Rat liver oxidative N-dealkylase and N-oxidase activities as a function of animal age

Developmental and tissue-specific expression of human flavin-containing monooxygenases 1 and 3

Substantial changes occur in drug and toxicant disposition during early life stages that can impact therapeutic efficacy and adverse reactions to drugs and toxicants. Of the many parameters involved, alterations in drug metabolism are of major importance. Although the cytochrome P450-dependent monooxygenases are accepted as playing a substantial role in drug and toxicant metabolism, the flavin-containing monooxygenases (FMOs) also have an important role. Apparently unique to the human, FMO3 is the most abundant FMO family member in the adult human liver, whereas FMO1 dominates in most animal models. However, early studies documented that FMO1 is the most abundant FMO enzyme in the human fetal liver, whereas FMO3 is essentially absent. This review focuses on recent studies characterising human FMO ontogeny and, in particular, the 'switch' in hepatic FMO enzyme expression. Because it is so closely related, tissue-specific expression patterns also are examined. Finally, a summary of what is known in animal models is presented as a point of comparison.

Physiological factors affecting the expression of FMO1 and FMO3 in the rat liver and kidney

FMO1 and FMO3, the main FMOs described in the rat, are highly expressed in the liver and the kidney. The age, from 3 to 11 weeks, and gender-dependent expression of FMO1 and FMO3 in the rat liver and kidney were investigated. Based on the enzyme activities, protein levels and mRNA levels, this study demonstrates an important increase in the expression of the FMO3 in the liver of male rats during a period that corresponds to the acquisition of the sexual maturity. Rat liver FMO1 remains unchanged during this period of observation. The evolutions of both isoforms in the kidney of the male rat are similar to those observed in the liver. On the contrary, the important decrease in the total flavin-containing monooxygenase (FMO) activity observed in the liver of female rat is linked to a considerable decrease in the FMO1-dependent activity, FMO1 protein and FMO1 mRNA levels as a function of age. The expression of the FMO3 in the liver does not seem to be affected by the age of the female rat. Inversely, the expression of FMO1 in the female rat kidneys does not seem to be modified as a function of age while the expression of FMO3 is strongly increased.

Developmental regulation of flavin-containing monooxygenase form 1 in the liver and kidney of fetal and neonatal rabbits

Flavin-containing monooxygenases (FMOs) comprise a multi-gene family and catalyze the oxygenation of soft nucleophilic sulfur, nitrogen, phosphorus, and selenium in xenobiotics. Previous studies have demonstrated that FMO is regulated developmentally and by the administration of certain steroid hormones. This study examined the expression of FMO form 1 in the livers and kidneys of fetal and neonatal rabbits, from day 25 of gestation through 3 weeks of age, by assaying FMO1 mRNA and protein levels, as well as catalytic activity. FMO1 mRNA and protein expression and FMO catalytic activity were present in fetal livers at the earliest time point measured (day 25 of gestation), although at levels approximately 10% of that found in adult livers. Hepatic FMO1 mRNA levels increased during and after gestation; levels were not significantly different from those measured in adult male livers. FMO1 protein content and activity rose rapidly after birth to reach 70-80% of adult levels by 3 weeks of age. The expression of FMO1 in fetal and neonatal kidneys was markedly lower than in liver. FMO1 mRNA levels never averaged more than 3.4% of adult male liver levels, but did not differ from adult kidney levels at any of the points measured. Protein levels and enzyme activity rose significantly after birth to approximately 30% of the level in adult kidneys by 3 weeks of age. The early developmental appearance of FMO1 suggests a possible role in the metabolism of xenobiotics through transplacental or lactational exposures.

