for OHA and an increase for NORA from 3 to 30 months, was observed. The Vmax values showed an increase for HMA from 3 to 24 months, a decrease for OHA from 12 to 30 months and no change for NORA. The absolute Vmax/Km ratios for all metabolites showed no significant changes during ageing. However, regarding the relative Vmax/Km ratios, an increase for HMA from 3 to 24 months, a decrease for OHA from 12 to 35 months and also a decrease for NORA from 3 to 24 months, was observed. It is concluded that ageing influenced the Km, Vmax and Km/Vmax ratio of the different P-450 isoenzymes involved in antipyrine metabolism, in different ways. However, the age-related changes are modest (10-100%), especially for the Vmax/Km ratio.     

Effects of aging and caloric restriction on hepatic drug metabolizing enzymes in the Fischer 344 rat. I: The cytochrome P-450 dependent monooxygenase system

The effects of long-term caloric restriction on the hepatic cytochrome P-450 dependent monooxygenase system were investigated in the 22-month-old Fischer 344 rat. Caloric restriction decreased the age-related changes in hepatic testosterone metabolism, which are associated with demasculinization of the liver. Caloric restriction also increased hepatic microsomal testosterone 6 beta-hydroxylase, lauric acid 12-hydroxylase and 4-nitrophenol hydroxylase activities over corresponding values in both ad libitum fed 22-month and 60-day-old control male rats. This suggests that cytochrome P-450 isozymes, P-450 pcn1&2, P-452 and P450j may be induced by caloric restriction. Such changes in cytochrome P-450 isozyme profiles could result in altered carcinogen activation, radical formation or drug detoxication in the calorically restricted rat.     

Ethoxyresorufin O-deethylase activity in rat brain subcellular fractions

Non-polar, lipid-soluble drugs or xenobiotics are able to cross the blood-brain barrier. The brain cytochrome P-450-dependent monooxygenases may oxidize these molecules to more polar and somewhat hazardous metabolites responsible for neurotoxicity. In order to characterize the cytochrome P-450 dependent aryl hydrocarbon hydroxylase activity in brain subcellular fractions, we used 7-ethoxyresorufin as a substrate, as its O-deethylation reflects specifically the activity of the cytochrome P-450 isoform which metabolizes and is induced by polycyclic aromatic hydrocarbons. The results reported here show that this enzymatic activity occurs in both microsomal and mitochondrial fractions, and that the induction after 3-methylcholanthrene treatment remains limited, thus preventing the formation of high levels of harmful metabolites.     

Metabolic activation of aflatoxin B1 by liver tissue from male fischer F344 rats of various ages

The effect of aging on the ability of the liver to activate chemical procarcinogens was studied using 12-, 18-, and 27-month-old male Fischer F344 rats. The cytochrome P-450 content of the S9 and microsomal fractions of the liver decreased approximately 30% between 12 and 18 months of age. The structural conformation of cytochrome P-450 in microsomes from 12-, 18-, and 27-month-old rats was studied using electron-spin resonance spectroscopy. An age-related decrease in the amount of cytochrome P-450 ferric iron in the liver microsomes was observed. The conversion of the chemical procarcinogen aflatoxin B1 to mutagenic compounds by the S9 and microsomal fractions of liver was measured using the Salmonella typhimurium bioassay. A 40-50% decrease in the metabolic activation of aflatoxin B1 was observed between 12 and 18 months of age. However, the activation of aflatoxin B1 did not change after 18 months of age. The age-related decrease in the activation of aflatoxin B1 by liver appears to be due to a decrease in the metabolic activity of the mixed-function oxidase system.     

Cytochrome P-450 metabolic-intermediate complex formation with a series of diphenhydramine analogues

A series of diphenhydramine analogues have been studied with regard to their formation of a metabolic intermediate (MI) during their biotransformation in phenobarbital induced rat hepatic microsomes. The MI forms a complex with reduced cytochrome P-450. MI complexation of cytochrome P-450 may result in drug-drug interactions and/or in cumulation of the parent compound. The extent of MI complex formation could be correlated with the lipophilicity of the substrates in a parabolic manner. A hydrophobic pocket of limited dimensions in cytochrome P-450 for the N-alkyl substituent of the substrates can be assumed. Moreover our data indicate a role for the O-atom in the diphenhydramine analogues for the interaction with cytochrome P-450.     

