Reaction of microsomal and cytosolic enzymes with N-arylacetohydroxamic acids

Reduction of hydroxamic acids to the corresponding amides catalyzed by rabbit blood

1. The hydroxamic acids N-hydroxyphenacetin and N-hydroxy-2-acetylaminofluorene were reduced to the corresponding amides, phenacetin and 2-acetylaminofluorene respectively by rabbit blood supplemented with both NAD(P)H and FAD. These reducing activities were found in erythrocytes but not in plasma, and were sensitive to inhibition by carbon monoxide and oxygen. When blood or erythrocytes were boiled, these activities were not abolished. 2. Haemoproteins such as haemoglobin and catalase exhibited the reductase activity in the presence of both NAD(P)H and FAD under anaerobic conditions. The activity was not abolished when the haemoproteins were boiled. 3. Haematin showed a significant reducing activity in the presence of these cofactors. The activity of haematin was also observed with the photochemically reduced form of FAD. 4. The reduction system in blood was composed of NAD(P)H, FAD and haemoglobin. Reduction appears to proceed in two steps, i.e. the reduction of FAD by NADH or NADPH, followed by the non-enzymatic reduction of the hydroxamic acids to the amides by reduced FAD, catalyzed by the haem group of haemoglobin in rabbit erythrocytes.

Formation of N-Arylacylhydroxamic Acids from Nitroso Aromatic Compounds in Isolated Spinach Leaf Cells

The formation of N-arylacetohydroxamic acids from nitroso aromatic compounds in the presence of pyruvate was investigated using isolated spinach leaf cells. The activity was enhanced by the addition of TPP, MgSO(4), and pyruvate, requirements for pyruvate dehydrogenase complex (PDHC). Measurement of the kinetic parameters revealed that the K(m) values of nitroso aromatic compounds tested were identical and that electron-donating ring substituents decreased the catalytic efficiency. The activation energy of the formation of N-phenylacetohydroxamic acid was lower than that reported for porcine heart PDHC. With alpha-oxo acids tested, alpha-oxobutyrate served as a substrate to give the corresponding N-phenylpropionylhydroxamic acid. The activity of spinach leaf cells in N-phenylacetohydroxamic acid formation was found in both mitochondria and chloroplasts. The contribution of chloroplast PDHC to total activity in the formation of N-phenylacetohydroxamic acid was estimated to be 50% under the conditions used.

Hemoglobin binding of monocyclic aromatic amines: molecular dosimetry and quantitative structure activity relationships for the N-oxidation

Aromatic amines are important intermediates in industrial manufacturing. N-oxidation to the N-hydroxyarylamines is a key step determining the genotoxic properties of aromatic amines. N-hydroxyarylamines can form adducts with DNA, with tissue proteins and with the blood proteins albumin and hemoglobin in a dose-dependent manner. The determination of hemoglobin adducts is a useful tool for biomonitoring exposed populations. We have established the hemoglobin binding index (HBI) [(mmol compound/mol Hb)/(mmol compound/kg body wt)] of several aromatic amines in female Wistar rats. Including the values obtained by other researchers in the same rat strain, the logarithm of hemoglobin binding (log HBI) was plotted against the following parameters: the sum of the Hammett constants (sigma sigma = sigma p + sigma m), pKa, log P (octanol/water), the half wave oxidation potential (E1/2) and the electronic descriptors of the amines and their corresponding nitrenium ions obtained by semiempirical calculations (MNDO, AM1 and PM3), such as atomic charge densities, energies of the HOMO and LUMO and their coefficients, the C-N bond order, the dipole moments and the 'reaction enthalpy' [MNDOHF, AM1HF or PM3HF = Hf(nitrenium) - Hf(amine)]. The correlation coefficients were determined from the plots of all parameters against log HBI for all amines by means of linear regression analysis. The amines were classified into three groups: group 1, all para-substituted amines, group 2, all amines with halogens and group 3, all amines with alkyl groups. For the amines of group 1, log HBI correlates with sigma sigma, MNDOHF, E1/2, the pKa and the log P with r = 0.84, 0.71, 0.73, - 0.69 and 0.50, respectively. For the amines of group 2, log HBI correlates with pKa, sigma sigma, MNDOHF, E1/2 and log P with r = 0.81, -0.76, -0.55, -0.46 and -0.20, respectively. For the amines of group 3, log HBI correlates with the E1/2, PM3HF, sigma sigma, pKa and log P with r = 0.92, 0.89, 0.76, 0.19 and 0.12, respectively. The apparent Michaelis-Menten constants Km and Vmax of the N-acetyltransferase of liver cytosol were determined for several amines. Km and Vmax do not correlate with any of the electronic descriptors. Female Wistar rats were dosed with nitroarenes. Hemoglobin binding of nitroarenes correlates with the energy levels of the LUMO. This investigation determines for a large variety of aromatic amines the bioavailability of the N-hydroxyarylamine--the genotoxic metabolite--and the utility of electronic descriptors for prediction of the N-oxidation.

