Destruction of cytochrome P-450 by ethylene. Structure of the resulting prosthetic heme adduct

Structure of the Green Heme Isolated from Allylbenzene-Modified Chloroperoxidase: A Novel Heme Architecture Implicating the Mechanisms of CPO Inactivation and Epoxidation

The chemical identification of the modified heme (the green heme) during chloroperoxidase catalyzed epoxidation of allylbenzene remains unestablished due to its high instability within the protein matrix, the absence of paramagnetically shifted signals, and the difficulty in obtaining crystals of the modified enzyme. We have established the unambiguous structure of the modified prosthetic heme group, which was extracted from the protein matrix using ²D NMR spectroscopy and LC-MS spectrometry. The modified heme was isolated as a µ-oxo dimer that can be quantitatively converted to the corresponding monomer. The depolymerized green heme displayed characteristic NMR signatures of iron porphyrin complexes, but no Nuclear Overhauser Effect was observable to assist in signal assignment. An alternative strategy was applied by removing the iron center of the green heme, resulting in a stable demetallated green porphyrin species. Complete assignment of all the NMR resonances in the demetallated green heme allowed us to establish the molecular architecture of the modified species as a novel N-alkylated heme. Decisive space correlations between the propyl protons of allylbenzene and the γ meso proton coupled with clear dipolar connectivities between the propyl-2H of the substrate and the β proton in the side chain of the propionic acid at carbon-6 of the porphyrin ring, clearly indicate that allylbenzene was covalently attached to the nitrogen atom of the pyrrole ring III of the prosthetic heme. In this study, the mechanism of green CPO formation and its relation to CPO catalyzed chiral transformations are also discussed. It is concluded that the double-phenyl clamp formed by two phenylalanine residues at the distal heme pocket plays a critical role in fine-tuning substrate orientation that determines the outcome of CPO catalyzed epoxidation of substituted styrenes.

Computationally designed tandem direct selective oxidation using molecular oxygen as oxidant without coreductant

A newly designed two-step selective oxidation process was computationally tested for propene epoxidation using molecular oxygen as oxidant without co-reductant.

Inhibition of human cytochrome P450 2E1 and 2A6 by aldehydes: structure and activity relationships

The purpose of this study was to probe active site structure and dynamics of human cytochrome P4502E1 and P4502A6 using a series of related short chain fatty aldehydes. Binding efficiency of the aldehydes was monitored via their ability to inhibit the binding and activation of the probe substrates p-nitrophenol (2E1) and coumarin (2A6). Oxidation of the aldehydes was observed in reactions with individually expressed 2E1, but not 2A6, suggesting alternate binding modes. For saturated aldehydes the optimum chain length for inhibition of 2E1 was 9 carbons (KI=7.8 ± 0.3 μM), whereas for 2A6 heptanal was most potent (KI=15.8 ± 1.1 μM). A double bond in the 2-position of the aldehyde significantly decreased the observed KI relative to the corresponding saturated compound in most cases. A clear difference in the effect of the double bond was observed between the two isoforms. With 2E1, the double bond appeared to remove steric constraints on aldehyde binding with KI values for the 5-12 carbon compounds ranging between 2.6 ± 0.1 μM and 12.8 ± 0.5 μM, whereas steric effects remained the dominant factor in the binding of the unsaturated aldehydes to 2A6 (observed KI values between 7.0 ± 0.5 μM and >1000 μM). The aldehyde function was essential for effective inhibition, as the corresponding carboxylic acids had very little effect on enzyme activity over the same range of concentrations, and branching at the 3-position of the aldehydes increased the corresponding KI value in all cases examined. The results suggest that a conjugated π-system may be a key structural determinant in the binding of these compounds to both enzymes, and may also be an important feature for the expansion of the active site volume in 2E1.