NADPH-dependent and -independent loss of cytochrome P-450 in control and phenobarbital-induced rat hepatic microsomes incubated with carbon tetrachloride

Carbon tetrachloride-mediated loss of cytochrome P-450 has been compared in hepatic microsomes from untreated and phenobarbital-treated rats. At concentrations of carbon tetrachloride greater than 2.5 mM, a direct effect (i.e., NADPH- independent) on cytochrome P-450 was observed. This apparently arose from the "solvent" properties of carbon tetrachloride as this effect could be duplicated with the physically similar alkyl halide 1,2-dibromo-3-chloropropane. NADPH-dependent loss of cytochrome P-450 occurred at lower concentrations with maximal response occurring at 2.5-5.0 mM. Residual cytochrome P-450 at these concentrations was similar in untreated and phenobarbital-treated microsomes. Mixed-function oxidase activities in phenobarbital-treated microsomes were reduced to levels below those of uninduced controls. The 52-kDa polypeptide(s) in untreated microsomes and that specifically induced in phenobarbital-treated microsomes were susceptible to NADPH-dependent carbon tetrachloride incubation. These data suggest that the susceptibility of specific forms of cytochrome P-450 to carbon tetrachloride can be duplicated in in vitro incubation. Furthermore, data on the direct action of carbon tetrachloride suggest that this route of damage must be taken into consideration when concentrations of carbon tetrachloride of 2.5 mM or greater are used in in vitro incubation systems.     

Hepatic drug metabolism during development in food-restricted female Sprague-Dawley rats.

Hepatic drug metabolism was investigated in female Sprague-Dawley rats fed ad libitum (A) or a restricted diet (R) (implemented from age 1 month), at 1.5, 4.5 and 12 months to determine the short- and long-term effects of caloric restriction. Microsomal cytochrome P-450 content and NADPH cytochrome c reductase activity were not modified by age. While dietary restriction did not affect cytochrome P-450, it significantly increased NADPH cytochrome c reductase activity at all time periods when compared to corresponding A-fed groups. Aniline hydroxylase and aminopyrine N-demethylase activity tended to decrease with age in the A-fed groups but the differences did not prove to be statistically significant. A significant decrease of aminopyrine N-demethylase was observed with age in R rats. A significant reduction of aniline hydroxylase activity was noted in the R groups compared to age-matched A-fed controls. In contrast, aminopyrine N-demethylase activity increased significantly, but only at 1.5 months of age. Glutathione S-transferase activity was augmented between 1.5 and 4.5 months of age, and this was followed by a significant decrease at age 12 months in both A and R groups. Dietary restriction had no effect on this enzymatic activity. The microsomal cholesterol and phospholipid content as well as the cholesterol/phospholipid molar ratio changed significantly between 1.5 and 4.5 months of age but not between 4.5 and 12 months of age. These parameters were unaltered by dietary restriction. In conclusion, in the female Sprague-Dawley rat there are no statistically significant changes in hepatic microsomal components and drug metabolizing capacity between 1.5 and 12 months of age. Dietary restriction resulted in significant changes in enzymes related to drug metabolism which varied with the enzyme examined. In general, these changes were similar after short- or long-term dietary intervention.     

Effect of traditional Chinese herbal medicines on the pharmacokinetics of western drugs in Sprague-Dawley rats of different ages (II): Aminophylline-huan shao tan and aminophylline-pu chung yi chi tang.

