Cytochrome P4501A1 genotypes and smoking- and hypertension-related ischemic stroke risk

This study aimed to determine whether the coding (A4889G) and noncoding region (T6235C) polymorphisms of the gene coding for cytochrome P4501A1 (CYP1A1), a xenobiotic-metabolizing enzyme responsible for the metabolism of carcinogenic polycyclic aromatic hydrocarbons, are involved in the pathogenesis of ischemic stroke in Turkish population. Study group consisted of 226 ischemic stroke patients and 113 controls. Genotypes were attained by allele-specific polymerase chain reaction (PCR) for A4889G and PCR/restriction fragment length polymorphism analysis for T6235C. Frequency of 6235C allele was significantly lower in patients (0.151) compared with controls (0.226, P = 0.015). Prevalence of hypertension and hypertension-associated ischemic stroke risk was lower for 6235C allele carriers. This allele decreased ischemic stroke risk twofold (adjusted odds ratio = 0.48, P = 0.005). There was almost no difference in 4889G allele frequencies in patients (0.445) and controls (0.425). However, prevalence of hypertension was lower in 4889G allele carriers when compared with the wild-type genotypes. In addition, risk of ischemic stroke for smoker and hypertensive individuals was lower when they have at least one 4889G allele. The present study demonstrated that CYP1A1 genetic variants contribute to interindividual variability in smoking- and hypertension-induced ischemic stroke risk.

Stimulation of aniline, p-nitrophenol and N-nitrosodimethylamine metabolism in kidney by pyridine pretreatment of rabbits

Pyridine has been shown to cause liver and kidney damage in animals and in humans. In a previous study we examined the effects of pyridine on rabbit liver and lung microsomal drug-metabolizing enzymes. In this study, in vivo i.p. administration of pyridine to rabbits caused a significant 3.4-fold increase in kidney N-nitrosodimethylamine (NDMA) N-demethylase activity as compared to the activity in control rabbits. The same treatment also significantly stimulated the activity of other cytochrome P4502E1-associated enzymes. The activities of p-nitrophenol hydroxylase and aniline 4-hydroxylase in kidney microsomes were increased 4.9-and 4.5-fold, respectively. Pyridine treatment increased the P450 content of the kidney 1.6-fold (P<0.05). SDS-PAGE of both kidney and liver microsomes of pyridine-treated rabbits showed a protein band of enhanced intensity at 51,000 Mr migrating in the region of cytochrome P4502E1. p-Aminophenol, a 4-hydroxylation product of aniline, has been shown to be nephrotoxic and NDMA, a procarcinogen, has been shown to be carcinogenic following bioactivation by NDMA N-demethylase in a number of tissues including the kidney. Since pyridine was shown to be nephrotoxic, it is expected that pyridine potentiates the toxic and/or carcinogenic effects of aniline, p-nitrophenol and NDMA through induction of their metabolism by the cytochrome P450-dependent drug-metabolizing enzymes.

Induction of N-nitrosodimethylamine metabolism in liver and lung by in vivo pyridine treatments of rabbits

N-Nitrosodimethylamine is a procarcinogen that is activated by cytochrome P450 dependent N-nitrosodimethylamine N-demethylase to labile alpha-carbon hydroxylated products further resulting in active methylating agents. In vivo intraperitoneal administration of pyridine to rabbits significantly increased N-nitrosodimethylamine N-demethylase activity by 6.9- and 5.2-fold in liver and lung microsomes, respectively. Although, p-nitrophenol hydroxylase and aniline 4-hydroxylase activities were markedly enhanced by pyridine treatment in liver about 4.4- and 5.8-fold, respectively, no change was observed in the activities of these enzymes in lung microsomes. Pyridine treatment also elevated P450 contents of liver and lung by 2.04- and 1.4-fold, respectively. SDS-PAGE of pyridine-induced liver microsomes revealed a protein band of enhanced intensity having Mr of 51,000 migrating in the region of cytochrome P4502E1. The results obtained in this study demonstrated for the first time, a significant 5.2-fold induction of NDMA N-demethylase activity in the rabbit lung over the controls. Pyridine is readily absorbed by inhalation and is a constituent of tobacco and tobacco smoke. Thus induction of NDMA N-demethylase suggests that in the lung, as in the liver, pyridine may stimulate the metabolic activation of this nitrosamine significantly.