Cometabolism of chlorinated solvents by nitrifying bacteria: kinetics, substrate interactions, toxicity effects, and bacterial response

Pure cultures of ammonia-oxidizing bacteria, Nitrosomonas europaea, were exposed to trichloroethylene (TCE), 1,1-dichloroethylene (1,1-DCE), chloroform (CF), 1,2-dichloroethane (1,2-DCA), or carbon tetrachloride (CT), in the presence of ammonia, in a quasi-steady-state bioreactor. Estimates of enzyme kinetics constants, solvent inactivation constants, and culture recovery constants were obtained by simultaneously fitting three model curves to experimental data using nonlinear optimization techniques and an enzyme kinetics model, referred to as the inhibition, inactivation, and recovery (IIR) model, that accounts for inhibition of ammonia oxidation by the solvent, enzyme inactivation by solvent product toxicity, and respondent synthesis of new enzyme (recovery). Results showed relative enzyme affinities for ammonia monooxygenase (AMO) of 1,1-DCE approximately TCE > CT > NH(3) > CF > 1,2-DCA. Relative maximum specific substrate transformation rates were NH(3) > 1,2-DCA > CF > TCE approximately 1,1-DCE > CT (=0). The TCE, CF, and 1,1-DCE inactivated the cells, with 1,1-DCE being about three times more potent than TCE or CF. Under the conditions of these experiments, inactivating injuries caused by TCE and 1,1-DCE appeared limited primarily to the AMO enzyme, but injuries caused by CF appeared to be more generalized. The CT was not oxidized by N. europaea while 1,2-DCA was oxidized quite readily and showed no inactivation effects. Recovery capabilities were demonstrated with all solvents except CF. A method for estimating protein yield, the relationship between the transformation capacity model and the IIR model, and a condition necessary for sustainable cometabolic treatment of inactivating substrates are presented. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 520-534, 1997.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Synthesis of substituted phenyl diaziridines and characterization as mechanism-based inactivators of human cytochrome P450 2B6

The metabolism of arylhydrazines by cytochromes P450 (P450s) has previously been shown to yield aryl-iron complexes that inhibit P450 enzymes as a result of heme modification. These modifications of the heme have been used to probe the topology of the active site of several P450s. Therefore, diaziridines containing one or more substitutions on the phenyl ring were synthesized and evaluated as potential mechanism-based inactivators of P450 2B enzymes that could be used to elucidate the active site topology. Five of the six trifluoroaryldiaziridines tested selectively inactivated P450 2B6 in the reconstituted system in a time-, concentration-, and NADPH-dependent manner as measured using the 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation assay. The kinetic parameters for P450 2B6 inactivation by the five compounds were calculated. Analysis of the P450 heme from P450s inactivated by the five substituted diaziridines suggested that the activity loss was not due to heme destruction as measured by the reduced-CO spectrum or high-performance liquid chromatography of the P450 heme. Dialysis experiments indicated the irreversible nature of the inactivation and the reaction between the diaziridine compounds and the P450 enzyme. Interestingly, a thiomethyl-substituted phenyl diaziridine had no effect on the activity of P450 2B6 in the reconstituted system, but competitively inhibited the O-debenzylation activity of P450 3A4 with 7-benzyloxy-4-(trifluoromethyl)coumarin as substrate. Binding spectra suggest that this compound bound reversibly to P450 2B6, and preliminary results indicate that 3-(4-methylthiophenyl)-3-(trifluoromethyl)diaziridine is metabolized by P450 2B6.

Multistate reactivity in styrene epoxidation by compound I of cytochrome p450: mechanisms of products and side products formation

Density functional theoretical calculations are used to elucidate the epoxidation mechanism of styrene with a cytochrome P450 model Compound I, and the formation of side products. The reaction features multistate reactivity (MSR) with different spin states (doublet and quartet) and different electromeric situations having carbon radicals and cations, as well as iron(III) and iron(IV) oxidation states. The mechanisms involve state-specific product formation, as follows: a) The low-spin pathways lead to epoxide formation in effectively concerted mechanisms. b) The high-spin pathways have finite barriers for ring-closure and may have a sufficiently long lifetime to undergo rearrangement and lead to side products. c) The high-spin radical intermediate, (4)2(rad)-IV, has a ring closure barrier as small as the C--C rotation barrier. This intermediate will therefore lose stereochemistry and lead to a mixture of cis and trans epoxides. The barriers for the production of aldehyde and suicidal complexes are too high for this intermediate. d) The high-spin radical intermediate, (4)2(rad)-III, has a substantial ring closure barrier and may survive long enough time to lead to suicidal, phenacetaldehyde and 2-hydroxostyrene side products. e) The phenacetaldehyde and 2-hydroxostyrene products both originate from crossover from the (4)2(rad)-III radical intermediate to the cationic state, (4)2(cat,z(2) ). The process involves an N-protonated porphyrin intermediate that re-shuttles the proton back to the substrate to form either phenacetaldehyde or 2-hydroxostyrene products. This resembles the internally mediated NIH-shift observed during benzene hydroxylation.