The effect of Chinese herbal medicines (Huan Shao Tan and Pu Chung Yi Chi Tang) and western drugs (sodium phenobarbital and cimetidine) on the serum concentration and pharmacokinetic parameters of theophylline and cytochrome P-450 of Sprague-Dawley (SD) rats of three different ages were examined. The older rats without pretreatment with Chinese herbal medicines and western drugs exhibited higher serum theophylline concentration and lower pharmacokinetic parameters of theophylline than middle-aged and younger rats (P < 0.05), but there was no difference in cytochrome P-450 activity among the three different ages of rats. All rats when pretreated with sodium phenobarbital showed lower serum theophylline concentration and higher pharmacokinetics parameters of theophylline. Also, the activity of cytochrome P-450 was higher (P < 0.05). When cimetidine was pre-administered in SD rats of three age groups, all rats exhibited lower serum theophylline concentration and higher pharmacokinetics parameters (P < 0.05), but the activity of cytochrome P-450 remained unchanged (P > 0.05). The results were opposite to other studies, probably because the dose and dosing intervals were different. No single effect occurred on the younger and middle-aged rats after pretreatment with Huan Shao Tan and Pu Chung Yi Chi Tang: their serum theophylline concentration, pharmacokinetics parameters and cytochrome P-450 activity were the same as the control group. However, the older rats after pretreatment with Huan Shao Tan or Pu Chung Yi Chi Tang showed lower serum theophylline concentration and higher pharmacokinetics parameters than the younger and middle-aged rats pretreated with similar Chinese herbal medicines. This indicates that Huan Shao Tan and Pu Chung Yi Chi Tang may perhaps improve the elimination of theophylline in older rats. This might be attributed to the increase in hepatic blood flow or in liver volume, since the activity of cytochrome P-450 was not affected by the administration of Chinese herbal medicines.     

Role of dietary corn oil in the function of hepatic drug and carcinogen metabolizing enzymes of starved-refed rats: response to the mixed function oxidase inducer, 3-methylcholanthrene

Hepatic microsomes from rats starved 48 hours and refed diets containing zero, 3 or 20% corn oil metabolized benzo(a)pyrene, aniline and N-nitrosodimethylamine in proportion to the quantity of corn oil in the diet. No diet-related changes in apparent Km for these reactions were evident. The content of microsomal cytochrome P-450 was also clearly dependent upon the content of corn oil in the refed diets. When metabolism of these three substrates is expressed as product formed per unit of cytochrome P-450, the activities are least in microsomes from rats fed the 20% corn oil diet, suggesting that P-450 species responsible for metabolizing substrates other than these are enhanced preferentially. Cytosolic glutathione S-transferase activities are also increased with increasing corn oil in the diet. The administration of 3-MC increased cytochrome P-448 content of microsomes from all rats, regardless of diet, however highest content was present in microsomes from rats fed the 20% corn oil diet. Induction of benzo(a)pyrene hydroxylase was not influenced by dietary corn oil and, as anticipated, 3-MC caused significant repression of DMN N-demethylase in microsomes from rats fed the 20% corn oil diet. In like manner, 3-MC induced glutathione S-transferase only in cytosol from rats fed the fat-free diet.     

Effects of phospholipid-containing hepatoprotectors on cytochrome P-450-dependent antitoxic function of the liver during experimental toxic hepatitis

Phospholipid-containing hepatoprotectors essentiale and eplir inhibited conversion of cytochrome P-450 into cytochrome P-420 and restored aminopyrine N-demethylase and anilinen-hydroxylase activities of cytochrome P-450 in rats during acute hepatitis induced by CCl₄ and allyl alcohol. The polyphenol phytopreparation legalon did not prevent degradation of cytochrome P-450. Differences in the effects of hepatoprotectors on the impaired antitoxic function of the liver are probably associated with the abilities of essentiale and eplir to provide phospholipids for regeneration of endoplasmic reticulum membranes of hepatocytes. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 127, No. 4, pp. 392–394, April, 1999     

Effects of inhibition and induction of cytochrome P-450 isozymes on hyperoxic lung injury in rats.

Pulmonary oxygen toxicity most likely results from excessive production of reactive oxygen species. The role of the cytochromes P-450 in this process is controversial because these enzymes have been reported both to enhance hyperoxic lung injury and to protect from the damaging effects of 100% oxygen. We sought to further determine the role of the cytochromes P-450 in hyperoxic lung injury by inhibiting and inducing pulmonary cytochrome P-450 isozymes in rats. Treatment with the cytochrome P-450 inhibitor cimetidine or 8-methoxypsoralen did not improve survival or reduce lung edema in rats exposed to 100% oxygen. The activity of cytochrome P-450IIB1, the major pulmonary cytochrome P-450 isozyme in rats, was clearly inhibited by 8-methoxypsoralen. beta-Naphthoflavone (beta NF), a selective inducer of cytochrome P-450IA1, was administered in two-dose and five-dose regimens. The two-dose regimen produced significant and sustained induction of cytochrome P-450IA1 activity, but survival in these rats was not improved when exposed to 100% oxygen. In rats treated with five doses of beta NF, a small increase in survival time was found from 71.1 +/- 8.7 to 88.0 +/- 20.2 h; however, there was no difference in the induction of cytochrome P-450IA1 activity between this five-dose regimen and the two-dose regimen. The small improvement in survival after five doses of beta NF is thus unrelated to cytochrome P-450IA1 induction. We conclude that neither inhibition of cytochrome P-450IIB1 activity nor induction of cytochrome P-450IA1 activity protects adult rats against hyperoxic lung injury.     