The effect of arginine-428 mutation on modulation of activity of human liver flavin monooxygenase 3 (FMO3) by imipramine and chlorpromazine

This study was carried out to investigate the molecular basis for modulation of recombinant FMO3-catalyzed activity by the tricyclic antidepressants, imipramine and chlorpromazine. A mutant of human liver FMO3 (T428R) was formed by site-directed mutagenesis and characterized along with the native enzyme in order to elucidate a possible structure-function relationship. Functional properties of native and T428R human FMO3s were studied with methimazole as substrate. Both enzymes catalyzed the S-oxidation of methimazole with the same Km value. Imipramine modulated the activities of the native and T428R human FMO3s differently; the activity of the native FMO3 was increased at all concentrations, whereas the activity of the mutant enzyme was inhibited at concentrations above 300 microM. Chlorpromazine activated the native enzyme at all concentrations of methimazole but activated the mutant enzyme only at high substrate concentrations. The direction (activation or inhibition) and extend of modulation of FMO3 activity is not only dependent on the concentration of the modulator, it is also dependent on the substrate concentration. This study confirms our previous findings with FMO1 that position 428 is important in the interaction of the FMO with modulators.

Structure-function of cytochromes P450 and flavin-containing monooxygenases: implications for drug metabolism

This article is a report on a symposium held at Experimental Biology '98 in San Francisco, California. Recent developments in site-directed mutagenesis, computer-modeling, and mechanistic analysis of cytochromes P450 and flavin-containing monooxygenases are described. A unifying theme is the elaboration of general approaches for understanding and predicting the function of individual forms of these enzymes. A related goal is the production of soluble forms of mammalian cytochromes P450 for X-ray crystallography.

Modulation of human flavin-containing monooxygenase 3 activity by tricyclic antidepressants and other agents: importance of residue 428

Human flavin-containing monooxygenase 3 (FMO3) is subject to modulation by tricyclic antidepressants and other agents. Imipramine activates FMO3-catalyzed metabolism of methimazole at all substrate concentrations tested. This distinguishes FMO3 from rabbit FMO1 and FMO2, which are activated at high substrate concentration and inhibited at low substrate concentration, and pig FMO1, which is inhibited at all substrate concentrations. The response of FMO3 is also unique in that chlorpromazine is markedly more effective as a modulator than is imipramine. n-Octylamine, MgCl2, and HgCl2 all inhibit FMO3, the first two in a biphasic manner. Substitution of lysine for threonine at position 428 significantly alters the response of FMO3 to modulators without changing the kinetic parameters for the metabolism of the substrate. Activation by imipramine and chlorpromazine is reduced or abolished and inhibition, most obvious at low substrate concentrations, is observed. This is consistent with elimination of self-activation in the metabolism of imipramine. The mutation at 428 also eliminates the biphasic nature of the inhibition by n-octylamine and MgCl2, but does not alter the effect of HgCl2. Our findings show that the activity of FMO3 can be modulated by large drug molecules as well as short-chain amines and metal ions. This modulation can be markedly altered by changing a single amino acid in the enzyme.

Immunochemical and sub-structural characterization of sheep lung cytochrome P450LgM2

Sheep lung cytochrome P450LgM2 belonging to gene subfamily 2B, was obtained in highly purified form and antibodies against sheep lung cytochrome P450LgM2 were produced in rabbits by using the previously developed methods in our laboratory. Immunological and enzymatic studies showed that antibodies against lung cytochrome P450LgM2 inhibited benzphetamine N-demethylation, ethylmorphine N-demethylation and aniline 4-hydroxylation reactions in sheep lung microsomes about 99, 80 and 62%, respectively. Benzphetamine N-demethylation reaction in sheep lung microsomes was only catalyzed by cytochrome P450LgM2 isozyme while the other isozymes of P450 as well as P450LgM2 are also involved in the metabolism of ethylmorphine and aniline. Similar to lung microsomes, benzphetamine N-demethylase activity of the reconstituted systems containing purified sheep lung cytochrome P450LgM2 or phenobarbital (PB)-treated rabbit liver cytochrome P450LM2(2B4) was also inhibited by P450LgM2 antibodies about 95 and 82%, respectively. A 50% inhibitory effect of sheep P450LgM2 antibodies was also observed in ethylmorphine N-demethylase activity of the reconstituted system containing purified sheep lung cytochrome P450LgM2. SDS-PAGE peptide maps obtained following the partial proteolysis of purified sheep lung cytochrome P450LgM2 and PB-rabbit liver P450LM2(2B4) isozymes, using chymotrypsin and papain, were similar in general. However they showed some differences both qualitatively and quantitatively, suggesting that some differences exist among the amino acid sequences of sheep lung cytochrome P450LgM2 and rabbit liver cytochrome P4502B4.