The mechanism-based inactivation of p450 2B4 by tert-butyl 1-methyl-2-propynyl ether: structural determination of the adducts to the p450 heme

tert-Butyl 1-methyl-2-propynyl ether (tBMP) was analyzed for its ability to act as a mechanism-based inactivator of p450 2B4. tBMP inactivated p450 2B4 in a time-, concentration-, and NADPH-dependent manner. Losses in activity occurred with concurrent losses in the reduced CO spectrum and native p450 heme; however, there was a greater loss in activity than could be accounted for by reduced CO spectra or native heme loss. LC/MS analysis demonstrated that the losses in native heme were accompanied by the appearance of two modified hemes with m/z values of 705Da, consistent with tBMP adducted hemes. Both adducts had identical fragmentation patterns when analyzed by LC/MS/MS. The spectra were consistent with a tBMP molecule and an oxygen atom attached to iron-depleted heme. Proton NMR studies suggest that the two modified hemes in p450 2B1 are N-alkylated on pyrrole rings A and D.

Cytochrome P450 oxidations in the generation of reactive electrophiles: epoxidation and related reactions

Much of the interest in the cytochrome P450 (P450) enzymes has been because of oxidation of chemicals to reactive products. The epoxides (oxiranes) have been a major topic of interest with olefins and aryl compounds. Epoxides vary considerably in their reactivity, with t(1/2) varying from 1s to several hours. The stability and reactivity influences not only the overall damage to biological systems but also the site of injury. Transformations of some xenobiotic chemicals may involve products other than epoxides. Chemicals considered here include olefins, aromatic hydrocarbons, heterocycles, vinyl halides, ethyl carbamate, vinyl nitrosamines, and aflatoxin B(1). These compounds either are unsaturated or are transformed to unsaturated products. The epoxides and other products provide a view of the landscape of P450-generated reactive products and the myriad of chemistry involved in the metabolism of drugs and protoxicants. Understanding the chemical nature of reactive products is necessary to develop rational strategies for intervention.

Mechanism-based inactivation of bovine cytochrome P-450(11beta) by 18-unsaturated progesterone derivatives

Two 18-unsaturated progesterone derivatives, 18-vinylprogesterone (18-VP) and 18-ethynylprogesterone (18-EP) have proved to be potent inhibitors of the bovine cytochrome P-450(11beta), the enzyme involved in the last steps of aldosterone biosynthesis [Delorme, C., Piffeteau, A., Viger, A. & Marquet, A. (1995) Eur. J. Biochem. 232, 247-256]. In the present study, we demonstrate that these two compounds exhibit the characteristics of mechanism-based inactivators of this enzyme. Inactivation followed pseudo-first-order and saturation kinetics. The kinetic parameters of inactivation were k(i) = 0.11 min(-1) and Ki = 4 microM for 18-VP, and k(i) = 0.12 min(-1) and 22 microM for 18-EP. Inactivation of P-450(11beta) activity was strictly dependent on the presence of NADPH. Protection by the substrate deoxycorticosterone was observed, demonstrating a selective modification at the substrate-binding site. With radiolabeled 18-VP, inactivation was shown to be irreversible with a stoichiometry of 1.4 mol bound [3H]18-VP/mol inactivated cytochrome P-450(11beta). SDS/PAGE analysis of the [3H]18-VP-inactivated enzyme showed that, under conditions preventing heme dissociation, the P-450(11beta) band was labeled, while no labeling of the apoprotein was observed under denaturating conditions. Furthermore, the loss of catalytic activity could be correlated with the destruction of the P-450 chromophore evaluated by the FeII-CO versus FeII difference spectra. These arguments led us to propose that 18-vinylprogesterone inactivates cytochrome P-450(11beta) by heme destruction rather than by protein modification.

