Studies of paracetamol/phenacetin toxicity: isolation and characterization of p-aminophenol-glutathione conjugate

Phytochemical Investigation of Egyptian Riverhemp: A Potential Source of Antileukemic Metabolites

As part of our research group’s continuous efforts to find alternative treatments for cancer, the aqueous ethanol extract of Sesbania sesban L. Merr. (SS, Egyptian riverhemp) demonstrated an antileukemic activity against K562 cell line. Bioguided fractionation of SS leaves hydroethanolic extract resulted in the isolation of one new compound (33) named as hederatriol 3-O-β-D-glucuronic acid methyl ester as well as 34 known compounds. Seven compounds ((34), (22), (20), (24), (21), (19), and (35)) showed high antiproliferative effects (IC50 = 22.3, 30.8, 31.3, 33.7, 36.6, 37.5, and 41.5 μM, respectively), while four compounds ((32), (5), (29), and (1)) showed milder activities (IC50 = 56.4, 67.6, 83.3, and 112.3 μM, respectively). A mechanistic study was further carried out on a molecular genetics level against several transcription factors signaling pathways that are incorporated in the incidence of cancer. The results showed that compounds (22) and (21) demonstrated a specific inhibition of Wnt pathway (IC50 = 3.8 and 4.6 μM, respectively), while compound (22) showed a specific inhibition of Smad pathway (IC50 = 3.8 μM). Compound (34) strongly altered the signaling of Smad and E2F pathways (IC50 = 5 μM). The bioactive metabolites were further investigated in silico by docking against several targets related to K562 cell line. The results showed that compounds (22) and (34) exhibited a strong binding affinity towards topoisomerase (docking score = −7.81 and −9.30 Kcal/Mole, respectively). Compounds (22) and (34) demonstrated a strong binding affinity towards EGFR-tyrosine kinase (docking score = −7.12 and −7.35 Kcal/Mole, respectively). Moreover, compound (34) showed a strong binding affinity towards Abl kinase (docking score = −7.05 Kcal/Mole).

Analysis of the chemical toxicity effects using the enrichment of Gene Ontology terms and KEGG pathways

BACKGROUND

Chemical toxicity is one of the major barriers for designing and detecting new chemical entities during drug discovery. Unexpected toxicity of an approved drug may lead to withdrawal from the market and significant loss of the associated costs. Better understanding of the mechanisms underlying various toxicity effects can help eliminate unqualified candidate drugs in early stages, allowing researchers to focus their attention on other more viable candidates.

METHODS

In this study, we aimed to understand the mechanisms underlying several toxicity effects using Gene Ontology (GO) terms and KEGG pathways. GO term and KEGG pathway enrichment theories were adopted to encode each chemical, and the minimum redundancy maximum relevance (mRMR) was used to analyze the GO terms and the KEGG pathways. Based on the feature list obtained by the mRMR method, the most related GO terms and KEGG pathways were extracted.

RESULTS

Some important GO terms and KEGG pathways were uncovered, which were concluded to be significant for determining chemical toxicity effects.

CONCLUSIONS

Several GO terms and KEGG pathways are highly related to all investigated toxicity effects, while some are specific to a certain toxicity effect.

GENERAL SIGNIFICANCE

The findings in this study have the potential to further our understanding of different chemical toxicity mechanisms and to assist scientists in developing new chemical toxicity prediction algorithms. This article is part of a Special Issue entitled "System Genetics" Guest Editor: Dr. Yudong Cai and Dr. Tao Huang.

Antigenicity of sulfanilamide and its metabolites using fluorescent-labelled compounds

In order to clarify the onset mechanisms of drug-induced allergies, three fluorescent-labelled compounds were synthesized by subjecting sulfanilamide (SA), a base compound for sulfonamides, and its active metabolites, i.e. sulfanilamide hydroxylamine and sulfanilamide nitroso, to dansylation using dansylchloride. In other words, 5-dimethylamino-N-(4-aminobenzyl)-naphthalenesulfonamide (DNS-4ABA), 5-dimethylamino-N-(4-hydroxylaminobenzyl)-1-naphthalenesulfonamide (DNS-4HABA) and 5-dimethylamino-N-(4-nitrosobenzyl)-1-naphthalenesulfonamide (DNS-4NSBA) were synthesized as model haptens. When analysed by HPLC, a conjugate of DNS-4HABA and glutathione (GSH) with nucleophilic amino acids had two peaks (P-1 and P-2). FAB-MS and 1H-NMR revealed that the DNS-4HABA-GSH conjugate consisted of sulphinamide and semimercaptal. The reactivity of GSH to DNS-4ABA, DNS-4HABA and DNS-4NSBA was quantified by HPLC using an oxidization system (horseradish peroxidase/H2O2). The results show that production of DNS-4NSBA-GSH-conjugate was four to eight times higher than that of DNS-4HABA-GSH conjugate, but that DNS-4ABA did not bind with GSH. Skin reactions were assessed using guinea pigs, and strong delayed erythema was seen with DNS-4NSBA, which bound most strongly with GSH, whereas weak delayed erythema was seen with DNS-4ABA, which did not bind with GSH. This suggests a correlation between GSH conjugate production and skin reactions. DNS-4HABA enzymatically bound with proteins in rat and guinea pig liver cytosol and microsomal fractions. The proteins that bound to DNS-4HABA were purified by HPLC and then subjected to N-terminal amino acid analysis. Ubiquitin (10 kDa) and fatty acid binding protein (30 kDa) were detected in the rat liver cytosol fraction; retinol-dehydrogenase (35 kDa) in the rat microsomal fraction; and glutathione-S-transferase B (mmu) (25 kDa) in the guinea pig liver cytosol fraction. When DNS-4HABA or DNS-4NSBA binds to proteins that play important roles in the body, unexpected adverse reactions may occur. Furthermore, by utilizing our technique using model compounds, it may be possible to identify the carrier proteins of various compounds, including pharmaceutical agents.