Effect of endotoxin to differentially affect cytochrome P-450 monooxygenase activities of untreated rats and animals induced with phenobarbital or 3-methylcholanthrene

The effect of endotoxin in decreasing the cytochrome P-450-dependent metabolism of aniline, aminopyrine and ethoxycoumarin was examined in untreated rats, and in rats pretreated with either phenobarbital or 3-methylcholanthrene. Ethoxycoumarin metabolism was determined at two substrate concentrations (5 microM and 500 microM) to determine the effect of endotoxin on the high and low affinity enzyme activities. In untreated animals, endotoxin depressed both aniline and ethoxycoumarin metabolism by the high and low affinity enzymes by approximately 70%, but aminopyrine was decreased by only 47%. In phenobarbital pretreated rats, endotoxin decreased enzyme activities less than in untreated animals. Aniline metabolism and low affinity ethoxycoumarin metabolism were decreased by only 24%, and aminopyrine metabolism was decreased by 35%. The high affinity ethoxycoumarin metabolism was least affected, being decreased by only 12%. In 3-methycholanthrene pretreated rats, aniline and ethoxycoumarin (500 microM) metabolism were decreased by approximately 45%, but aminopyrine metabolism was only decreased by 20%. In these animals, endotoxin did not significantly affect the activity of ethoxycoumarin metabolism assayed with the low substrate concentration. Endotoxin decreased total cytochrome P-450 level of untreated rats by 32%, of phenobarbital pretreated rats by 39%, and in 3-methylcholanthrene pretreated animals the decrease was only 21%. Heme oxygenase activity of untreated animals was induced most by endotoxin administration and least in phenobarbital treated rats. The data suggest that endotoxin may differentially affect the various isozymes of cytochrome P-450 associated with the metabolism of aniline, aminopyrine and ethoxycoumarin. The results also suggest that the isozymes associated with these activities in untreated, phenobarbital or 3-methylcholanthrene pretreated rats may differ in their sensitivity to the effect of endotoxin.     

Immunohistochemical localization of cytochrome P-450 in rat brain

The presence of cytochrome P-450 in rat brain was studied by immunohistochemistry, using antibodies to cytochrome P-450 purified from livers of phenobarbital- or 3-methylcholanthrene-treated rats. Immunoreactive nerves were observed only in brain sections incubated with immunoglobulin-G to 3-methylcholanthrene-induced cytochrome P-450. This immunoreactivity was abolished by preabsorption of the antibody with highly purified rat liver cytochrome P-450c, the major cytochrome P-450 isozyme induced by 3-methylcholanthrene, but was not affected by other cytochrome P-450 isozymes induced by phenobarbital, isosafrole or pregnenolone-16-carbonitrile.

The most abundant concentration of nerve fibers with cytochrome P-450 immunoreactivity was observed in the globus pallidus. Immunoreactive fibers were also observed in the caudate putamen, amygdala, septum, ventromedial nucleus of the hypothalamus, medial forebrain bundle, ansa lenticularis, and ventromedial portion of the internal capsule and crus cerebri. Cell bodies with cytochrome P-450 immunoreactivity were observed in the caudate putamen and in the perifornical area of the hypothalamus. The cytochrome P-450 immunoreactive fibers in the globus pallidus and caudate putamen do not appear to emanate from cell bodies in the substantia nigra, since there was no reduction in the density of these fibers after unilateral stereotaxic electrolytic destruction of the substantia nigra (zona compacta and reticulata). Our data suggest that these striatal nerve processes are derived from cell bodies within the caudate putamen itself.