Specific inhibition of the last steps of aldosterone biosynthesis by 18-vinylprogesterone in bovine adrenocortical cells

18-Vinylprogesterone (18-VP), designed for mechanism-based specific inhibition of the last steps of the aldosterone biosynthesis, was used to characterize the mechanism of the 11 - and 18-hydroxylase activities of bovine cytochrome P450(11beta). In the present work, its action was studied by observations on a primary culture of bovine adrenocortical cells. First, we investigated the effects of 18-VP on the different enzymatic steps of the biosynthesis of cortisol and aldosterone. The production of cortisol, baseline or hormone-stimulated (ACTH or AII), was inhibited by 18-VP in a dose-dependent manner with a maximal inhibition at 5 microM. Supply of different exogenous substrates to support steroidogenesis revealed an inhibition of the last step of cortisol or corticosterone biosynthesis. We then used specific blockers to measure individual activities and conclude that 11beta-hydroxylation was the only enzymatic activity affected. Aldosterone, as well as 18-hydroxycorticosterone, was also measured following addition of corticosterone. The 18-hydroxylation of corticosterone was inhibited by 18-VP, with 50% inhibition occurring at 0.04 microM compared with the 50% inhibition value of 0.3 microM obtained for 11-hydroxylation. Surprisingly, 18-ethynyl-progesterone (18-EP), which has a structure very similar to 18-VP, only weakly inhibits 11beta-hydroxylation. The inhibition of aldosterone formation was also much lower with 18-EP than with 18-VP. These studies demonstrate that 18-VP inhibits only the later steps of aldosterone biosynthesis and more specifically 18- than 11-hydroxylation activity.

Inhibition of bovine cytochrome P-450(11 beta) by 18-unsaturated progesterone derivatives

The last step of aldosterone biosynthesis, an 11 beta-hydroxylation followed by two 18-hydroxylations, are catalyzed, in the bovine system, by the same enzyme, the cytochrome P-450(11 beta) (deoxycorticosterone (DOC)-->corticosterone-->18-hydroxycorticosterone-->aldosterone). The 11 beta- and 18-hydroxylase activities were studied separately with a reconstituted enzymic system, using 11-deoxy[14C]corticosterone and [3H]corticosterone, respectively, as substrates. The inhibition of 11 beta-hydroxylase activity by corticosterone was competitive (Ki = 60 microM) showing that transformation of both substrates occurs at the same site. Double-label/double-substrate experiments, using an equimolar mixture of 11-deoxy[14C]corticosterone and [3H]corticosterone, suggested that 18-hydroxycorticosterone is directly formed from 11-deoxycorticosterone without the intermediate corticosterone leaving the enzyme. Inhibitions by 18-vinylprogesterone and 18-ethynylprogesterone, potent inhibitors of aldosterone biosynthesis [Viger, A., Coustal, S., Pérard, S., Piffeteau, A. & Marquet, A. (1989) J. Steroid Biochem. 33, 119-124], were characterized for both activities (11 beta- and 18-hydroxylase). The value of reversible Ki for the 18-hydroxylation (Ki = 5 microM for 18-vinylprogesterone and 30 microM for 18-ethynylprogesterone) is lower than that for the 11 beta-hydroxylation (30 microM and 100-150 microM, respectively); the former inhibitor is stronger than the latter for both steps. The binding of substrates and inhibitors to the active site was also examined by difference absorption spectroscopy. 18-Vinylprogesterone gave rise to a type I spectrum with a Ks value of 35 microM close to that of progesterone, while 18-ethynylprogesterone showed a reverse type I spectrum with a much higher Ks value (140 microM). Based on these results, a hypothetical model, involving a conformational change of the enzyme for the second step, is proposed.

Haloalkene oxidation by the soluble methane monooxygenase from Methylosinus trichosporium OB3b: mechanistic and environmental implications

The soluble, three-protein component methane monooxygenase purified from Methylosinus trichosporium OB3b is capable of oxidizing chlorinated, fluorinated, and brominated alkenes, including the widely distributed ground-water contaminant trichloroethylene (TCE). The oxidation rates for the chloroalkenes were observed to be comparable to that for methane, the natural substrate, and up to 7000-fold higher than those reported for other well-defined biological systems. The competitive inhibitor tetrachloroethylene was found to be the only chlorinated ethylene not turned over. However, this appears to be due to steric effects rather than electronic effects or the lack of an abstractable proton because chlorotrifluoroethylene was efficiently oxidized. As evidenced by the formation of diagnostic adducts with 4-(p-nitrobenzyl)pyridine, the halogenated alkenes were oxidized predominantly by epoxidation. Stable acidic products resulting from subsequent hydrolysis were identified as the major products. However, additional aldehydic products resulting from intramolecular halide or hydride migration were observed in 3-10% yield during the oxidation of TCE, vinylidene chloride, trifluorethylene, and tribromoethylene. Product analysis of the hydrolysis reaction of authentic TCE epoxide showed little or no 2,2,2-trichloroacetaldehyde (chloral) formation, indicating that atomic migration occurred prior to product dissociation from the enzyme. The occurrence of atomic migration products shows that an intermediate in the substrate to product conversion carries significant cationic character. Such a species could be generated through interaction with a highly electron-deficient activated oxygen in the active site.(ABSTRACT TRUNCATED AT 250 WORDS)

