Destruction of cytochrome P-450 by 2-isopropyl-4-pentenamide and methyl 2-isopropyl-4-pentenoate: mass spectrometric characterization of prosthetic heme adducts and nonparticipation of epoxide metabolites

Study of the Metabolic Profiles of "Indazole-3-Carboxamide" and "Isatin Acyl Hydrazone" (OXIZID) Synthetic Cannabinoids in a Human Liver Microsome Model Using UHPLC-QE Orbitrap MS

Unregulated core structures, "isatin acyl hydrazones" (OXIZIDs), have quietly appeared on the market since China legislated to ban seven general core scaffolds of synthetic cannabinoids (SCs). The fast evolution of SCs presents clinical and forensic toxicologists with challenges. Due to extensive metabolism, the parent compounds are barely detectable in urine. Therefore, studies on the metabolism of SCs are essential to facilitate their detection in biological matrices. The aim of the present study was to elucidate the metabolism of two cores, "indazole-3-carboxamide" (e.g., ADB-BUTINACA) and "isatin acyl hydrazone" (e.g., BZO-HEXOXIZID). The in vitro phase I and phase II metabolism of these six SCs was investigated by incubating 10 mg/mL pooled human liver microsomes with co-substrates for 3 h at 37 °C, and then analyzing the reaction mixture using ultrahigh-performance liquid chromatography-quadrupole/electrostatic field orbitrap mass spectrometry. In total, 9 to 34 metabolites were detected for each SC, and the major biotransformations were hydroxylation, dihydrodiol formation (MDMB-4en-PINACA and BZO-4en-POXIZID), oxidative defluorination (5-fluoro BZO-POXIZID), hydrogenation, hydrolysis, dehydrogenation, oxidate transformation to ketone and carboxylate, N-dealkylation, and glucuronidation. Comparing our results with previous studies, the parent drugs and SC metabolites formed via hydrogenation, carboxylation, ketone formation, and oxidative defluorination were identified as suitable biomarkers.

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.

Porphyrinogenic xenobiotic-induced N-alkylprotoporphyrin IX formation: a bioassay utilizing chick embryo hepatic ferrochelatase

INTRODUCTION

The porphyrinogenicity of some xenobiotics results from mechanism-based inactivation of selected cytochrome P450 (CYP) enzymes accompanied by conversion of prosthetic heme groups to N-alkylprotoporphyrins (N-alkylPPs), some of which inhibit ferrochelatase (FC). Problems have arisen in extrapolating xenobiotic porphyrinogenicity observed in test animals to humans, due in part to differences among CYP enzymes. Our goal was to develop a bioassay to detect N-alkylPPs formed following interaction of porphyrinogenic xenobiotics with rat liver microsomal CYP.

METHODS

Seventeen-day-old chick embryo livers were homogenized, and the mitochondrial fraction was isolated. The FC activity of this fraction was determined by means of the pyridine hemochromogen method. Inhibition of FC was used to detect N-alkylPP formation following interaction of porphyrinogenic xenobiotics with rat liver microsomes.

RESULTS

The 17-day-old chick embryo hepatic mitochondrial preparation served as a stable source of FC activity, which was linear with respect to time and protein concentration. FC activity was higher than previously reported in a homogenate of 17-day-old chick embryo hepatocytes in culture and in an aqueous extract of 17-day-old chick embryo mitochondria. The EC(50) of N-methylprotoporphyrin IX in the chick embryo liver mitochondrial preparation was similar to that in the homogenate of chick embryo liver cell culture. The FC bioassay could detect N-alkylPPs formed following the interaction of porphyrinogenic xenobiotics with rat liver microsomes containing 2.4-9.0 nmol of CYP.

DISCUSSION

In future studies investigating N-alkylPP formation following interaction of xenobiotics with CYP enzymes, we recommend using a combination of a fluorescence technique and the chick embryo hepatic mitochondrial FC assay. This would provide information both on the formation of N-alkylPPs and distinguish between those N-alkylPPs that produced porphyrin accumulation via FC inhibition and those that do not.

Isolation of regioisomers of N-alkylprotoporphyrin IX from chick embryo liver after treatment with porphyrinogenic xenobiotics