Characterization of 2-amino-4,5-dichlorophenol (2A45CP) in vitro toxicity in renal cortical slices from male Fischer 344 rats

2-Amino-4,5-dichlorophenol (2A45CP) is a major, aromatic ring hydroxylated metabolite of the renal toxicant, 3,4-dichloroaniline. 3,4-Dichloroaniline is nephrotoxic with primary damage located to the proximal tubules. The purpose of this study was to first characterize the in vitro toxicity of 2A45CP in renal cortical slices. Second, the effect of antioxidants and sulfhydryl containing agents on the severity of 2A45CP toxicity was explored since part of the mechanism of toxicity for aminophenols may involve redox cycling. Renal tissue was isolated from male Fischer 344 rats (190--220 g). Renal slices were rinsed three times for 3 min each in 5-ml Krebs buffer. Tissues were then incubated for 90--120 min with varying concentrations of 2A45CP between 0 and 0.5 mM. In a separate series of experiments, the slices (50--100 mg) were preincubated for 30 min with 1 mM dithiothreitol (DTT), 1 mM glutathione (GSH) or 2 mM ascorbic acid prior to exposure to 0, 0.05, 0.1 or 0.25 mM 2A45CP. 2A45CP produced a concentration and time dependent increase in LDH leakage from renal cortical slices. Total glutathione levels were diminished by 0.5 mM 2A45CP within 30 min. Renal slices incubated for 60 and 120 min with 0.05 and 0.1 mM 2A45CP had lower malondialdehyde levels than control. Pretreatment with DTT did not alter 2A45CP toxicity. Pretreatment of renal cortical slices with GSH or ascorbic acid reduced 2A45CP toxicity. These findings indicate that 2A45CP is directly toxic to renal cortical slices and that cytotoxicity is at least partially mediated by a reactive intermediate.

Lipoxygenase-mediated biotransformation of p-aminophenol in the presence of glutathione: possible conjugate formation

This study tested a hypothesis that soybean lipoxygenase (SLO), a model enzyme, may be capable of generating a glutathione (GSH) conjugate(s) from p-aminophenol (PAP). Horseradish peroxidase was employed as a positive control. GSH depletion or an increase in the absorption at 327 nm with time due to GS-PAP formation was used to quantitate the reaction. The rate of GS-PAP formation was dependent on the incubation time and the amount of SLO and exhibited Km values of 0.44 and 0.71 mM for PAP and H2O2, respectively. Classical inhibitors of lipoxygenase and free radical scavengers markedly decreased the rate of GS-PAP formation in a concentration-dependent manner. PAP-dependent GSH depletion from the reaction medium occurred at a rate of 2.37 +/- 0.18 micromol/min/mg protein. Collectively, the results suggest that lipoxygenase pathway may be involved in the enzymatic formation of GSH conjugate(s) from PAP.

Lipoxygenase-another pathway for glutathione conjugation of xenobiotics: A study with human term placental lipoxygenase and ethacrynic acid

In this study, we examined the ability of human term placental lipoxygenase (HTPLO) to catalyze glutathione (GSH) conjugate formation from ethacrynic acid (EA) in the presence of linoleic acid (LA) and GSH. HTPLO purified by affinity chromatography was used in all the experiments. The results indicate that the process of EA-SG is enzymatic in nature. The reaction shows dependence on pH, the enzyme, and the concentration of GSH, LA, and EA. The optimal assay conditions to observe a maximal rate of EA-SG formation required the presence of 0.3 mM LA, 0.2 mM EA, 2.0 mM GSH, and approximately 300 microg HTPLO in the reaction medium buffered at pH 9.0. Under the experimental conditions employed, the reaction exhibited K(m) values of 1.1 mM, 200 microM, and 130 microM for GSH, LA, and EA, respectively. The estimated specific activity of HTPLO-catalyzed EA-GS formation was approximately 4.4 +/- 0.4 micromol/min/mg protein. This rate is more than twofold greater than the rate noted for the reaction mediated by the purified human term placental glutathione transferase. Under physiologically relevant conditions (20 microM LA, 2.0 mM GSH, at pH 7.4), HTPLO produced EA-SG at 56% of the maximal rate noted under optimal assay conditions. Nordihydroguaiaretic acid, the classical inhibitor of different lipoxygenases, significantly blocked the reaction. It is proposed that free radicals are involved in the process of EA-SG formation by HTPLO. The evidence gathered in this in vitro study suggests for the first time that lipoxygenase present in the human term placenta is capable of EA-SG formation.

