Inhibition of glutathione excretion, bile flow, and alterations of the glutathione status by 4-nitrosophenetol during perfusion of rat livers

Cytotoxicity of aromatic amines in rat liver and oxidative stress

A possible role of oxidative stress in producing acute toxicity in rat liver by aromatic amines and nitroarenes was tested. Oxidative stress was assessed by measuring the excretion of oxidized glutathione (GSSG) into the bile in isolated perfused livers and in female Wistar rats with cannulated bile ducts. The liver perfusion system was calibrated with t-butylhydroperoxide (t-BH) and menadione. The minimal concentration in the perfusate of t-BH necessary to observe a significant effect was 18 microM for 5 min. It was calculated that rat liver is able to cope with an extra production of about 70 nmol GSSG per min and g liver before GSSG is excreted into bile. No effect was observed when 2-aminofluorene, 2-acetylaminofluorene (AAF), trans-4-aminostilbene, and trans-4-acetylaminostilbene were added to the perfusate at 50 microM for 20 min. Moreover, 2-aminofluorene, trans-4-aminostilbene, 2-nitrofluorene and trans-4-nitrostilbene did not increase GSSG excretion when administered simultaneously with effective concentrations of t-BH. AAF was not acutely toxic, blood transaminases and lipid peroxidation were not increased with AAF doses as high as 1 mmol/kg. Since the dose rate of aromatic amines, like AAF, in feeding studies for tumor formation is about 100 times below that examined in the isolated perfused livers, it is highly unlikely that oxidative stress is generated by metabolites able to undergo redox cycling and that reactive oxygen contributes to acute toxic effects.

In vivo EPR studies of the metabolic fate of nitrosobenzene in the mouse

We report the first demonstration of EPR spectroscopy to study free radical reactions in live mice and excised muscle tissue resulting from the metabolism of nitrosobenzene. A broad three-line EPR spectrum (aN = 11.6 G) appeared in the buttocks region of a mouse place in an L-band loop gap resonator after intramuscular or intraperitoneal injection of 0.2 mmol/kg nitrosobenzene. The signal intensity reached a maximum at 20 to 30 min and remained constant well beyond 2 h. If muscle tissue was dosed with nitrosobenzene and excised within 5 min, a similar three-line X-band EPR spectrum was obtained which was preceded by the rapid growth and subsequent decay of an EPR spectrum identical with that of the phenylhydronitroxide radical, which was presumably generated from reactions between nitrosobenzene and reducing agents in the blood or tissue such as NADH or ascorbic acid. A model system containing nitrosobenzene and unsaturated fatty acids (olive oil or animal fat) yielded an identical three-line spectrum resulting from radical adducts of nitrosobenzene across the double bond. Overall, these results suggest that the most probable mechanism in vivo was nitrosobenzene covalently adding ("binding") to polyunsaturated fatty acid clusters in fat or membranes.

N-hydroxy-N-arylacetamides--IV. Differences in the mechanism of haemoglobin oxidation in vitro between N-hydroxy-N-arylacetamides and arylhydroxylamines

In solutions of purified human haemoglobin N-hydroxy-4-chloroacetanilide (N-hydroxy-4ClAA] was one of the most active compounds and N-hydroxy-acetanilide (N-hydroxy-AA) was the least active compound among the six N-hydroxy-N-arylacetamides tested for ferrihaemoglobin (HbFe3+)-forming activity. Co-oxidation of haemoglobin by N-hydroxy-4-chloroacetanilide was compared with that of N-hydroxy-4-chloroaniline(N-hydroxy-4ClA) and found to differ in the kinetics of HbFe2+-oxidation, in the catalytic activity of the two compounds, in the activation energy, and in the product pattern, indicating that the mechanism by which N-hydroxy-N-arylacetamides oxidize oxyhaemoglobin in vitro is different from that of arylhydroxylamines. Attempts have failed to detect by EPR spectroscopy acetyl 4-chlorophenyl nitroxide radical, the postulated catalytically-active oxidation product of N-hydroxy-4-chloroacetanilide.

