Determination of partition ratios for allylisopropylacetamide during suicidal processing by a phenobarbital-induced cytochrome P-450 isozyme from rat liver

The oxidative inactivation of cytochrome P450 in monooxygenase reactions

Possible mechanisms of cytochrome P450 self-inactivation during catalytic turnover have been considered. Two ways of hemoprotein inactivation are so far known. The first, studied extensively by many authors, is the formation of active substrate intermediates, capable of modifying heme and apoenzyme. The second way, revealed quite recently and resulting from uncoupled cytochrome P450-catalyzed monooxygenase reactions, is yet to be clarified. Briefly, it involves formation of hydrogen peroxide in the hemoprotein active center, which interacts with the enzyme associated Fe2+, thereby generating hydroxyl radicals that bleach the heme and modify the apoenzyme. This mechanism operates with substrates and cytochrome P450 forms with partially coupled monooxygenase reactions, thus causing the formation of hydrogen peroxide as a byproduct.

The mechanism of the suicidal, reductive inactivation of microsomal cytochrome P-450 by carbon tetrachloride

1. Stoichiometric losses of microsomal haem and cytochrome P-450 were observed when carbon tetrachloride (CCl4) was incubated anaerobically with rat liver microsomes using NADPH or sodium dithionite as a reducing agent. A rapid destruction of haem was also observed during the non-enzymatic reductive incubation of CCl4 with soluble haem preparations (methaemalbumin) in presence of sodium dithionite. The results indicate that haem is both the site and the target of the suicidal activation of CCl4 by cytochrome P-450. 2. When an additional, fluorimetric assay for haem determination was used, an equimolar loss of protoporphyrin IX fluorescence was also observed in both the enzymatic and non-enzymatic system, indicating that the haem moiety of cytochrome P-450 has undergone a structural change, involving either loss or labilization of the porphyrin tetrapyrrolic structure. In both systems the loss of porphyrin was prevented by carbon monoxide (CO). 3. A dichlorocarbene-cytochrome P-450 ligand complex is partially responsible for the difference spectrum obtained on addition of CCl4 to anaerobically reduced rat liver microsomes. A molar extinction coefficient for this complex has been calculated. The carbene trapping agent 2,3-dimethyl-2-butene (DMB) strongly inhibited (greater than 95%) the formation of this spectrum but did not modify the loss of haem in reduced CCl4-supplemented microsomal incubations. The results suggest that dichlorocarbene (:CCl2) is not significantly involved in CCl4-dependent haem destruction. 4. Pretreatment of rats with different microsomal enzyme inducers was responsible for similar but not identical patterns of :CCl2 and CO formation and haem loss during incubation of CCl4 with reduced microsomes. This indicates a critical role of CCl4 metabolism in the suicidal destruction of cytochrome P-450 haem and suggests that the apoprotein of cytochrome P-450 is capable of modulating not only the metabolism of CCl4 to :CCl2 but also the hydrolysis of :CCl2 to CO. 5. Inactivation of cytochrome P-450 by CCl4 with reduced microsomes from Aroclor-pretreated rats was saturable and followed pseudo first-order kinetics. This provides further evidence to conclude that CCl4 activation is a suicidal process where the reactive metabolite(s) formed bind to haem, we predict, in a one to one stoichiometry. 6. The partition ratio between loss of cytochrome P-450 haem and CCl4 metabolism by liver microsomes from Aroclor pretreated rats has been investigated using limiting concentrations of CCl4. It was calculated that approximately 26 molecules of CCl4 had to be metabolised to achieve the loss of one molecule of haem.

Inactivation of cytochrome P-450 by 2-isopropyl-4-pentenamide and other xenobiotics leads to heme-derived protein adducts

When cytochrome P-450 in phenobarbital-induced rat liver microsomes was destroyed by 2-isopropyl-4-pentenamide (AIA) in vitro, 50% of the degraded heme was recovered as heme-derived products irreversibly bound to microsomal proteins. In contrast, less than 50% of the degraded heme was accounted for as N-alkylated porphyrins. Furthermore, 64% of the irreversibly bound products was bound specifically to a 54-kD form of cytochrome P-450. Several other compounds which have been reported to destroy cytochrome P-450 by forming N-alkylated porphyrins also produced heme-derived protein adducts. These findings indicate that the formation of heme-derived protein adducts may represent an important pathway for the irreversible degradation of cytochrome P-450 by many xenobiotics.

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.

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.

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.

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.