Several porphyrinogenic xenobiotics cause mechanism-based inactivation of cytochrome P450 (P450) isozymes with concomitant formation of a mixture of four N-alkylprotoporphyrin IX (N-alkylPP) regioisomers, which have ferrochelatase inhibitory properties. To isolate the four regioisomers of N-methylprotoporphyrin IX (N-methylPP), 3,5-diethoxycarbonyl, 1-4-dihydro-2,4,6-trimethylpyridine (DDC) was administered to untreated, β-naphthoflavone-, phenobarbital-, and glutethimide-pretreated 18-day-old chick embryos. Separation of the N-methylPP regioisomers by high pressure liquid chromatography (HPLC) revealed no marked difference in the regioisomer pattern among the different treatments. After administration of griseofulvin, allylisopropylacetamide (AIA), or 1-[4-(3-acetyl-2,4,6-triemethylphenyl)-2,6-cyclohexanedionyl]-O-ethyl propionaldehyde oxime (ATMP) to untreated and glutethimide-pretreated 18-day-old chick embryos, an N-alkylPP was isolated after AIA administration only. This finding strengthened previous reports of the species specificity of N-alkylPP formation with griseofulvin and ATMP. A series of dihydropyridines, namely 4-ethylDDC, 4-hexylDDC, and 4-isobutylDDC were administered to untreated and glutethimide-pretreated 18-day-old chick embryos and hepatic N-alkylPPs were isolated and separated by HPLC into regioisomers. The regioisomer patterns obtained did not support a previous proposal of masked regions above both rings B and C in the heme moieties of the P450 isozymes responsible for N-alkylPP formation. However, the data support the hypothesis of a partially masked region above ring B alone. The regioisomer patterns were in agreement with results previously obtained in rats showing that the percentage of NCand (or) NDregioisomers in the regioisomer mixture increases as the length and bulk of the 4-alkyl substituent of a DDC analogue increase. Differences in the regioselectivity of heme N-alkylation may be due to intrinsic chemical features of DDC analogues themselves or to differences in the P450 isozymes inactivated.Key words: mechanism-based inactivation, cytochrome P450, N-alkylprotoporphyrin IX, experimental porphyria, dihydropyridine.

Epoxide formation from diallyl sulfone is associated with CYP2E1 inactivation in murine and human lungs

We tested the hypothesis that an epoxide formed from diallyl sulfone (DASO(2)) is responsible for inactivation of CYP2E1 in murine and human lungs. An epoxide (1,2-epoxypropyl-3,3'-sulfonyl-1'-propene [DASO(3)]) was synthesized from DASO(2) and conjugated with glutathione (GSH) to produce the conjugates S-(1R, S-[[1-hydroxymethyl-2,3' -sulfonyl]-1' -propenyl]ethyl)glutathione (diastereomers) and S-(1-[[2R,S-hydroxypropyl]-3, 3'-sulfonyl]-1'-propenyl)glutathione (diastereomers). Analysis of these conjugates by high performance liquid chromatography revealed a major peak eluting at 20.5 min. This peak was detected in incubations of murine and human lung microsomes containing DASO(2) and nicotinamide adenine dinucleotide phosphate (NADPH), and was not detected in incubations performed in the absence of DASO(2) or NADPH. The amounts of epoxide-derived GSH conjugates formed in the incubations were concentration-dependent and achieved saturation at 0.75 mM DASO(2). Formation of the conjugates was also time-dependent and peaked at 2.0 h after DASO(2). The peak containing the GSH conjugates was also detected in incubations of CYP2E1-expressed lymphoblastoid microsomes, NADPH, and DASO(2). Maximal amounts of DASO(3), as estimated by formation of a 4-(p-nitrobenzyl)pyridine derivatized product, were detected in murine lung microsomes incubated for 35 min with 1 mM DASO(2). The derivatized DASO(3) was not detectable in incubations of human lung microsomes. p-Nitrophenol hydroxylation, a catalytic activity associated with CYP2E1, was reduced in murine and human lung microsomes incubated with DASO(2), with decreases that were concentration-dependent. Dose-dependent decreases in hydroxylase activity were also found in microsomes from mice treated in vivo with DASO(2) (25 to 200 mg/kg). These results supported the premise that an epoxide formed from DASO(2) mediates inactivation of lung CYP2E1. Furthermore, the findings suggested that the mouse model is relevant for studies of DASO(2) in human lung.

Use of recombinant human ferrochelatase as a sensitive bioassay for N-alkylprotoporphyrin IX formed after interaction of porphyrinogenic xenobiotics with rat liver microsomes