Modified acetylenic steroids as potent mechanism-based inhibitors of cytochrome P-450SCC

Synthesized 20-(4-tetrahydropyranyl-1-butynyloxy)-5-pregnen-3 alpha,20 beta- diol [steroid I] and 20-(3-tetrahydropyranyl-1-propargyloxy)-5-pregnen- 3 alpha,20 beta-diol [steroid III] have been found to inactivate purified adrenocortical cytochrome P-450SCC. When incubated with the enzyme under turnover conditions, steroid I inactivated cytochrome P-450SCC by about 85% in 40 min. This is in contrast to the free triol analog, steroid II which inactivated the enzyme by only 45% within the same incubation period. A comparison of steroid III with its free triol analog, steroid IV, also showed that the diol is a more effective inactivator of the enzyme than the triol. The partition ratio was calculated by two different methods. Each of the steroids I-IV bound to the enzyme with spectrophotometric dissociation constant (Ks) in the micromolar range, producing Type II low spin spectra changes during titration of the enzyme. In addition, it was found that the binding of each of the compounds to the enzyme occurred without inactivation of the enzyme and that the inactivation under turnover condition, is not as a result of conversion to the denatured P-420 species. This demonstrated that steroids I and III could correctly be designated as mechanism-based (suicide) inhibitors. The kinetic studies demonstrated that steroids with the tetrahydropyranyl substituent are more potent inhibitors of cytochrome P-450SCC as shown by an initial turnover rate of 0.06 min-1, an inactivation rate constant of 0.05 min-1, and a partition ratio of about 1.0 for steroid I. Based on our finding, possible mechanisms of inactivation of cytochrome P-450SCC by these acetylenic steroids are proposed.

Chronic inhalation effects of ethylene oxide on porphyrin-heme metabolism

The effects of chronic ethylene oxide (EtO) inhalation on porphyrin-heme metabolism were investigated. When Wistar male rats were exposed to 500 ppm EtO for 6 h a day, 3 times a week for 13 weeks, hemoglobin content significantly decreased, and a normocytic and normochromic anemia was found. In the liver, cytochrome P-450 and protoheme significantly decreased but wet weight, microsomal protein and cytochrome b5 were not affected. The activity of delta-aminolevulinic acid (ALA) synthase increased while ALA dehydratase did not change. The activity of hepatic ferrochelatase decreased time-dependently. Uroporphyrin increased 37% and coproporphyrin tended to increase in the liver. The concentration of protoporphyrin in the liver and erythrocytes tended to increase. Coproporphyrin excretion in the urine showed a 5-6-fold increase while there was no significant increase in urinary ALA excretion. These results indicate that chronic inhalation of EtO causes alterations of hepatic porphyrin-heme metabolism as well as anemia and may affect mechanisms of adaptation to xenobiotics.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

Ethylene metabolism in Pisum sativum L

The possible role of C2H4 metabolism in mediating the responses of plants to C2H4 is re-examined. It is demonstrated that (i) the effects of inhibitors upon C2H4 action do not correspond with their effects on metabolism, (ii) elicitors of C2H4 effects do not have appropriate effects on C2H4 metabolism, (iii) inhibitors of C2H4 metabolism do not affect the response of plants to C2H4. It is concluded that metabolism of C2H4 is not linked to the mode of action of the growth regulator.