Oxidation of the 2-hydroxyamino derivative of 2-amino-6-methyl-dipyrido[1,2-a: 3',2'-d] imidazole (Glu-P-1) to its 2-nitroso form, an ultimate form reacting with hemoglobin thiol groups

The binding to hemoglobin of synthetic 2-hydroxyamino-6-methyldipyrido[1,2-a: 3',2'-d] imidazole from the carcinogenic product of L-glutamic acid pyrolysis 2-amino-6-methyldipyrido[1,2-a: 3',2'-d] imidazole were investigated in vitro. The hydroxylamine required oxidation to its nitroso derivative to bind to rat hemoglobin through thiol groups. Oxidation of the hydroxylamine to the nitroso form was found to be enhanced by oxyhemoglobin and superoxide dismutase at pH 7.4 under aerobic conditions. Since these conditions might also enhance this oxidation in vivo, the conversion of the DNA-reactive arylhydroxylamines to the DNA-non-reactive nitroso compounds and their subsequent binding to highly abundant thiol groups of proteins could be considered as a process for detoxification of toxic arylhydroxylamines.

Non-enzymatic glutathione conjugation of 2-nitroso-6-methyldipyrido [1,2-a: 3',2'-d] imidazole (NO-Glu-P-1) in vitro: N-hydroxy-sulfonamide, a new binding form of arylnitroso compounds and thiols

In order to study the possible detoxification mechanisms of the carcinogenic arylamine, 2-amino-6-methyldipyrido[1,2-a: 3',2'-d]imidazole (Glu-P-1), the in vitro non-enzymatic reaction of 2-nitroso-6-methyldipyrido[1,2-a: 3',2'-d]imidazole (NO-Glu-P-1) with reduced glutathione (GSH) was examined at pH 7.4 under both aerobic and anaerobic conditions. Two GSH-arylamine adducts were isolated and found to contain the Glu-P-1 and GSH moieties in a 1:1 molar ratio via an N-S linkage. Their structures were assigned as sulfinamide (-NH-SO-) and N-hydroxy-sulfonamide (-N(OH)-SO2-) by their behaviour under acidic and basic conditions and by UV-VIS, 1H-NMR, infrared and mass spectrometries. Also, a N-hydroxy-sulfonamide adduct was produced when NO-Glu-P-1 and cysteine were reacted at pH 7.4. The N-hydroxy-sulfonamide structure is a new binding form between arylnitroso compounds and thiols. The formation of these adducts may also take place in vivo as a detoxification of toxic arylamines since GSH is abundant in organs such as liver or kidney.

The metabolism of N-hydroxyphenacetin in vitro and in vivo

N-Hydroxyphenacetin (100 mg/kg) injected i.p. into rats rapidly appeared in the blood and disappeared with a t1/2 of 14 min; phenacetin and 4-acetamidophenol were major metabolites in blood. Ferrihaemoglobin was formed, but 4-nitrosophenetole was not detected in blood. N-Hydroxyphenacetin injected i.p. into rats was excreted in the urine unchanged (partly conjugated 2.1% of the dose, 2% was excreted as phenacetin, 19% as 4-acetamidophenol) and 1.8% as 2-hydroxyphenacetin. In addition, small amounts of 3-hydroxyphenacetin (0.4%) and traces of N-[4-(2-hydroxyethoxy)phenyl]acetamide (beta-HAP) (0.05%) were found. Time-course kinetics have shown that N-hydroxyphenacetin is metabolized in vitro to phenacetin, 2- and 3-hydroxyphenacetin, and 4-acetamidophenol by microsomal and cytosolic preparations of rat and rabbit liver. However, after the initial reaction, the formation of phenacetin and 2- and 3-hydroxyphenacetin did not continue with time, indicating that these products were not formed enzymically. N-Hydroxyphenacetin incubated with rat erythrocytes formed ferrihaemoglobin; the relationship between ferrihaemoglobin, phenacetin and 4-nitrosophenetole concn indicated that N-hydroxyphenacetin was oxidized by oxyhaemoglobin to acetyl 4-ethoxyphenyl nitroxide, which yielded phenacetin and 4-nitrosophenetole spontaneously.