Several porphyrinogenic xenobiotics elicit mechanism-based inactivation of cytochrome P450 (CYP) isozymes, leading to the formation of N-alkylprotoporphyrin IX (N-alkylPP), a potent inhibitor of ferrochelatase, the terminal enzyme in heme biosynthesis. Recognizing their role in experimental porphyria, our long term objective is the establishment of an appropriate in vitro system for the detection and quantification of N-alkylPPs, formed in human liver after the administration of potential porphyrinogenic compounds. In a previous study, we used a combination of thin-layer chromatography and UV-visible spectrophotometry to isolate and identify N-alkylPPs after incubating porphyrinogenic compounds with rat liver microsomes. However, the overall yield of N-alkylPPs was low, and it was concluded that in vitro systems, such as human lymphoblastoid microsomal preparations containing single cDNA-expressed human cytochrome P450 (CYP) isozymes, do not contain sufficient CYP for in vitro studies designed to isolate N-alkylPP. In the present study we demonstrate that purified recombinant human ferrochelatase (FC) provides an extremely sensitive bioassay system for N-alkylPPs and is capable of detecting N-alkylPP in the 10-6nmol range. Therefore, we propose that this bioassay system might allow the use of human lymphoblastoid microsomal preparations containing single cDNA-expressed human CYP isozymes to detect N-alkylPP produced after mechanism-based (catalysis-based) CYP inactivation. If this is found to be correct it will facilitate identification of potentially porphyrinogenic drugs prior to administration to humans.Key words: ferrochelatase, N-alkylprotoporphyrin IX, porphyria, mechanism-based inactivation.

Drug-mediated inactivation of cytochrome P450

1. Multiple forms of cytochrome P450 (CYP) catalyse the oxidation of chemicals of endogenous and exogenous origin, including drugs, carcinogens, steroids and eicosanoids. However, this unusual low substrate specificity also makes CYP susceptible to inhibition by a wide range of drugs, leading to pharmacokinetic interactions of potential clinical significance. 2. Some drugs are converted by CYP to reactive metabolites that bind covalently to sites within the active centre of the same CYP. Such mechanism-based inhibition leads to CYP inactivation or complexation. These processes give rise to long-term effects on drug pharmacokinetics, as the inactivated or complexed CYP must be replaced by newly synthesized CYP protein. 3. Drugs that inactivate CYP generally possess recognizable functional groups that are oxidized to reactive products. Thus, drugs with side chains containing unsaturated carbon-carbon bonds and furan ring systems are associated with CYP inactivation. Nitrogen-containing systems may also inactivate CYP. 4. Metabolites formed from drugs containing alkylamino and methylenedioxy functionalities can trap CYP as inert complexes without eliciting inactivation. However, the functional effects of inactivation and complexation on drug pharmacokinetics are indistinguishable. Drugs that elicit CYP complexation include the first generation macrolide antibiotics, but newer analogues appear much safer. Some antidepressants, antiepileptics and tuberculostatic agents have been associated with CYP complexation.

Identification of the heme adduct and an active site peptide modified during mechanism-based inactivation of rat liver cytochrome P450 2B1 by secobarbital

The olefinic barbiturate secobarbital (SB) is a sedative hypnotic known to be a relatively selective mechanism-based inactivator of rat liver cytochrome P450 2B1. Previous studies have demonstrated that such inactivation results in prosthetic heme destruction and irreversible drug-induced protein modification, events most likely triggered by P450 2B1-dependent oxidative activation of the olefinic pi-bond. However, the precise structure of the SB-modified heme and/or the protein site targeted for attack remained to be elucidated. We have now isolated the SB-heme adduct from P450 2B1 inactivated by [14C]SB in a functionally reconstituted system and structurally characterized it by electronic absorption spectroscopy and tandem collision-induced dissociation (CID), matrix-assisted laser desorption ionization on time of flight (MALDI-TOF), and liquid secondary ion mass spectrometry in the positive mode (+ LSIMS) as the N-(5-(2-hydroxypropyl)-5-(1-methylbutyl)barbituric acid)protoporphyrin IX adduct. The [14C]SB-modified 2B1 protein has also been isolated from similar inactivation systems and subjected to lysyl endopeptidase C (Lys-C) digestion and HPLC-peptide mapping. A [14C]SB-modified 2B1 peptide was thus isolated, purified, electrotransferred onto a poly-(vinylidene) membrane, and identified by micro Edman degradation of its first N-terminal 17 residues (S277NH(H)TEFH(H)ENLMISLL293) as the Lys-C peptide domain comprised of amino acids 277-323. This peptide thus includes the peptide domain corresponding to the distal helix I of P450 101, a region highly conserved through evolution, and which is known not only to flank the heme moiety but also to intimately contact the substrates. This finding thus suggests that SB-induced protein modification of P450 2B1 also occurs at the active site and, together with heme N-alkylation, contributes to the SB-induced mechanism-based inactivation of P450 2B1.