Hormonal regulation of microsomal flavin-containing monooxygenase activity by sex steroids and growth hormone in co-cultured adult male rat hepatocytes

To investigate the hormonal control of the expression of flavin-containing monooxygenase (FMO; EC 1.14.13.8) under defined in vitro conditions, adult male rat hepatocytes were isolated by collagenase perfusion and co-cultured with rat liver epithelial cells of primitive biliary origin. The direct effect of 17beta-estradiol, testosterone, 5alpha-dihydrotestosterone (5alpha-DHT) and human growth hormone (hGH) on FMO activity was studied using this in vitro model. Optimal, non-cytotoxic hormonal concentrations were determined by measuring the lactate dehydrogenase (LDH) index. In addition, the microsomal protein content of the cultured hepatocytes was determined as a function of culture time. The female sex hormone 17beta-estradiol caused a significant decrease in FMO as a function of culture time. After 14 days of exposure, FMO activity decreased by 56%. Neither of the male sex hormones or human growth hormone had an effect on FMO activity. These results in co-cultured male rat hepatocytes support in vivo observation that 17beta-estradiol is a potent hormone involved in the negative regulation of the expression of FMO in male rat liver.

Species differences in the stereoselectivity of N-oxygenation of N-ethyl-N-methylaniline in vitro

The prochiral tertiary amine N-ethyl-N-methylaniline (EMA) is known to be stereoselectively N-oxygenated in the presence of hepatic microsomal preparations. This biotransformation is thought to be mediated predominantly by the flavin-containing monooxygenase (FMO) enzyme system. In order to characterise this reaction further, the in vitro metabolism of EMA in the presence of hepatic microsomal preparations derived from a number of laboratory species has been examined. EMA N-oxide formation was stereoselective with respect to the (-)-S-enantiomer in the presence of microsomal preparations from all species examined, with the degree of selectivity decreasing in the order of rabbit > rat approximately LACA mouse approximately DBA/2Ha mouse > guinea-pig > dog. The enantiomeric composition of the metabolically derived EMA N-oxide appeared to be determined solely by the differential rate of formation of the two enantiomers as opposed to any differences in affinities for the substrate in its pro-R and pro-S conformations. The use of enzyme inhibitors, activators and inducers indicated that EMA N-oxide formation was predominantly mediated by FMO in the presence of rabbit hepatic microsomes and that these agents did not generally affect the stereochemical outcome of the biotransformation.

Developmental regulation of flavin-containing monooxygenase (FMO) isoforms 1 and 2 in pregnant rabbit

Mammalian flavin-containing monooxygenase (FMO, EC 1.14.13.8) metabolizes a vast number of structurally diverse xenobiotics containing a soft-nucleophile, typically a nitrogen or sulfur. FMO is not inducible by the classical cytochrome P450 (CYP) inducers, such as phenobarbital, polycyclic aromatic hydrocarbons, ethanol or macrolide antibiotics. Evidence does exist from a number of laboratories, however, for developmental and hormonal regulation of FMO. Our laboratory has confirmed previous observations of enhanced FMO activity during mid- and late-gestation in maternal rabbit lung and have demonstrated that this response is due to increased protein and catalytic activity associated with FMO2. The time course of expression of FMO2 during mid- and late-gestation correlates to plasma peaks of progesterone or cortisol. FMO2 also peaks at parturition in maternal kidney, coincident with plasma cortisol levels. FMO2 is induced by s.c. administration of either progesterone or dexamethasone in lung, or by dexamethasone in kidney. Correlation of plasma progesterone or cortisol levels during gestation and postpartum support a role for progesterone, but not cortisol in regulation of FMO2 in maternal rabbit lung. The levels of FMO1 also appear to be increased during mid- and late-gestation in liver. FMO1 in liver may also be regulated during gestation by progesterone or glucocorticoids as administration of these steroids enhanced FMO1 mRNA levels 4-fold.