Tissue- and species-dependent expression of sheep lung microsomal cytochrome P4502B(LgM2)

Expression of sheep lung microsomal cytochrome P4502B(LgM2) isozyme was determined in lung and liver preparations from rabbits, rats, sheep, cows and humans. Several tissues of sheep such as lung, liver, adrenal, kidney, ovary, intestine, muscle, spleen, pancreas and brain were also examined. Tissue homogenates were analyzed by Western-blotting with an antibody raised against purified sheep lung P4502B(LgM2) isozyme in rabbits. This isozyme was detected in lung samples from every species examined. Homologues of cytochrome P4502B(LgM2) were expressed in livers of sheep and rabbits but not in those of humans and cows. Although homologous forms of P4502B(LgM2) were not expressed in sheep brain, ovary, pancreas, spleen and muscle tissues, they were expressed in sheep adrenal, intestine and kidney.

Different influences of two fractions of lung cytochrome b5 on reconstituted lung benzphetamine N-demethylase system

Chromatography of lung microsomal cytochrome b5 obtained from DEAE-cellulose columns, yielded two distinct cytochrome b5 fractions. These cytochrome b5 fractions were further purified by Sephadex G-100 gel filtration chromatography. The specific cytochrome b5 content of fraction 1 and fraction 2 was found to be 16.5 and 16.4 nmol/mg protein respectively. Both fractions were free of cytochrome P-450, NADPH-cytochrome P-450 reductase and NADH-cytochrome b5 reductase activities. The effects of lung cytochrome b5 (fraction 1 and fraction 2) and liver cytochrome b5 on benzphetamine N-demethylase activity were examined. Four different reconstitution systems were used. Lung cytochrome b5 fraction 2 and liver cytochrome b5 stimulated N-demethylase activity in all four systems when b5:P-450 molar ratio was low, i.e. 0.25 or 0.5. Both cytochrome b5 samples inhibited N-demethylase activity when b5:P-450 ratio exceeded 1:1 molar ratio. In contrast lung cytochrome b5 fraction 1 stimulated N-demethylase activity in all four systems. Maximal enhancement of the activity was obtained when b5:P-450 ratio was 0.5. The greatest increase in N-demethylation activity was observed in the reconstitution system with the lowest concentration of cytochrome P-450 reductase, conditions which most closely resemble intact microsomes.

Stimulatory effects of lung cytochrome b5 on benzphetamine N-demethylation in a reconstituted system containing lung cytochrome P450LgM2

Cytochrome b5 was partially purified from sheep lung microsomes in the presence of detergents Emulgen 913 and cholate by three consecutive DEAE-cellulose and Sephadex G-100 gel filtration chromatographies. The specific content of cytochrome b5 was 16.5 nmol/mg protein and purified cytochrome b5 fractions were free of cytochrome P450, NADPH-cytochrome P450 reductase and NADH-cytochrome b5 reductase activities. The influences of increasing concentrations of lung cytochrome b5 on benzphetamine N-demethylation reactions were examined in four different reconstitution systems containing lung cytochrome P450LgM2, lung cytochrome P450 reductase and lipid. In each system concentration of reductase was doubled with respect to former system. In all systems cytochrome b5 stimulated benzphetamine N-demethylase activity especially when cytochrome b5 was present at 0.5:1 molar ratio with respect to cytochrome P450LgM2. Besides, the greatest fold of increase in benzphetamine N-demethylation activity due to addition of cytochrome b5 was observed in System 1 with the lowest concentration of reductase.

Purification and characterization of two forms of soluble NADH cytochrome b5 reductases from human erythrocytes

1. Two forms of soluble NADH cytochrome b5 reductase were purified from human erythrocytes. Two distinct fractions both having the NADH cytochrome b5 reductase activity eluted from the second DEAE-cellulose column were further purified by ultrafiltration and 5'-ADP-agarose affinity chromatography. 2. The final preparations were purified 9070- and 4808-fold, respectively, over hemolysate. Both reductases exhibited identical electrophoretic patterns when subjected to SDS-PAGE and apparent monomer Mr of each reductase was determined to be 32,000 +/- 1300. 3. Vmax values of reductase II for the various electron acceptors, namely, 2,6-dichlorophenolindophenol, ferricyanide and cytochrome c through cytochrome b5 were found to be 1.9, 1.8 and 2 times higher than those of reductase I. 4. Some differences were noted for reductase I and reductase II fractions. Their elution profiles from a second DEAE-cellulose column were quite different and that suggested that reductase II is more acidic than reductase I. Reductase II was found to be more sensitive to heat treatment than reductase I.