Regulation of cytosolic guanylyl cyclase by porphyrins and metalloporphyrins

The experimental evidence is convincing that cytosolic guanylate cyclase is a hemoprotein containing stoichiometric amounts of heme, which functions as a prosthetic group for enzyme activation by NO. Nearly all of the studies described in this chapter were conducted before we began to appreciate in 1986 that mammalian vascular endothelial cells could synthesize their own NO. We know now that many different cell types synthesize NO, and that in most instances the NO interacts in a paracrine manner with adjacent target cells to activate cytosolic guanylate cyclase and elevate intracellular levels of cyclic GMP (Ignarro, 1990). The studies on endothelium-derived relaxing factor and authentic NO have shown clearly that heme and hemoproteins have a very high binding affinity for, and inhibit the actions of, these substances (Ignarro, 1989). The interaction between NO and the heme prosthetic group of guanylate cyclase appears to constitute an important signal transduction mechanism whereby NO raises intracellular cyclic GMP levels. This novel signal transduction mechanism is highly conducive to the efficient functioning of NO as a paracrine mediator of cellular function. As a small, lipophilic, and chemically labile molecule, NO diffuses out of its cells of origin and into nearby target cells. The very high binding affinity of enzyme-bound heme for NO ensures interaction of the two to cause guanylate cyclase activation and cyclic GMP formation. Thus, relatively uncomplicated mechanism can account for the paracrine function of endogenous NO in transcellular communication.

Thin-layer chromatographic separation of the ferrochelatase-inhibitory ring A and ring B regioisomers of N-ethylprotoporphyrin from a mixture of the four regioisomers

When N-alkylprotoporphyrins are prepared synthetically or biologically, a mixture of four regioisomers is obtained. For our studies, separation of the potent ferrochelatase-inhibitory ring A (NA) and ring B (NB) regioisomers from the ring C (NC) and ring D (ND) regioisomers of low potency is required. Previously this separation required two successive high-performance liquid chromatography procedures. We now report that the separation of the zinc complexes of the NA and NB regioisomers from the NC and ND regioisomers can be achieved by a rapid and inexpensive thin-layer chromatography procedure.

Effect of some antineoplastics on metabolic heme pathway

1. The porphyrinogenic ability of several antineoplastics used in the therapy of the different cancers was evaluated. The action of cyclophosphamide, busulfan and 5-fluorouracil on the amount and nature of the accumulated hepatic porphyrins and on the activity of delta-aminolaevulinate synthase (ALA-S), were estimated at different doses and times of drug treatment in 17-day-old chick embryos. 2. It was observed that cyclophosphamide produces a significant increase in the accumulation of hepatic porphyrins at different doses as well as in the activity of the ALA-S, at all the incubation times. Cyclophosphamide alters the pattern of porphyrins accumulated in the liver, where a coproporphyrin: protoporphyrin ratio higher than in the controls can be observed. 3. Busulfan increased the hepatic porphyrins accumulated in the liver but to a lesser degree than cyclophosphamide. 4. 5-Fluorouracil did not modify the hepatic porphyrin content when it was administered at doses up to 40 mg/embryo. 5. When the embryos were injected with busulfan or 5-fluorouracil no significant differences were observed in the activity of ALA-S up to 11 hr of incubation. 6. These results indicate that cyclophosphamide has a remarkable porphyrinogenic capacity in chick embryo while busulfan, notwithstanding the fact that it alters the haem pathway, it does so to a degree that does not impair the regulation of ALA-S activity. Fluorouracil seems to be non porphyrinogenic in this system, up to 40 mg/embryo.

The effects of haem arginate and haematin upon the allylisopropylacetamide induced experimental porphyria in rats

Biochemical disorders caused by allylisopropylacetamide in various animal species resemble human acute intermittent porphyria. The antiporphyrogenic efficacy and potency of haem arginate, a new haem compound, were compared with those of haematin in experimental porphyria of rats. Both haem arginate and haematin dose-dependently decreased the urinary excretions of porphyrin precursors. They inhibited significantly the induction of hepatic delta-aminola-evulinic acid synthase. Haem arginate and haematin could restore the activity of haem oxygenase and after higher doses they increased the activity. The dose-effect relationships of the two haem compounds were demonstrated.

Mechanisms regulating cell resistance to adriamycin. Evidence that drug accumulation in resistant cells is modulated by phosphorylation of a plasma membrane glycoprotein

Incubation of adriamycin resistant Chinese hamster lung cells with the calmodulin inhibitor trifluoperazine (TFP) resulted in a significant increase in the cellular accumulation of drug. When resistant cells were prelabeled with 32Pi and then treated with TFP, a major increase also occurred in the phosphorylation of a plasma membrane glycoprotein (P-180). The concentration of TFP required for inducing the superphosphorylation of this protein correlated well with the TFP concentration required for inducing an increase in drug accumulation in resistant cells. In addition to TFP, the Ca2+ channel blocker verapamil also induced drug uptake and enhanced the phosphorylation level of P-180. Additional studies showed that, when resistant cells reverted to drug sensitivity, there was a parallel loss in the TFP-induced P-180 phosphorylation. The results of this study indicate that the trifluoperazine-induced uptake of drug in resistant cells is mediated by a mechanism which involves an enhanced phosphorylation of P-180. It is suggested that, when this protein is superphosphorylated, it becomes biologically inactive, and that this results in the conversion of the resistant cell to one having a drug sensitive phenotype.