The present results indicate that rat brain contains a form of cytochrome P-450 with antigenic relatedness to the hepatic 3-methylcholanthrene-inducible cytochrome P-450c. This cytochrome P-450 isozyme was detected in brain areas which metabolize morphine and convert estradiol and estrone into catecholestrogens, which suggests an important role for this enzyme in the metabolism of both ex´ogenous and endogenous compounds in brain.     

Effects of ethanol on microsomal drug metabolism in aging female rats. I. Induction

The purpose of this study was to determine how aging affects the induction by ethanol or acetone of the hepatic microsomal monooxygenase system of female Fischer 344 rats. Young-adult, middle-aged and old rats (4, 14 and 25 months) were fed an ethanol-containing or control liquid diet for 15 days. Cytochrome P-450, cytochrome c reductase, aniline hydroxylase, nitrophenol hydroxylase, nitroanisole O-demethylase and benzphetamine N-demethylase activities were measured in hepatic microsomes. All of the drug metabolism activities except benzphetamine N-demethylase were 20-35% lower in old than in young-adult rats fed the control diet. In addition, the increase in drug metabolism produced by feeding the regular ethanol diet (36% of calories as ethanol) was 50-60% lower in the old rats. However, there was no difference in the magnitude of ethanol induction when ethanol intakes were matched. The effects of chronic acetone consumption (1.2g/day per kg body weight for 15 days) paralleled those of ethanol consumption, except that the extent of induction was greater with acetone. Acetone-induced levels of hepatic microsomal cytochrome P-450, nitrophenol hydroxylase, nitroanisole O-demethylase and aniline hydroxylase were similar in all three age groups. The results of this study indicate that induction of hepatic microsomal drug metabolism by ethanol or acetone is unaffected by the aging process.     

Immunohistochemical localization of cytochrome P-450 in the rat brain

Immunohistochemical localization of cytochrome P-450 in the brains of normal untreated male rats has been demonstrated with polyclonal antibodies raised against hepatic P-450C. These antibodies recognize P-450C and D, two closely related forms of P-450. We have identified P-450C,D immunoreactivity (P-450C,D-i) within both neurons and glial cells. Neuronal cell bodies showing P-450C,D-i were present in several forebrain areas, including the olfactory bulb, the caudate nucleus, the cingulate, the frontal and the entorhinal cortex, the hippocampus and the thalamus. In the brainstem, P-450C,D-i neurons were detected in the substantia nigra, the nucleus locus coeruleus, raphe nucleus, reticular formation and the trigeminal nerve nucleus and the dorsal motor nucleus of the vagus. Glial cells containing P-450C,D-i were situated within myelinated fiber tracts in the forebrain (e.g. the corpus callosum, the internal capsule) and the brainstem (e.g. the superior cerebellar peduncle, the medial longitudinal fasciculus). Some, but not all, of the P-450C,D-i glial cells also stained with an antibody against glial fibrillary acidic protein. The present findings represent the first immunohistochemical demonstration of cytochrome P-450 in the brain and suggest that cytochrome P-450C,D is constitutive in neurons and glial cells of most brain regions throughout the rat neuraxis.     

Drug metabolism in rats with arthritis induced by 6-sulfanilamidoindazole (6-SAI)

Hexobarbital sleeping time was prolonged and ethylmorphine N-demethylation was inhibited after a single dosage or seven administrations of 6-SAI to old rats. These effects were independent of the development of arthritis. Changes in cytochrome P-450 concentration after 6-SAI treatment were insignificant and thus not responsible for the decrease in drug metabolism.In vitro 6-SAI inhibited ethylmorphine N-demethylation; the inhibition was of a mixed type. 6-SAI bound to cytochrome P-450 and induced a type II spectrum. The magnitude of hexobarbital-induced type I spectral changes was diminished by 6-SAI.It is concluded that 6-SAI inhibits cytochrome P-450-dependent drug metabolism by binding to cytochrome P-450.     

Endogenous and exogenous factors modifying the activity of human liver cytochrome P-450 enzymes.