Substrate inactivation of enzymes in vitro and in vivo

Some enzymes are inactivated by their natural substrates during catalytic turnover, limiting the ultimate extent of reaction. These enzymes can be separated into three broad classes, depending on the mechanism of the inactivation process. The first type is enzymes which use molecular oxygen as a substrate. The second type is inactivated by hydrogen peroxide, which is present either as a substrate or a product, and are stabilized by high catalase activity. The oxidation of both types of enzymes shares common features with oxidation of other enzymes and proteins. The third type of enzyme is inactivated by non-oxidative processes, mainly reversible loss of cofactors or attached groups. Sub classes are defined within each broad classification based on kinetics and stoichiometry. Reaction-inactivation is in part a regulatory mechanism in vivo, because specific proteolytic systems give rapid turnover of such labelled enzymes. The methods for enhancing the stability of these enzymes under reaction conditions depends on the enzyme type. The kinetics of these inactivation reactions can be used to optimize bioreactor design and operation.

Mechanism-based inhibitors of dopamine beta-hydroxylase

The copper-containing monooxygenase dopamine beta-hydroxylase catalyzes the hydroxylation of dopamine at the benzylic position to form norepinephrine. Mechanism-based inhibitors for dopamine beta-hydroxylase have been used as probes of the mechanism of catalysis. The variety of such inhibitors that have been developed for this enzyme can be divided into three groups: (i) those in which the inactivating species is formed by abstraction of a hydrogen atom to form a radical intermediate; (ii) those in which the inactivating species is formed by abstraction of an electron to form an epoxide-like intermediate; and (iii) those in which the product is the inactivating species. A mechanism consistent with inactivation by all three groups of inhibitors which proposes that hydroxylation of dopamine by dopamine beta-hydroxylase involves formation of a benzylic radical has been developed. The benzylic radical is formed by abstraction of a hydrogen atom from the substrate by a high-potential copper-oxygen species.

Metabolic activation of olefins. Conversion of 1-octene to a putative reactive intermediate 1-octen-3-one: an alternative pathway to epoxidation

The enzymic activation of a model olefin oct-1-ene was studied in rat liver microsomal systems in vitro. An active metabolite was trapped using N-acetylcysteine and identified by means of capillary GLC/mass spectrometry and 360 MHz 1H NMR as S-3-oxo-octyl-N-acetylcysteine. A two step pathway for the formation of this adduct was proposed involving first the production of oct-1-en-3-ol by NADPH dependent mixed function oxidases and secondly a NADP or NAD linked oxidation, independent of cytochrome P-450, to yield the putative reactive intermediate oct-1-en-3-one. Under physiological conditions, oct-1-en-3-one, prepared chemically, reacted non-enzymically with N-acetylcysteine with a t1/2 of about 6 sec. Enzymes catalysing the NADP-dependent oxidation of octen-3-ol were present in microsomal preparations from a number of organs apart from the liver, those from adrenal and intestinal epithelia showing the next highest levels of activity. Unlike the activation of octene, rates of hepatic activation of octen-3-ol were not induced by pretreatment of rats with phenobarbitone or 3-methylcholanthrene. Using 1-octene as the substrate, comparisons were made of alternative routes of hepatic metabolism activation. Relative to the rate of formation of the 3-oxo intermediate trapped with N-acetylcysteine, epoxidation of octene and subsequent hydrolysis to octane-1,2-diol was over 40 times more rapid. The rate of formation of a presumptive oxirane precursor trapped with the haem of cytochrome P-450 as N-(2-hydroxyoctyl)protoporphyrin IX was about 17-fold lower.

Effects of acetylenic and olefinic pyrenes upon cytochrome P-450 dependent benzo[a]pyrene hydroxylase activity in liver microsomes

1-Ethynylpyrene, trans-, & cis-1-(2-bromovinyl)pyrene, methyl 1-pyrenyl acetylene, and phenyl 1-pyrenyl acetylene are substrates for cytochrome P-450 dependent monooxygenases and also inhibitors of cytochrome P-450 dependent benzo[a]pyrene hydroxylase activities in liver microsomes from 5,6-benzoflavone or phenobarbital pretreated rats. 1-Ethynylpyrene, trans-1-(2-bromovinyl)pyrene, and methyl 1-pyrenyl acetylene cause a mechanism based inhibition (suicide inhibition) of the benzo[a]pyrene hydroxylase activities in microsomes from 5,6-benzoflavone or phenobarbital pretreated rats, while cis-1-(2-bromovinyl)pyrene only causes suicide inhibition of the hydroxylse activities in the 5,6-benzoflavone induced microsomes and phenyl 1-pyrenyl acetylene does not cause a detectable suicide inhibition of these activities in either type of microsome. Incubation with NADPH and 1-ethynylpyrene, trans-, or cis-1-(2-bromovinyl)pyrene causes a loss of the P-450 content in the microsomes from 5,6-benzoflavone or phenobarbital pretreated rats, but incubations with methyl 1-pyrenyl acetylene or phenyl 1-pyrenyl acetylene did not cause a loss of the P-450 content of either microsomal preparation.