Evidence for the in vitro metabolism of allylisopropylacetamide to reactive intermediates. Mechanistic studies with oxygen-18

Metabolism of allylisopropylacetamide (AIA) (1) in microsomal preparations from phenobarbital-pretreated rats is shown to proceed by way of three cytochrome P-450-dependent pathways: (i) aliphatic (C-3') hydroxylation, (ii) allylic (C-3) hydroxylation and (iii) olefin oxidation. The latter represents the major route of biotransformation and leads ultimately to the formation of the gamma-butyrolactone 2. In order to elucidate the mechanism by which AIA is converted to this gamma-lactone, and to gain information on the nature of chemically reactive intermediates in the process, the metabolism of AIA to 2 was investigated in 18O2 or H218O and the pattern of label incorporated into the product was determined by gas chromatography/mass spectrometry (GC/MS). The results support the formation of AIA epoxide as an initial product of olefin oxidation and indicate that this species undergoes rapid intramolecular rearrangement to a protonated iminolactone which, in turn, is hydrolysed to the stable gamma-lactone. On the other hand, the 'dihydrodiol' metabolite of AIA, which would be expected to result from direct hydrolysis of AIA epoxide, was not detected in incubation products and, furthermore, the 18O labeling data specifically exclude the possibility that it served as a precursor of 2. It may be concluded, therefore, that AIA epoxide and the protonated iminolactone to which it gives rise represent reactive intermediates in the oxidation of AIA which may play a key role in the alkylation of certain cellular constituents which accompanies metabolism of AIA by liver enzymes.

Epoxidation of olefins by cytochrome P-450 model compounds: mechanism of oxygen atom transfer

The mechanism of the Mn(III) porphyrin-catalyzed epoxidation of olefins by lithium hypochlorite is examined. The active oxidant is thought to be a high-valent manganese-oxo complex. It is shown that a relatively stable intermediate is reversibly formed upon interaction of the olefin and the oxo complex. The decomposition of this intermediate to Mn(III) porphyrin and epoxide is the rate-determining step of the catalytic cycle. Some analogies to the biochemical epoxidation of olefins catalyzed by cytochrome P-450 are discussed.

Regulation of heme metabolism and cytochrome P-450 levels in primary culture of rat hepatocytes in a defined medium

Liver cells were prepared from adult Sprague-Dawley rats and used for the determination of delta-aminolevulinic acid synthetase (ALAS) activity and cytochrome P-450 concentrations at different time intervals in tissue culture in a serum-free synthetic medium. During the first 24 hr in culture, the level of cytochrome P-450 decreased to 30-40% of the level in isolated liver cells from untreated animals. The disappearance of cytochrome P-450 was especially fast in hepatocytes obtained from female phenobarbital-treated rats where only 40% of the original cytochrome P-450 was present after 2 hr in culture and 80% had disappeared in 2 days. The activity of ALAS increased 3- to 4-fold when measured 2 hr after plating, and it reached the maximum level in 19-24 hr when its activity was about eight times the original activity. In 2-4 days in culture, the activity of ALAS was four to five times above the original level. When the amount of delta-aminolevulinic acid (ALA) in the medium was increased from 1 to 100 microM, a decrease in ALAS was obtained, but no significant increase in cytochrome P-450 level was observed. Addition of heme to the medium gave a dose-dependent decrease in the activity of ALAS. Our data indicate that during the first 24 hr in culture the increase of ALAS activity was prevented by exogenous heme. This effect may be due to inhibition of the catalytic activity, suppression of the synthesis of the enzyme, or accelerated breakdown of the enzyme by heme.

Effect of heme on allylisopropylacetamide-induced changes in heme and drug metabolism in the rhesus monkey (Macaca mulatta)

In rhesus monkeys, in which porphyria was induced by the administration of allylisopropylacetamide (AIA), hepatic delta-aminolevulinic acid synthase (ALA-S) was increased. Cytochrome P-450 and associated monooxygenase activities and microsomal heme oxygenase activity were decreased in these animals. Administration of heme for 4 days concurrently with AIA prevented the induction of hepatic ALA-S but produced further decreases in cytochrome P-450 and monooxygenase activities. The decrease in heme oxygenase activity elicited by AIA alone was partially reversed. Administration of heme alone caused an impairment of hepatic drug metabolism but had no significant effect on heme metabolism. The porphyric monkeys showed elevation of porphyrin levels in blood and urine. When heme was administered concurrently with AIA, blood porphyrin levels were further elevated, while the urinary excretion of porphyrins was lower than that following treatment of monkeys with AIA. Following the administration of heme alone, blood and urinary porphyrin levels were minimally affected. These results suggest that repeated heme administration in the primate may adversely affect drug metabolism by the liver.