Cytochrome P-450 dependent monooxygenases play a dual role for xenobiotic metabolism. On one hand they initiate the primary rate limiting step for the elimination of a bulk of drugs and organic chemicals. On the other hand they catalyze the formation of toxic metabolites from chemical carcinogens and many other toxic chemicals. Numerous studies have shown that their activity in animals is subject to the influence of various modifying factors, such as strain, species, sex, age, diurnal rhythm and the effect of enzyme inducers. Less is known about the influence of these factors on human cytochrome P-450 enzymes. Here we report the results of an extended study on human liver cytochrome P-450 performed with liver biopsies of 178 individuals taken for diagnostic purposes. The enzymatic activity was determined by the aldrin epoxidase assay indicating a variety of enzymes inducible by phenobarbital and by glucocorticoid and androgenic hormones. The frequency histogram of individual aldrin epoxidase activities showed a unimodal distribution and a variation factor of 100 between maximal and minimal activity. Individuals with severe liver diseases, such as cirrhosis and fatty liver, exhibited a 50% loss of enzyme activity. Age and sex did not significantly influence the enzyme activity. No significant correlation was observable between the rate of aldrin epoxidation and debrisoquine 4-hydroxylation, a prototype of a genetically controlled cytochrome P-450 reaction. We assume that the broad interindividual variation of epoxidase activities is more likely due to the influence of exogenous and endogenous inducers rather than to a genetic polymorphism.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of enzyme imprinting of liver microsomal monooxygenases upon lifespan of rats.

Effect of the enzyme imprinting by phenobarbital upon alterations of hepatic microsomal monooxygenase activities and lifespan of Wistar rats has been studied. Phenobarbital-sodium (3.5 mg/100 g body weight per day, i.p.) was injected during 1-3 days after birth. This resulted in the enzyme imprinting of the liver microsomal monooxygenases, however, this effect being observed in female but not male rats. In the phenobarbital treated female rats of different age the duration of sleeping time was significantly lower than that in control animals, whereas it did not differ substantially in male rats. The cytochrome P-450 content increased by 34.5% in phenobarbital treated female rats in the age of 12 months in comparison with control animals. A mean lifespan of experimental female rats increased by 17.5% compared to the level of control animals and did not change in male rats. The analysis of survival of animals in Gompertz equation coordinates showed that enzyme imprinting by phenobarbital caused changes in the mortality patterns at different stages of ontogenesis in experimental female but not male rats. An inverse correlation was found between the duration of pentobarbital sleeping time and lifespan of female and male rats.     

Resistance of different forms of cytochrome P-450 of rat liver to factors destabilizing the microsomal membrane

The effect of factors destabilizing the membrane of the liver microsomes on the spectral properties of cytochrome P-450 (P-448) was investigated in intact rats and rats receiving phenobarbital (PB) or 3-methylcholanthrene (MC). Considerable resistance of microsomes induced by PB and MC to enzymic and nonenzymic peroxidation of polyunsaturated fatty acids of membrane phospholipids was discovered. A clear difference was shown in the sensitivity of cytochrome P-448 and cytochrome P-450 of intact rats and rats receiving PB to in vitro treatment with sodium deoxycholate. The results indicate structural changes in the microsomal membrane during induction by PB and MC, which are two different types of inducers of the monooxygenases of the liver.Institute of Clinical and Experimental Medicine, Siberian Branch, Academy of Medical Sciences of the USSR, Novosibirsk. (Presented by Academician of the Academy of Medical Sciences of the USSR V. P. Kaznacheev.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 83, No. 5, pp. 553–555, May, 1977.     

The effects of cimetidine,ranitidine and famotidine on rat hepatic microsomal cytochrome P-450 activities

It has been questioned whether the interaction of H₂-antagonists with cytochrome P-450 that is observedin vitro is also relevant for thein vivo situation. Until now the possibility that cytochrome P-450 may function with different modes of action has been neglected in this respect. We studied the effect of cimetidine, ranitidine and famotidine on the monoxygenase, the oxidase and the peroxidase action of cytochrome P-450. Biotransformation catalyzed by the monoxygenase and oxidase action of cytochrome P-450 was affected by cimetidine (probably via its ligand interaction with cytochrome P-450), whereas metabolism by the peroxidase mode of action of cytochrome P-450 was hardly influenced. Ranitidine and famotidine (both pharmacodynamically more potent than cimetidine) only slightly affected cytochrome P-450 activities.