Inactivation of hepatic cytochrome P-450 by allenic substrates

Improved methods for the detection of heme and protoporphyrin IX adducts and quantification of heme B from cytochrome P450 containing systems

Heme B is a critical prosthetic group for the function of numerous proteins including the cytochrome P450 (CYP) family of enzymes. CYP enzymes are involved in the metabolism of endogenous and xenobiotic molecules that are of central interest in drug development. Formation of reactive metabolites by CYPs can lead to heme modification and destruction of the enzyme. The structure of the adducted heme can provide key information on the mechanism of inactivation, which is of great interest during preclinical drug discovery. Historically, techniques to extract the modified heme or protoporphyrin IX species involved harsh extraction conditions and esterification of propionate groups to aid chromatography. We have developed a simplified extraction method and LC/MS chromatography system that does not require derivatization to quantify heme B and identify modified heme B species from multiple CYP-containing matrices. The method uses mass defect filter triggered data dependent MS2 scans to rapidly identify heme and protoporphyrin IX adducts. These methods may also be useful for the analysis of other heme variants and hemoproteins.

Substrate-dependent irreversible inactivation of cytochrome P-450: conversion of its haem moiety into modified porphyrins

2-Allyl-2-isopropylacetamide and other drugs containing either an allyl, a vinyl or an ethynyl unsaturated side chain are metabolized by liver cytochrome P-450 to reactive derivatives that irreversibly inhibit the haemoprotein by a suicidal type of inactivation. The main target is the haem moiety of cytochrome P-450 which is converted into abnormal porphyrins. These have been isolated from the liver of treated rats, extensively purified and compared with model porphyrins. The abnormal porphyrins incorporate metal ions in vitro much more readily than does their parent porphyrin, protoporphyrin. They are also much more basic than protoporphyrin, and on titration with a strong acid they readily give rise to a porphyrin monocation which then requires relatively large amounts of acid for conversion to the porphyrin dication. In all these respects and also in the intensity of their bathochromic shifts these abnormal porphyrins closely resemble N-alkylated porphyrins and they markedly differ from porphyrins that are substituted at one of their meso-carbon positions or which bear electron-withdrawing substituents at the beta-positions of the pyrrole rings. This suggests strongly that reactive derivatives of the unsaturated drugs act as electrophilic reagents and alkylate one of the pyrrole nitrogen atoms of cytochrome P-450. A model centred on the apoprotein of cytochrome P-450 is considered for the degradation of liver haem caused by unsaturated drugs. The apocytochrome may accept exchangeable pools of liver haem for degradation, leading to a state of haem depletion and to activation of delta-aminolaevulinate synthase.

Suicidal destruction of cytochrome p-450 by ethynyl substituted compounds

Compounds containing a terminal carbon-carbon triple bond, ranging in structure from the 17α-ethynyl substituted contraceptive steroids to acetylene gas, when administered to rats cause a selective and rapid time dependent loss of up to 50% of hepatic cytochrome P-450. Cytochrome b5 is not affected. Metabolic activation of the acetylenic substituent by the phenobarbital inducible, NADPH dependent, mixed function oxidases results in the formation of a reactive species which alkylates one of the tetrapyrrole nitrogen atoms of heme to form a 1 : 1 covalent adduct. Either cytochrome P-450 destruction or the formation of N-alkylated porphyrins (green pigments) have been used to assess factors affecting the extent of metabolic activation of the acetylenic group in rats, man, and other laboratory species. The chemical identity of a number of green pigments have been resolved. Those formed from 1-octyne consist of the protoporphyrin IX ring of heme substituted with the saturated 2-oxo-octyl group. In contrast, reactive metabolites of 1-octyne trapped with N-acetylcysteine contain the unsaturated 3-oxo-octenyl substituent. Two independent routes of activation of terminal acetylenes have been described to account for these results. Both pathways can lead to cytochrome P-450 loss but only one, probably involving an oxirene intermediate, leads to green pigment formation.

Metabolism of vinylcyclooctane and partition ratio between epoxide formation and cytochrome P-450 destruction

Vinylcyclooctane (VCO), which binds to the active site of cytochrome P-450 (P-450) giving a type I difference spectrum, has been found to form the corresponding epoxide as the main metabolite on treatment with liver microsomal monooxygenase obtained from phenobarbital-treated or untreated mice. During this metabolic process about 40% of the microsomal P-450 isozymes are destroyed, but the remainder still demethylates aminopyrine. Approx. 180 molecules of VCO are turned over and 132 of epoxyethylcyclooctane (EECO) are formed for each destructive event.

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'.

Cytochrome P-450 mediated interaction between hydroperoxide and molecular oxygen. A proposed method for P-450 kinetic studies

Rat liver cytochrome P-450 mediates a novel reaction between equimolar quantities of dissolved oxygen and organic hydroperoxides. The reaction shares some of the properties of both NADPH-O2 dependent hydroxylation and NADPH-O2 independent peroxidase reactions, but does not require either NADPH, phosphatidylcholine, or any substrates other than hydroperoxide and oxygen. It proceeds at a rate approximately 100 times faster than other well known P-450 hydroxylation reactions. Monitoring the rate of O2 consumption in this novel reaction may be a simple and rapid means for studying the kinetics of cytochrome P-450.

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.

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.

Destruction of liver haem by norethindrone. Conversion into green pigments

1. Factors affecting the norethindrone-mediated conversion of hepatic haem into green pigments have been studied in the rat. Concentrations of haem and green pigments were estimated spectrophotometrically after esterification and separation by silica gel high-pressure liquid chromatography (h.p.l.c.). 2. Accumulation of green pigments in the liver was dependent on the dose of steroid and the time after dosing, maximum values being reached after 4-8h. Phenobarbitone pretreatment of rats resulted in an 8-fold increase in the concentration of green pigments at these times. 3. In microsomal systems in vitro, the formation of green pigments in the presence of NADPH and norethindrone was also dependent on the concentration of steroid and incubation times. Reaction rates very rapidly became non-linear with time, consistent with the self-catalysed destruction of the form(s) of cytochrome P-450 responsible for the metabolic activation of norethindrone. Microsomal mixtures incubated for a short period of time (1min) with norethindrone gave only one green-pigment peak after h.p.l.c. Longer incubation times gave four or five additional green pigments. Results suggested that multiple green pigments may arise by metabolic transformation of a single precursor. 4. When liver haem was prelabelled with (14)C by using 5-amino[4-(14)C]laevulinic acid, subsequent dosing with norethindrone in vivo gave rise to three major (14)C-labelled-green-pigment peaks on h.p.l.c. None of these components had the same retention times as the green pigments produced by microsomal fractions in vitro. 5. When liver haem was prelabelled with (59)Fe by using (59)FeCl(3), norethindrone administration resulted in the detection of (59)Fe-labelled green pigments if subsequent esterification was carried out under neutral conditions with trimethyloxonium tetrafluoroborate, but not when carried out under acidic conditions with methanol/H(2)SO(4). These results suggested that green pigments normally contain chelated iron and that metal-free green pigments are not produced by the liver.