Activation of 3,4,3',4'-tetrachlorobiphenyl to protein-bound metabolites by rat liver microsomal cytochrome P-448-containing monooxygenase system

The specificity of two major types of cytochrome P-450 of rat liver microsomes induced by phenobarbital (PB) and 3-methylcholanthrene (MC) toward activation of three 14C-labeled tetrachlorobiphenyl (TCB) isomers to protein-bound metabolites was examined by intact microsomal and reconstituted monooxygenase systems. The monooxygenase system containing PB-inducible form(s) of cytochrome P-450 was much more active than those inducible by MC in activating two TCB isomers, i.e., 2,4,2',5'-[14C]TCB, for the binding reaction. However, the binding of 3,4,3',4'-[14C]TCB was catalyzed most actively by the system containing a MC-inducible (P-448 type) cytochrome P-450 from rat liver microsomes. Thus two forms of cytochrome P-450 had different substrate specificities for the metabolism of TCB. In another series of experiments, the site of protein molecule bound to TCB metabolites was examined by the cytochrome P-448-containing reconstituted system with 3,4,3',4'-[14C]TCB as substrate. The following lines of evidence supported the hypothesis that the active metabolites of TCB bind covalently to cysteine residues of protein molecules a) TCB-binding activity was strongly inhibited by thiol compounds such as cysteine and glutathione and b) of the various kinds of proteins tested as target for TCB metabolites, only those which contained free sulfhydryl groups on the protein molecule were radiolabeled. Moreover, we found that during the metabolism of 3,4,3',4'-TCB, the heme content of the reconstituted system was lowered, probably reflecting the alkylation of the sulfhydryl groups in the active sites of cytochrome P-448.

The effect in vivo and in vitro of allylisopropylacetamide on the content of hepatic microsomal cytochrome P-450 2 of phenobarbital treated rabbits

Rabbits treated with phenobarbital were given a single injection of allylisopropylacetamide (AIA) s.c. and/or heme i.v. Hepatic microsomes were isolated 1, 5 and 24 hours post injection and the microsomal contents of both total cytochrome P-450 chromophore, and the protein moiety of P-450 2 were determined by spectrophotometric and immunochemical methods respectively. AIA caused the levels of total P-450 chromophore and of P-450 2 protein to decline to 30% of the control values at 5 hours post-injection. Concurrent administration of heme with AIA diminished the decrease in the total microsomal content of P-450 chromophore but not in that of P-450 2 protein. These findings suggest that the destruction of the heme prosthetic group of P-450 by suicide substrates such as AIA may lead to an enhanced degradation of the apo-P-450.

Cytochrome P-450 metabolic-intermediate complex formation and induction by macrolide antibiotics; a new class of agents

1. By several criteria, macrolide antibiotics constitute a new class of nitrogenous cytochrome P-450 metabolic-intermediate complex-forming compounds. 2. Macrolide antibiotic metabolic-intermediate complexes are only formed in livers induced with phenobarbital or with the macrolide antibiotics themselves. The extent of metabolic-intermediate complex formation in microsomes from phenobarbital-induced rats is lower than that seen for members of the amphetamine and SKF 525-A classes of compounds. 3. Cytochrome P-450 induced by macrolide antibiotics, of which troleandomycin is the most potent, is extensively sequestered as a metabolic intermediate complex in vivo. 4. Cytochrome P-450 induced by troleandomycin differs, using several criteria, from those induced by phenobarbital, beta-naphthoflavone or SKF 525-A, and those present in uninduced rats.

Tryptophan pyrrolase in haem regulation. The mechanisms of enhancement of rat liver 5-aminolaevulinate synthase activity by starvation and of the glucose effect on induction of the enzyme by 2-allyl-2-isopropylacetamide