Mixed-function oxidase system induction and propylene hepatotoxicity

Propylene is hepatotoxic to male Charles River COBS Sprague-Dawley rats pretreated with polychlorinated biphenyls (PCB: Aroclor 1254). Four-hour inhalation exposure to 50,000 ppm propylene increased liver weight/body weight ratios and elevated serum enzyme activities in PCB-pretreated animals. Hepatic microsomal cytochrome P-450 content of PCB-pretreated rats dropped profoundly during propylene exposure and remained depressed for at least 24 h. In addition, PCB-pretreated, propylene-exposed rats exhibited a decrease in the specific activity of hepatic microsomal aniline hydroxylase. However, there was no change in activities of either hepatic microsomal aminopyrine demethylase or glucose-6-phosphatase. Propylene exposure of rats pretreated with beta-naphthoflavone (BNF), phenobarbital (PB), or a mixture of BNF and PB was not hepatotoxic. However, there was, in these animals, a substantial decline in hepatic microsomal cytochrome P-450 levels 24 h after the start of propylene exposure. Hence, the propylene-dependent process resulting in hepatic cytochrome P-450 destruction is qualitatively or quantitatively different from the process that causes acute hepatotoxicity. Preexposure fasting had no effect on the hepatotoxicity resulting from a 4-h exposure of PCB-pretreated rats to 50,000 ppm propylene. Administration of SKF-525A to PCB-pretreated rats immediately prior to propylene exposure completely prevented elevations in serum enzyme activities and liver weight/body weight ratios. In vitro incubation of hepatic microsomes prepared from either BNF-, PB-, or PCB-pretreated rats with an atmosphere of 20% propylene/80% air produced in NADPH-dependent decrease in cytochrome P-450 content. These results suggest that PCB pretreatment is a prerequisite for propylene hepatotoxicity in the rat. Cytochrome P-450-dependent bioactivation of propylene is associated with this hepatotoxicity, but further studies are needed to characterize the mechanism of the PCB-propylene interaction.

Exposure to toxic agents: the heme biosynthetic pathway and hemoproteins as indicator

The heme biosynthetic pathway is closely controlled by levels of the end product of the pathway, namely, heme, and porphyrins are normally formed in only trace amounts. When control mechanisms are disturbed by xenobiotics, porphyrins accumulate and serve as a signal of the interaction between a xenobiotic and the heme biosynthetic pathway. For example, an increase in erythrocyte protoporphyrin is a useful measurement for early detection of exposure to lead and porphyrinuria was an early manifestation of a hexachlorobenzene-induced porphyria in Turkey. In recent years a variety of additional xenobiotics has been shown to interact with the heme biosynthetic pathway, namely, halogenated aromatic hydrocarbons, pesticides, sulfides, and a variety of metals. Moreover, different xenobiotics (e.g., dihydropyridines and compounds containing unsaturated carbon-carbon bonds) interact with the prosthetic heme of cytochrome P-450 forming novel N-alkylporphyrins.

Oxidation of ethylene by cotyledon extracts from Vicia faba L. : Cofactor requirements and kinetics

Improved rates of ethylene oxidation by cell-free preparations from cotyledons of Vicia faba L. have been obtained using cryogenic storage techniques and by developing a method for the hydrolysis of ethylene oxide. Gel permeation chromatography showed that a low-molecular-size fraction was required for activity; accordingly, the kinetics of ethylene oxidation in the presence of this fraction were studied. Reduced pyridine nucleotides could substitute for the low-molecular-size fraction. Activity under a nitrogen atmosphere was 60% lower than in air. The need for reduced nicotinamide adenine dinucleotide phosphate (NADPH) and oxygen indicated that the enzyme might be a mixed-function oxidase. Using sufficient NADPH to approach saturation, the apparent Michaelis constant (K m) for ethylene was 1.94±0.38 · 10(-8) M (aqueous phase), and when ethylene was saturating, the K m for NADPH was 3.7 · 10(-5) M. Carbon monoxide was found to inhibit by competing with ethylene, and the inhibitor constant was 5.97 · 10(-7) M in solution. In the presence of excess ethylene and NADPH, activity was highest in phosphate-buffered medium pH 7.9. The bulk of the activity was found in a microsomal fraction.