Immunological characterization of flavin-containing monooxygenases from the liver of rainbow trout (Oncorhynchus mykiss): sexual- and age-dependent differences and the effect of trimethylamine on enzyme regulation

Polyclonal antibodies raised against flavin-containing monooxygenase (FMO) enzymes purified from pig liver and rabbit lung were used in conjunction with N,N-dimethylaniline (DMA) N-oxidase to better characterize FMO from the liver of rainbow trout (Oncorhynchus mykiss). Two proteins reacted with polyclonal antibodies raised against pig liver FMO (PL-1 and PL-2) and anti-rabbit lung FMO (RL-1 and RL-2). Although there was no difference in DMA N-oxidase observed between sexually mature male and female trout liver microsomes, RL-2 and PL-2 were significantly less than RL-1 and PL-1, respectively, in sexually mature females. FMO activity and protein content increased as fish aged. DMA oxidase and FMO isozymes were unaltered after pretreatment with the endogenous substrate trimethylamine. Since antibodies to the purified mammalian enzymes react with proteins of similar MW in trout, some forms of FMO appear to be structurally conserved through evolution.

The flavin-containing monooxygenase of mouse kidney. A comparison with the liver enzyme

Flavin-containing monooxygenase (FMO; EC 1.14.13.8) was purified from mouse kidney microsomes and compared to that isolated from mouse liver microsomes. The purified enzymes from kidney and liver appeared as a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent molecular weight of 58,000 daltons. On wide range (pH 3.5 to 9.0) isoelectric focusing, FMOs from kidney and liver resolved as a single band with an isoelectric point of 8.2. The enzymes from both kidney and liver have a pH optimum of 9.2. Thiobenzamide-S-oxidation catalyzed by both enzymes was sensitive to inhibition by the competitive inhibitors thiourea and methimazole. At an n-octylamine concentration of 3 mM, thiobenzamide-S-oxidation by the kidney FMO was increased by 122% and that by the liver FMO by 148%. Km and Vmax values were determined and compared between the two tissue enzymes for xenobiotic substrates containing nucleophilic nitrogen, sulfur or phosphorus atoms. In general, for most FMO substrates, Km and Vmax values were similar between kidney and liver FMO with only a few exceptions. The Km and Vmax values for fenthion for kidney were only half of those observed for liver FMO. Fonofos was unusual in having a low Km as well as a low Vmax for both tissue enzymes. Anti-sera developed to the FMO purified from kidney and liver showed cross-reactivity with each purified enzyme as well as with a protein with the same molecular weight as the purified FMO present in both kidney and liver microsomes. These bands showed equal intensity based on an equivalent amount of protein. Analysis of kidney and liver FMO by proteolytic digestion followed by visualization of peptides by silver staining or immunoblotting showed only minor differences between the enzymes of the two tissues. The amino acid composition of both mouse kidney and liver FMO was low in methionine and histidine and rich in aspartate/asparagine, glutamate/glutamine, leucine, valine and glycine. Edman degradation of the purified mouse kidney and liver FMO provided a single amino acid sequence of the NH2-terminus. This sequence matched exactly with the cDNA-deduced sequence reported for the pig and rabbit liver beginning with the fifth amino acid and contained the highly conserved FAD-binding domain Gly-X-Gly-X-X-Gly, commonly found in a number of other FAD-binding proteins. These studies indicate that the renal and hepatic forms of FMO from mouse are similar enzymes that are immunologically related and show only a few minor differences.

Distinct pulmonary and hepatic forms of flavin-containing monooxygenase in sheep

1. Flavin-containing monooxygenase (FMO) in pulmonary and hepatic microsomes from sheep was analyzed by western blotting by probing with antibodies raised against FMO purified from rabbit lung and pig liver. 2. Pulmonary microsomes from sheep contain a single major protein which cross-reacts with the antibody to rabbit lung FMO, but no band can be observed when probed with the antibody to the pig liver enzyme. Likewise, sheep liver microsomes contain a protein which cross-reacts with the antibody to pig liver FMO, but no significant staining is observed following incubation with antibody to the lung enzyme. 3. Sheep pulmonary and hepatic microsomal FMO also display a difference in activity toward chlorpromazine and n-dodecylamine. 4. Preliminary evidence suggests that sheep FMO may be induced (liver) or repressed (lung) during pregnancy. 5. Sheep are similar to rodents (rat, mouse, guinea pig, hamster and rabbit) in having distinct forms of pulmonary and hepatic FMO. The immunochemical and catalytic difference between sheep liver and lung FMO is similar to that of rabbit.