1. Rat liver tryptophan pyrrolase activity is enhanced by a hormonal-type mechanism during the first 2 days of starvation and by a substrate-type mechanism during the subsequent 2 days. 5-Aminolaevulinate synthase activity is also enhanced during the first 2 days of starvation, but returns thereafter to values resembling those observed in the fed rat. Treatments that prevent or reversé the enhancement of tryptophan pyrrolase activity in 24-48h-starved rats also abolish that of 5-aminolaevulinate synthase activity. Starvation of guinea pigs, which does not enhance the pyrrolase activity, also fails to alter that of the synthase. It is suggested that the decrease in 5-aminolaevulinate synthase activity in 72-96h-starved rats represents negative-feedback repression of synthesis, possibly involving tryptophan participation, whereas the enhancement observed in 24-48h-starved animals is caused by positive-feedback induction secondarily to increased utilization of the regulatory-haem pool by the newly synthesized apo-(tryptophan pyrrolase). 2. Glucose, fructose and sucrose abolish the 24h-starvation-induced increases in rat liver tryptophan pyrrolase and 5-aminolaevulinate synthase activities. Cortisol reverses the glucose effect on 5-aminolaevulinate synthase activity, presumably by enabling pyrrolase to re-utilize the regulatory-haem pool after induction of synthesis of this latter enzyme. 3. The impaired ability of 2-allyl-2-isopropylacetamide to enhance markedly 5-aminolaevulinate synthase activity in 24h-starved rats treated with glucose is associated with a failure of the porphyrogen to cause loss of tryptophan pyrrolase haem. Cortisol restores the ability of the porphyrogen to destroy tryptophan pyrrolase haem and to enhance markedly 5-aminolaevulinate synthase activity, presumably by enhancing tryptophan pyrrolase synthesis and, thereby, its re-utilization of the regulatory-haem pool. It is tentatively suggested that 2-allyl-2-isopropylacetamide destroys the above pool only after it has become bound to (or utilized by) apo-(tryptophan pyrrolase).

Oxidation of trichloroethylene by liver microsomal cytochrome P-450: evidence for chlorine migration in a transition state not involving trichloroethylene oxide

Trichloroethylene (TCE) was metabolized by cytochrome P-450 containing mixed-function oxidase systems to chloral (2,2,2,-trichloroacetaldehyde), glyoxylic acid, formic acid, CO, and TCE oxide. TCE oxide was synthesized, and its breakdown products were analyzed. Under acidic aqueous conditions the primary products were glyoxylic acid and dichloracetic acid. The primary compounds formed under neutral or basic aqueous conditions were formic acid and CO. TCE oxide did not form chloral in any of these or other aqueous systems, even when iron salts, ferriprotoporphyrin IX, or purified cytochrome P-450 was present. Ferric iron salts catalyzed the rearrangement of TCE oxide to chloral only in CH2Cl2 or CH3CN. A 500-fold excess of iron was required for complete conversion. A kinetic model involving the zero-order oxidation of TCE to TCE oxide by cytochrome P-450 and the first-order degradation of the epoxide was used to test the hypothesis that TCE oxide is an obligate intermediate in the conversion of TCE to other metabolites. Kinetic constants fo the breakdown of TCE oxide and for the oxidative metabolism of TCE to stable metabolites were used to predict epoxide concentrations required to support the obligate intermediacy of TCE oxide. The maximum levels of TCE oxide detected in systems using microsomal fractions and purified cytochrome P-450 were 5-28-fold lower than those predicted from the model. The kinetic data and the discrepancies between the observed metabolites and TCE oxide breakdown products support the view that the epoxide is not an obligate intermediate in the formation of chloral, and an alternative model is presented in which chlorine migration occurs in an oxygenated TCE-cytochrome P-450 transition state.

The destruction of cytochrome P-450 by alclofenac: possible involvement of an epoxide metabolite

The metabolism of alclofenac (4-allyloxy-3-chlorophenyl acetic acid) and its ability to induce destruction of cytochrome P-450 was studied in mouse hepatic microsomes obtained from control animals or animals pretreated with phenobarbitone (PB) or 3-methyl-cholanthrene (3-MC). No evidence was obtained for metabolism of alclofenac in control microsomes although in induced microsomes alclofenac was metabolised to both the dihydroxy (DHA) and phenolic (4-hydroxy-3-chlorophenyl acetic acid: HCPA) metabolites. Significant destruction of cytochrome P-450 was observed when alclofenac was incubated with microsomes from mice pretreated with PB but not from untreated or 3-MC-treated mice. This destruction is dependent on the presence of a NADPH-regenerating system and in inhibited in the presence of SKF-525A, metyrapone, glutathione and cysteine. The stable metabolites DHA and HCPA caused no loss of cytochrome P-450 whereas the reactive intermediate, alclofenac epoxide, was a potent inducer of destruction in the absence of NADPH. These results suggest that destruction of cytochrome P-450 by alclofenac in vitro is mediated, at least in part, through the formation of a reactive epoxide metabolite.

Cytochrome P-450 inactivation: structure of the prosthetic heme adduct with propyne

Hepatic microsomal cytochrome P-450 from phenobarbital-pretreated rats is destroyed by propyne in a reduced nicotinamide adenine dinucleotide dependent process which also results in vivo in the accumulation of an abnormal green porphyrin. The green porphyrin has been identified by its electronic absorption, mass spectrometric, and nuclear magnetic resonance properties as the isomer of N-(2-oxopropyl)protoporphyrin IX in which the alkylated nitrogen is that of pyrrole ring A. Alkylation of the other nitrogens in the parent heme is quantitatively unimportant, although evidence for traces of the resulting adducts has been obtained. The green porphyrin exhibits a circular dichroism spectrum and is therefore the result of a chirally selective or specific interaction. The structure of the green porphyrin implicates a cytochrome P-450 destructive mechanism in which a species formed by catalytic oxidation of the acetylenic moiety reacts with the nitrogens of prosthetic heme. The possible nature of the reactive intermediate is discussed.