Stimulatory effects of benzene on rabbit liver and kidney microsomal cytochrome P-450 dependent drug metabolizing enzymes

Treatment of rabbits with benzene (880 mg/kg/day), s.c. for 3 consecutive days, caused 3.8- and 5.7-fold increases in aniline 4-hydroxylation rates of liver and kidney microsomes, respectively. Benzene treatment markedly enhanced hydroxylation rats of p-nitrophenol by liver and kidney by 7.2- and 4.2-fold, respectively. Both of these enzymes are associated with cytochrome P-450 LM3a. In contrast, the activity of benzphetamine N-demethylase, associated with P-450 LM2, was not altered significantly in either liver or kidney microsomes. Although the total cytochrome P-450 contents of liver and kidney microsomes were not altered significantly by the benzene treatment, in the case of liver microsomes, formation of a new cytochrome P-450 with an apparent Mr of 51,400 was observed on SDS-PAGE. On the other hand, in the kidney microsomes, the intensity of the bands corresponding to approximate Mr of 50,000 and 51,400 was markedly increased. The results of the present work, in combination with those of the previous work (Arinç et al. 1988), indicate the existence of tissue specificity in the induction of rabbit P-450 isozyme by benzene.

Electrophoretic, spectral, catalytic and immunochemical properties of highly purified cytochrome P-450 from sheep lung

1. Cytochrome P-450LgM2 was purified from sheep lung microsomes in the presence of detergents, Emulgen 913 and cholate. 2. The purification procedure involved the chromatography of the detergent solubilized microsomes on DEAE-cellulose and hydroxylapatite. 3. Cytochrome P-450LgM2 was further purified on second DEAE-cellulose and hydroxylapatite columns. 4. The specific content of the highly purified P-450LgM2 was 16-18 nmol P-450/mg protein and purified 164-fold. 5. The yield was 16% of the initial content in microsomes. 6. The SDS-polyacrylamide slab gel electrophoresis (PAGE) of the purified lung cytochrome P-450LgM2 showed one protein band having the monomer molecular weight of 49,500. 7. The absolute CO-difference spectrum of dithionate-reduced P-450LgM2 gave a peak at 451 nm. 8. When sheep lung cytochrome P-450LgM2 and P-450LM2 purified from liver of phenobarbital (PB)-induced rabbit were subjected to Western Blotting and visualized immunochemically with anti-P-450LM2, they showed identical mobilities. 9. P-450LgM2 was found to be very active in N-demethylation of benzphetamine in a reconstituted system containing purified sheep lung reductase and synthetic lipid. 10. Turnover numbers (min-1) for benzphetamine, aniline, ethylmorphine and p-nitrophenol were determined to be 273, 1.2, 15.5 and 1.05, respectively, in a reconstituted microsomal lung monooxygenase system. 11. Spectral, electrophoretic, biocatalytic and immunochemical properties of sheep lung P-450LgM2 were found to be similar to those of P-450 isozyme 2, purified from PB-treated rabbit liver and of rabbit lung microsomes.

Solubilization and partial purification of two forms of cytochrome P-450 from trout liver microsomes

Liver microsomal cytochrome P-450 content, NADPH-cytochrome c reductase, aniline 4-hydroxylase and ethylmorphine N-demethylase activities of rainbow trout, Salmo gairdneri, were found to be 0.16 (n = 10) nmol P-450/mg protein, 38 (n = 5) units/mg protein, 0.04 (n = 4) nmol p-aminophenol/min/mg protein and 0.174 (n = 3) nmol formaldehyde/min/mg protein, respectively. Trout liver cytochrome P-450 was solubilized by treatment of microsomes with sodium cholate. Chromatography on DEAE-cellulose column yielded two distinct cytochrome P-450 fractions from solubilized microsomes. Cytochrome P-450-I was eluted with Emulgen 913-containing buffer. Application of 0.08 M KCl in Emulgen 913-containing buffer to the DEAE-cellulose column eluted cytochrome P-450-II fraction. Cytochrome P-450-I was further purified on hydroxylapatite column. CO-difference spectrum of dithionite-reduced cytochrome P-450-I gave a peak at 449 nm while the similar spectrum of cytochrome P-450-II showed a maximum absorbance at 451 nm. Monomer molecular weights of cytochrome P-450-I and cytochrome P-450-II were determined by sodium dodecyl sulfate gel electrophoresis and found to be 56,000 and 48,500, respectively.