The metabolism of N-benzyl-4-substituted anilines: factors influencing in vitro C- and N-oxidation

N-Benzyl-4-substituted anilines are metabolized by N-debenzylation, aniline-ring hydroxylation and N-oxidation. Factors affecting metabolism of these secondary anilines have been studied in vitro. Enzymes responsible for C- and N-oxidation are present mainly in the liver, but also occur in lung and kidney and are localized in the endoplasmic reticulum. Quantitative and qualitative species differences were observed in microsomal N-benzylaniline metabolism. In hamsters and mice N-debenzylation is the major route of metabolism. In guinea pig and rabbit, ring hydroxylation is the major pathway whereas the rat uses both pathways to equal extents. The hamster has particularly high N-oxidase activity. A sex difference in microsomal N-benzylaniline metabolism is evident in rats but not mice. Kinetic constants for C- and N-oxidation of N-benzylanilines are reported.

The metabolism of aromatic amines

Aromatic amines are of general interest in drug metabolism and some are a health hazard, particularly as bladder carcinogens. Conditions for the biological ring- and N-oxidation of aniline and its derivatives are reviewed. The metabolism of 2-naphthylamine and aminobiphenyls and the involvement of metabolites of aromatic amines in bladder cancer is discussed.

Microsomal N- and C-oxidation of 4-substituted N-benzylanilines

Using direct specific analytical techniques microsomal metabolism of N-benzyl-4-substituted anilines has been investigated and found to include both N- and C-oxidation. N-Debenzylation was observed with all substrates and species examined. N-Oxidation usually yielded aryl nitrones, although the N-hydroxy derivative of the 4-chloro-substituted substrate was identified in some species. This is the first direct evidence of microsomal N-hydroxylation of a secondary aniline. The metabolic formation of amides from these secondary amines was observed and is believed to be a novel class of metabolite for these substrates.

The metabolism of 4-substituted-N-ethyl-N-methyl-anilines. II. Some factors influencing alpha-C- and N-oxidation

A g.l.c. method for the quantification of N-demethylated and/or N-de-ethylated (alpha-C-oxidation) and N-oxidation products of several 4-substituted-N-ethyl-N-methylanilines is described. Factors affecting the metabolism of these tertiary anilines in vitro have been studied and conditions which allow maximal metabolism established. The alpha-C- and N-oxidase activity was detected principally in the liver, lung, kidney and bladder microsomal fraction. A species difference in the extent of metabolism was evident, the order of activity not being the same for alpha-C- as that for N-oxidation. The pKa but not log P values determined for the teritary anilines appear to influence the apparent Km for both alpha-C- and N-oxidation.

Androgenic suppression of mouse hepatic FAD-containing monooxygenase activity

Sex-related differences in the activity of hepatic FAD-containing monooxygenase (FAD-M) were found in C3H/St mice. Adult female mice had enzyme activities nearly two-fold greater than male mice and these differences, which were absent in sexually immature mice, became apparent at the onset of puberty. The sex differences in hepatic FAD-M appeared to be mediated through the suppressive effect of testosterone; castration of male mice enhanced enzyme activity, while androgenic replacement returned activities to control levels. Testosterone's suppressive effect was found to be relatively specific for hepatic FAD-M. Treatment of castrated male mice with both the anti-androgen flutamide and testosterone returned enzyme activity to control levels, suggesting that testosterone's regulation of hepatic microsomal FAD-M is receptor-mediated. Female gonadectomy had no effect on this enzyme's activity.