Role of haem in the induction of cytochrome P-450 by phenobarbitone. Studies in chick embryos in ovo and in cultured chick embryo hepatocytes

The role of haem synthesis during induction of hepatic cytochrome P-450 haemoproteins was studied in chick embryo in ovo and in chick embryos hepatocytes cultured under chemically defined conditions. 1. Phenobarbitone caused a prompt increase in the activity of 5-aminolaevulinate synthase, the rate-limiting enzyme of haem biosynthesis, and in the concentration of cytochrome P-450. This induction response occurred without measurable initial destruction of the haem moiety of cytochrome P-450. 2. When intracellular haem availability was enhanced by exogenous haem or 5-aminolaevulinate, phenobarbitone-medicated induction of cytochrome P-450 was not affected in spite of the well known repression of 5-aminolaevulinate synthase by haem. These data are consistent with the concept that haem does not regulate the synthesis of cytochrome P-450 haemoproteins. 3. Acetate inhibited haem biosynthesis at the level of 5-aminolaevulinate formation. When intracellular haem availability was diminished by treatment with acetate, phenobarbitone-medicated induction was decreased. 4. This inhibitory effect of acetate on cytochrome P-450 induction was reversed by exogenous haem or its precursor 5-aminolaevulinate. These data suggest that inhibition of haem biosynthesis does not decrease synthesis of apo-cytochrome P-450. Moreover, they indicate that exogenous haem can be incorporated into newly formed aop-cytochrome P-450.

Differential responses to inducers of delta-aminolaevulinate synthase and haem oxygenase during pregnancy

The responses of hepatic delta-aminolaevulinate synthase and microsomal haem oxygenase to inducers were examined in pregnant rats. 2-Allyl-2-isopropylacetamide-mediated induction of delta-aminolaevulinate synthase was greatly decreased during pregnancy and in the early post-partum period. Administration of allylisopropylacetamide to pseudopregnant rats induced delta-aminolaevulinate synthase normally. Treatment of pregnant rats with cortisol failed to restore the drug-mediated induction of delta-aminolaevulinate synthase. Microsomal cytochrome P-450 content and the activities of drug-metabolizing enzymes such as aniline hydroxylase and ethylmorphine. N-demethylase were significantly lowered during pregnancy. In contrast with the greatly impaired induction of delta-aminolaevulinate synthase, the induction of haem oxygenase in response to CoCl2 remained unaltered in pregnant rats. The normal perturbations of delta-aminolaevulinate synthase, consisting of an initial inhibition followed by a rebound increase in the enzyme activity associated with CoCL2 treatment, were observed during pregnancy. These findings indicate that hormones and metabolic factors associated with gestation exert significant but differential controls on the induction patterns of delta-aminolaevulinate synthase and haem oxygenase.

Autocatalytic alkylation of the cytochrome P-450 prosthetic haem group by 1-aminobenzotriazole. Isolation of an NN-bridged benzyne-protoporphyrin IX adduct

Destruction of hepatic cytochrome P-450 during catalytic processing of 1-amino-benzotriazole is accompanied by an equal loss of microsomal haem but not by loss of cytochrome b5, or stimulation of lipid peroxidation. An abnormal porphyrin, tentatively identified as an NN-bridged benzyne-protoporphyrin IX adduct, appears to be formed by the addition of catalytically generated benzyne to prosthetic haem.

N-Methylprotoporphyrin IX: chemical synthesis and identification as the green pigment produced by 3,5-diethoxycarbonyl-1,4-dihydrocollidine treatment

The hepatic pigment accumulated in 3,5-diethoxycarbonyl-1,4-dihydrocollidine-treated rats, which has been reported to inhibit ferrochelatase, has been isolated and purified. The pigment has been resolved into one major, one minor, and two trace components, all of which appear to be isomeric porphyrins. The major fraction has been unambiguously identified by spectroscopic methods as the isomer of N-methylprotoporphyrin IX (isolated as the dimethyl ester) in which vinyl-substituted pyrrole ring A is methylated. The minor product appears to be an isomer of the same porphyrin with the N-methyl group on propionic acid-substituted ring C, and the trace components have the same high-pressure liquid chromatography retention times as the other two possible isomers of the porphyrin. The four isomers of N-methylprotoporphyrin IX have been chemically synthesized, independently characterized, and used to confirm the structures of the biologically products.