Formation of 4-ethoxy-4'-nitrosodiphenylamine in the reaction of the phenacetin metabolite 4-nitrosophenetol with glutathione

Phenacetin, a constituent of several analgesic and antipyretic formulations has been made responsible for a variety of toxic and carcinogenic actions. 4-Nitrosophenetol, the N-oxydation product of intermediate 4-phenetidine, forms methemoglobin and binds covalently to sulfhydryl groups of proteins and glutathione. In the reaction of 4-nitrosophenetol with glutathione and other thiols an intermediate so-called "semimercaptal" is formed from which N-(thiol-S-yl)-4-phenetidine S-oxide, N-(thiol-S-yl)-4-phenetidine and 4-phenetidine derive. Besides thiol adducts, a yellow compound is formed which was isolated as a pure crystalline product (elemental analysis) and identified by FAB-MS, EI-MS, 13C-, 1H-NMR, and UV-VIS spectroscopy as 4-ethoxy-4'-nitrosodiphenylamine. This nitrosoarene is formed by an unknown mechanism from 4-nitrosophenetol and 4-phenetidine under liberation of ethanol. In human erythrocytes this compound is easily reduced to 4-amino-4'-ethoxydiphenylamine (FAB-MS, EI-MS, 13C-NMR). During the reaction of 4-nitrosophenetol with red cells only traces of 4-ethoxy-4'-nitrosodiphenylamine were formed, whereas up to 10% appeared as the reduction product 4-amino-4'-ethoxydiphenylamine. This latter compound is unstable in red cells and is metabolized further to unidentified products.

Identification of a reactive intermediate of furazolidone formed by swine liver microsomes

Furazolidone (N-(5-nitro-2-furfurylidene)-3-amino-2-oxazolidone) is metabolized by swine liver microsomes under aerobic and anaerobic conditions (rate: 2.55 and 3.25 nmol/mg protein/min, respectively). Covalent binding to microsomal protein amounted aerobically to 0.29 nmol/mg protein/min. Of all amino acids tested, only addition of cysteine to the incubation mixture decreased microsomal protein binding of furazolidone, indicating that covalent binding may occur at protein thiol groups. Two known metabolites of furazolidone, 3-(4-cyano-2-oxobutylidene-amino)-2-oxazolidone and 2,3 dihydro-3-cyano-methyl-2-hydroxyl-5-nitro-1 alpha,2-di(2-oxo-oxazolidin-3-yl) iminomethyl-furo[2,3-b] furan, were minor metabolites. At least 50% of total metabolites is formed by swine liver microsomes via a reductive process of furazolidone as indicated by the formation of a furazolidone-mercaptoethanol conjugate after the addition of mercaptoethanol to the incubation mixture. The conjugate was identified as 3-(4-cyano-3-beta-hydroxyethylmercapto-2-oxobutylidene amino)-2-oxazolidone, indicating that the open-chain acrylonitrile-derivative is the reactive intermediate of furazolidone which also may be responsible for interaction with protein.

Evaluation of the micromethod for determination of glutathione using enzymatic cycling and Ellman's reagent

Evaluation of the kinetic parameters of the various reactions involved in the determination of glutathione provided the rationale for a modification of the frequently used assay (F. Tietze, 1969, Anal. Biochem. 27, 502-522) whereby the enzymatic reaction is no longer rate limiting. At pH 6.0, the nonenzymatic thiol interchange reaction of reduced glutathione (GSH) with Ellman's reagent becomes rate limiting, and inhibition of glutathione reductase up to 50% has no influence on the accuracy of the determination. The lower level of sensitivity is 10(-10) mol glutathione with a linear response up to 5 X 10(-9) mol. For determination of glutathione disulfide, GSH is alkylated by N-ethylmaleimide (NEM), and excess NEM is removed by extraction with ethyl acetate. Since the glutathione adduct is not stable, extracted samples are kept deep-frozen prior to analysis. Using this precaution, less than 0.05% of GSSG was determined in GSH-containing samples which had been previously freed from GSSG.