Studies on the metabolism of dimethylnitrosamine in vitro by rat-liver preparations. III. Effect of cobaltous chloride treatment

1. The effects of CoCl2 administration to rats on xenobiotic metabolism, dimethylnitrosamine (DMN) metabolism to formaldehyde and methanol, and monoamine oxidase (MAO) enzyme activities in hepatic subcellular fractions have been studied. 2. CoCl2 treatment markedly decreased hepatic mixed-function oxidase enzyme activities and microsomal cytochrome P-450 content. In contrast, the N-oxidation of N, N-dimethylaniline and the activity of microsomal NADPH-cytochrome c reductase was unaffected. 3. The metabolism of DMN to formaldehyde by postmitochondrial supernatant fractions was decreased at substrate concn. of 0 . 5, 5 and 50 mM by CoCl2 treatment but the metabolism of 5 and 50 mM DMN to methanol was affected less. 4. CoCl2 had little effect on MAO activities in whole homogenates, but microsomal MAO activities were markedly inhibited. 5. The inhibition of microsomal MAO indicates that CoCl2 is not a specific inhibitor of cytochrome P-450-dependent biotransformations and consequently the inhibition of DMN metabolism is not evidence of a wholly cytochrome P-450-dependent process.

Studies on the metabolism of dimethylnitrosamine in vitro by rat-liver preparations. II. Inhibition by substrates and inhibitors of monoamine oxidase

1. The metabolism of dimethylnitrosamine (DMN) to formaldehyde by rat-hepatic postmitochondrial supernatant fractions has been compared with the activities of several cytochrome P-450-dependent mixed-function oxidase enzymes and the Ziegler mixed-function amine oxidase enzyme (EC 1.14.13.8). 2. A variety of monoamine oxidase (MAO, EC 1.4.3.4) inhibitors of diverse chemical structure inhibited the metabolism of DMN. In parallel studies a number of MAO substrates, but not their deaminated products, also inhibited DMN metabolism, whereas substrates of diamine oxidase were ineffective. 3. At concentrations which inhibited DMN metabolism several MAO substrates and inhibitors did not inhibit the N-oxidation of N, N-dimethylaniline and an inhibitor and an activator of the Ziegler enzyme had no corresponding effect on DMN metabolism. 4. The metabolism of DMN and a number of MAO enzyme activities were stable to storage under conditions where mixed-function oxidase enzymes were not. 5. These results are consistent with the suggestion that DMN may, at least in part, be metabolized by hepatic enzyme(s) not dependent on cytochrome P-450 and that a microsomal amine oxidase enzyme, unrelated to the Ziegler enzyme, may be involved in the hepatic degradation of this nitrosamine. The present data does, however, suggest a role for microsomal NADPH-cytochrome c reductase in hepatic DMN metabolism.

The in vitro metabolism of N,N-dimethylaniline by guinea pig and rabbit tissue preparations

A study of the in vitro metabolism of N,N-dimethylaniline using guinea pig and rabbit preparations and GLC techniques has confirmed N-demethylation and N-oxidation and established ring hydroxylation as metabolic routes. Whereas N-demethylation and N-oxidation are major routes of metabolism, ring hydroxylation is a comparatively minor pathway. Like N-demethylase and N-oxidase, the 4-hydroxylase has been shown to be a microsomal enzyme. The major ring hydroxylated product of N,N-dimethylaniline is N,N-dimethyl-4-aminophenol; N-methyl-4-aminophenol is subsequently formed. The apparent Km and V max values for N-demethylation, N-oxidation and 4-hydroxylation, are presented for both the guinea pig and the rabbit.