Novel metabolites of trichloroethylene through dechlorination reactions in rats, mice and humans

The excretion and biotransformation of [14C]trichloroethylene (Tri) has been studied in female rats and mice. Seventy-two hours after a single oral dose of 200 mg/kg, rats exhaled 52% and mice 11% of the recovered radioactivity as unchanged Tri, and 1.9% and 6%, respectively, as 14CO2. Rats excreted 41.2% of the recovered radioactivity in the urine, in contrast to mice where urinary activity amounted to 76%. The isolation of urinary metabolites was accomplished by reversed-phase HPLC, using a water-methanol gradient. After chemical derivatization, a combination of radio-GC and GC/MS was used for identification. The metabolites identified in rat urine were: trichloroacetic acid (15.3%); trichloroethanol, free (11.7%) and as the glucuronide (61.9%); dichloroacetic acid (2.0%); oxalic acid (1.3%) and N-(hydroxyacetyl)-aminoethanol (HAAE) (7.2%). In mice, trichloroethanol (free and in several conjugated forms) is the main metabolite of Tri (94.3%), but small amounts of HAAE (4.1%) and oxalic acid (0.7%) are also excreted. Only traces of dichloro- and trichloroacetic acids were found in this species. In human male subjects, HAAE was also identified as a urinary metabolite of Tri after exposure of two volunteers to 200 ppm Tri for 6 hr. The identification of HAAE and oxalic acid as metabolites indicates hydrolytic dechlorination reactions in the metabolism of Tri.

Mg-2,4-divinyl pheoporphyrin a5: the product of a reaction catalyzed in vitro by developing chloroplasts

The major product of an aerobic reaction mixture containing developing chloroplasts, Mg-protoporphyrin IX, S-adenosylmethionine, and other cofactors was isolated and purified. Structural studies using nuclear magnetic resonance confirmed earlier reports, based on fluorescence and absorption spectra, that this compound is Mg-2,4-divinyl pheoporphyrin a5. The molecular weight determined by secondary-ion mass spectroscopy further confirmed the assigned structure. Absorption and fluorescence spectra indicate that this compound is identical to that reported previously by various workers in less-purified biological extracts. The nuclear magnetic resonance spectrum of the Mg-free base also supports the assigned structure.

Cytochrome P-450 and halothane metabolism. Decrease in rat liver microsomal P-450 in vitro

Anaerobic in vitro incubation of microsomes from phenobarbital(PB)-induced rats with halothane results in an irreversible decrease of measurable cytochrome P-450. There is a parallel decrease in heme content under the same incubation conditions. However, microsomes from 3-methylcholanthrene(3-MC)-induced or untreated animals do not show a reduction in cytochrome P-450 content. Aerobic incubation with halothane results in a decrease of cytochrome P-450 which can be completely reversed by dialysis or the addition of potassium ferricyanide. These latter treatments only partially restore the cytochrome P-450 levels following anaerobic incubations. The decrease in cytochrome caused by halothane is not associated with measureable heme N-alkyl adduct formation; lipid peroxidation does not play a role as indicated by the lack of effect of 1 mM EDTA or a decrease in glucose-6-phosphatase activity. Halothane metabolites are bound irreversibly to microsomal protein as determined by gel electrophoresis only when the oxygen concentration is very low. The mechanism of cytochrome P-450 decrease is consistent with the formation of a reactive metabolite which binds to the protein portion and also destroys heme.

Loss of hepatic monooxygenase activities, glutathione, and 'green pigment' formation after the administration of vinyl-cyclooctane to mice

Vinylcyclooctane, when administered to mice at 500 mg/kg, produced reduction of microsomal cytochrome P-450, heme, aminopyrine-N-demethylase and ethoxycoumarin-O-deethylase activities with respect to control values; furthermore the hepatic reduced glutathione level was depleted suggesting that glutathione is involved in the vinylcyclooctane metabolism. The reduction of cytochrome P-450 and monooxygenase activities was accompanied by the formation of abnormal 'green pigments'.