N-Hydroxy-N-arylacetamides. I. Toxicity of certain polycyclic and monocyclic N-hydroxy-N-arylacetamides in rats

Of the two carcinogenic N-hydroxy-N-arylacetamides tested, N-hydroxy-4-acetylaminobiphenyl was as active as the monocyclic analogs in the oxidation of hemoglobin, whereas N-hydroxy-2-acetylaminofluorene produced less ferrihemoglobin after IP injection into female and male rats. Monocyclic N-hydroxy-N-arylacetamides, such as N-hydroxy-4-chloroacetanilide or N-hydroxyphenacetin, were more toxic than the parent N-arylacetamides, LD50 in mice being 190 mg/kg for N-hydroxy-4-chloroacetanilide vs 755 mg/kg for 4-chloroacetanilide, and 702 mg/kg for N-hydroxyphenacetin versus 1,220 mg/kg for phenacetin. The higher acute toxicities are probably due, at least in part, to the production of more ferrihemoglobin by the N-hydroxy-N-arylacetamides. Chronic toxicity of N-hydroxy-4-chloroacetanilide was tested on 10 male and 10 female Sprague Dawley rats after IP or SC injection of 20 mg (0.11 mmol)/kg twice weekly for 16 weeks into two groups of 10 animals each (five males, five females, total dose: 3.5 mmol/kg). The experiment, which was terminated after 2 years, did not yield any hint that N-hydroxy-4-chloroacetanilide was carcinogenic in the rat. Subchronic toxicity of N-hydroxyphenacetin was tested in two experiments on male and female Sprague Dawley rats after IP or SC injection of 50 or 100 mg (0.26 or 0.51 mmol)/kg. In the first experiment, two groups of 15 rats each (seven males, eight females) were injected either IP or SC with 50 and 100 mg/kg twice weekly for 29 weeks, and in the second experiment groups of 10 males and 10 females were injected SC with 100 mg/kg twice daily on 5 days a week for 12 weeks. The experiments, which were terminated after 29 weeks and 12 weeks treatment, respectively, did not provide evidence for chronic interstitial nephritis or tumor growth in the kidney. N-Hydroxy-N-arylacetamides were found to be inferior to the corresponding arylhydroxylamines in their ferrihemoglobin-forming capabilities in female rats. Large differences in activity of the arylhydroxylamines and no close relation to the number of rings was observed, N-hydroxy-2-acetylaminofluorene being the least active and N-hydroxy-4-acetylaminobiphenyl being as active as the monocyclic compounds, and exceeding all in the duration of its activity.(ABSTRACT TRUNCATED AT 400 WORDS)

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

(1) Hemoglobin-free single-pass perfusion of isolated rat livers was carried out with various concentrations of 4-nitrosophenetol (NOPt). (2) NOPt, up to 2 mumol/min/g liver wet wt., was reduced by the liver with formation of N-hydroxy-4-phenetidine (NHOHPt), 4-phenetidine (NH2Pt), phenacetin and polar metabolites. (3) Three per cent of NOPt was irreversibly bound to liver proteins after a load of 20 mumol/g liver wet wt. After 30 min perfusion, 0.2 mumol of liver glutathione was lost by 1 mumol NOPt infused. (4) Bile flow and glutathione release by the bile decreased rapidly during NOPt perfusion. (5) Glutathione release of the livers into venous effluent was diminished by NOPt and was stimulated only slightly by t-butylhydroperoxide (BOOH). Because BOOH reduction and glutathione peroxidase were not altered and intracellular glutathione disulfide (GSSG) levels were elevated, inhibition of the GSSG excretory mechanism is assumed.