Some factors involved in the N-oxidation of 3-substituted pyridines by microsomal preparations in vitro

1. Pyridine-N-oxides have been identified as metabolites in vitro of several 3-substituted pyridines. 2. Factors affecting the metabolism of these pyridines in vitro have been studied, and conditions which give the most metabolism have been established. 3. 'Pyridine-N-oxidase' activity resides mainly in the hepatic and pulmonary microsomal fractions. 4. A species difference was evident with 'pyridine-N-oxidase' activity decreasing in the order hamster, rabbit, mouse, guinea-pig and rat. 5. A sex difference in 'pyridine-N-oxidase' activity was also established in rats and mice. 6. The appropriate kinetic factors, Km and Vmax for the N-oxidation of pyridine, 3-methylpyridine and 3-chloropyridine are reported.

Metabolism of norcocaine, N-hydroxy norcocaine and cocaine-N-oxide in the rat

1. The metabolism of [3H]norcocaine, N-hydroxy[3H]norcocaine and cocaine-N-oxide has been investigated in rats after i.v. injection. 2. The biological t 1/2 of norcocaine (dose 2 mg/kg i.v.) in plasma, liver and brain were 0.4, 1.6, 0.5 h, respectively and the compound was not detectable in the central nervous system 6 h after injection. The % dose of norcocaine excreted unchanged in urine and faeces in 96 h were 0.7 and 1.0, respectively. Benzoylnorecgonine, norecgonine, norecgonine methyl ester and an unidentified compound were excreted in urine. 3. The biological t 1/2 of N-hydroxynorcocaine (5 mg/kg i.v.) in brain and plasma were 0.3, 1.6 h respectively and only 1.3 and 1.6% of dose were excreted unchanged in urine and faeces in 96 h. N-Hydroxybenzoylnorecgonine and N-hydroxynorecgonine methyl ester were the major urinary metabolites. N-hydroxynorcocaine was not metabolized to norcocaine in vitro by liver microsomes. Doses of greater than 7.5 mg/kg i.v. resulted in death of rats by cardiorespiratory arrest. 4. Cocaine-N-oxide (50 mg/kg i.v.) yielded ecgonine-N-oxide methyl ester as its major metabolite; other minor metabolites were cocaine (0.5%), norcocaine (1%), benzoylecgonine, ecgonine, ecgonine-N-oxide, along with minor amounts of unmetabolized compound. Lethality of cocaine-N-oxide (100 mg/kg i.v.) was possibly due to metabolism to norcocaine and cocaine.

Isolation and identification of morphine n-oxide alpha- and beta-dihydromorphines, beta- or gamma-isomorphine, and hydroxylated morphine as morphine metabolites in several mammalian species

New morphine metabolites in the urine of guinea pigs, rats, rabbits, cats, monkeys, and humans were isolated with column chromatography, solvent extraction, and TLC and identified with TLC, GLC, and GLC-mass spectrometry. In addition to the known morphine metabolites, morphine N-oxide was isolated from the urine of guinea pigs, and alpha- and beta-dihydromorphines were isolated or detected in the urine of guinea pigs, rats, and rabbits. Monohydroxymorphine was identified tentatively in the urine of guinea pigs, rats, rabbits, and cats. Dihydroxymorphine was identified tentatively in the urine of guinea pigs, rats, and possibly, rabbits. Finally, beta- or gamma-isomorphine was identified tentatively in the urine of guinea pigs. The newly described morphine metabolites may be involved in some long lasting pharmacological effects of morphine.

Role of mixed-function amine oxidases in N-hydroxylation of 2-acetamidofluorene by hamster liver microsomal preparations

Pretreatment of hamsters with 3-methylcholanthrene (100mg/kg body wt.) 24h before death did not appreciably change the extent of N-oxide formation when hepatic microsomal preparations were incubated with NN-dimethylaniline as substrate. In contrast, the N-hydroxylation of 2-acetamidofluorene was increased severalfold in hepatic microsomal preparations from pretreated animals. Under these conditions there were no appreciable changes in cytochrome P-450 content and NADPH-cytochrome c reductase activity. On the basis of these comparative data, it is suggested that amine oxidase is not involved in N-hydroxylation of 2-acetamidofluorene.