Formation and disposition of nitrosochloramphenicol in rat liver

It has been suggested that in the chloramphenicol-induced aplastic anemia nitrosochloramphenicol may be involved as a toxic intermediate. We found that aminochloramphenicol, which reportedly is formed from chloramphenicol by intestinal bacteria, is N-oxygenated by liver microsomes of untreated rats with apparent Km = 0.4 mM and Vmax = 0.28 nmole/min/mg protein. These values are in close agreement with those reported for aniline N-oxygenation. Reductive reactions, however, eliminate the N-oxygenation products at markedly higher rates. As judged from hemoglobin-free single-pass liver perfusion experiments, N-hydroxy-chloramphenicol is reduced at rates faster than 300 nmole/min/g liver wet, and nitrosochloramphenicol is eliminated at rates faster than 1.5 mumole/min/g liver. At least two NADPH- and two NADH-dependent cytosolic enzymes are responsible for nitrosochloramphenicol reduction. Determination of the kinetic parameters of these enzymes by stop-flow analysis revealed the contribution of enzymes, one of it being alcohol dehydrogenase, with Michaelis constants in the micromolar range. Despite this high reducing capacity, about 10% of nitrosochloramphenicol reacted with GSH under formation of glutathionesulfinamidochloramphenicol and GSSG released from the liver into bile and venous effluent. At high nitrosochloramphenicol load these reactions led to glutathione depletion of the liver, caused membrane damage, and impaired bile production. At low nitrosochloramphenicol load, i.e. below 0.5 mumole/min/g, no relevant nitrosochloramphenicol passed the liver. These data together with the previously reported reactions of nitrosochloramphenicol within human blood suggest that nitrosochloramphenicol, if formed at all in the intestine or liver, is rather unlikely to be transferred to the critical target.

Reactions of nitrosochloramphenicol in blood

It has been suggested that nitrosochloramphenicol (NOCAP), a possible metabolite of chloramphenicol (CAP), may be involved in CAP-induced aplastic anemia. We found that NOCAP was rapidly eliminated from human blood in vitro (more than 90% in less than 15 sec). Analysis of the different reactions showed that 5% of NOCAP was covalently bound to plasma proteins, mainly to albumin, the remainder being metabolized in red cells. The most important reaction in red cells was the very rapid adduct formation with GSH (k = 5,500 M-1S-1), yielding presumably a semimercaptal which either isomerized to a sulfinamide (GSONHCAP, k = 0.05 s-1) or was thiolytically cleaved by another GSH molecule with formation of the hydroxylamine (NHOHCAP) and GSSG (k = 7.1 M-1S-1). Another important elimination reaction was the covalent binding of NOCAP to the SH groups of hemoglobin (k = 5M-1S-1), also yielding a sulfinamide. Besides these reactions with thiols, NOCAP was enzymatically reduced to NHOHCAP in the presence of NADPH (Km NADPH = 10(-5) M; Km NOCAP = 10(-4) M; Vmax = 2 mumole/min per ml). This reaction was only effective at NOCAP concentrations below 10(-4)M, probably because of limited NADPH-regeneration. Further reduction of NHOHCAP to NH2CAP was a slow process which did not exceed 0.5 nmole/min per ml. NH2CAP was mainly formed from GSONHCAP, a reaction which depended on NADPH and the presence of hemolysate, indicating an enzymatic reaction. In contrast to smaller nitrosoarenes, NOCAP was a poor ligand for ferrohemoglobin (probably due to steric hindrance by its bulky molecule) and was therefore much more exposed to biotransformation. NOCAP and NHOHCAP formed ferrihemoglobin at a rate 5000 times slower than did phenylhydroxylamine. In contrast to NOCAP, NHOHCAP penetrated slowly the red cell membrane (4 about 5 min), and its disposition in blood was quite ineffective. From these data, it seems likely that most of the NOCAP formed by microorganisms in the intestine or produced in the liver, will be degraded in blood before it can reach the bone marrow.