4-Amino-2,6-dichlorophenol nephrotoxicity in the Fischer 344 rat: protection by ascorbic acid, AT-125, and aminooxyacetic acid

A halogenated derivative of 4-aminophenol, 4-amino-2, 6-dichlorophenol (ADCP), is a potent nephrotoxicant and a weak hepatotoxicant in Fischer 344 rats. Although the mechanism of ADCP nephrotoxicity is unknown, ADCP could undergo oxidation to a reactive intermediate, such as a 4-amino-2,6-dichlorophenoxy radical or 2,6-dichloro-1,4-benzoquinoneimine, which can generate additional free radicals and/or covalently bind to cellular proteins. The toxic process might also be mediated by glutathione (GSH) conjugates of ADCP, as suggested for the mechanism of 4-aminophenol nephrotoxicity. In this study, the effects of modulators of oxidation and GSH conjugation-related metabolism or transport on ADCP-induced nephrotoxicity were examined. In one set of experiments, male Fischer 344 rats (four/group) were intraperitoneally (ip) administered ADCP (0.38 mmol/kg) only or coadministered an antioxidant, ascorbic acid (1.14 mmol/kg, ip) with ADCP. Administration of ascorbic acid markedly reduced both functional nephrotoxicity and morphological changes induced by ADCP. Administration of a gamma-glutamyltransferase (GGT) inhibitor, l-(alphaS, 5S)-alpha-amino-3-chloro-4,5-dihydroxy-5-isoxazoleacetic acid (10 mg/kg, ip), or a cysteine conjugate beta-lyase inhibitor, aminooxyacetic acid (0.5 mmol/kg, ip), 1 hr before ADCP (0.38 mmol/kg) challenge partially protected rats against ADCP nephrotoxicity. In contrast, administration of an organic anion transport inhibitor, probenecid (140 mg/kg, ip), 30 min before ADCP had little effect on ADCP nephrotoxicity. The GSH depletor, buthionine sulfoximine (890 mg/kg, ip), was given 2 hr prior to ADCP and only minimal protection was noted. In addition, the nonprotein sulfhydryl (NPSH) contents in renal cortex and liver were determined at 2 hr following the administration of ADCP only or ascorbic acid/ADCP. Ascorbic acid afforded complete prevention of the depletion of NPSH in the kidney and liver caused by ADCP administration and also prevented the elevation of renal glutathione disulfide content induced by ADCP. The results indicate that oxidation of ADCP appears to be essential to ADCP nephrotoxicity and that GSH or GSH-derived conjugates of ADCP may be partly responsible for the nephrotoxic effects of ADCP via a GGT-mediated mechanism.

Quantitative colorimetric determination of urinary p-aminophenol with an automated analyzer

We developed an automated colorimetric method for the quantitative determination of p-aminophenol with a Cobas Mira analyzer. The procedure can be used for the biological monitoring of human exposure to aniline. An absorbed aniline dose is extensively oxidized to p-aminophenol, which is excreted in urine mainly as glucurono- and sulfo- conjugates. After enzymatic hydrolysis, we reacted the free compound with resorcinol in the presence of manganese ions to form an indophenol dye, which is measured at 550 nm. Excellent accuracy (102.8%, 103.9%, and 96.8% at 2.5, 50, and 90 mg/L, respectively) and precision (7.7%, 2.1%, and 0.8% CV for within-run and 11.1%, 4.7%, and 4.6% for total reproducibility at 2.5, 50, and 90 mg/L, respectively) were achieved over a linear concentration range of 2.0 to 100 mg/L. The detection limit was 0.9 mg/L and no significant interference (except for o-aminophenol) was found for several investigated drugs and related compounds. The proposed method was used for a stability study and to analyze several samples from an occupational health screen.

A novel mechanism of glutathione conjugate formation by lipoxygenase: a study with ethacrynic acid

Ethacrynic acid (EA), a diuretic drug, is known to interact with glutathione transferases in the presence of reduced glutathione (GSH) to yield an EA-SG conjugate. Here we present evidence for a new mechanism for the formation of EA-SG conjugate by a soybean lipoxygenase (SLO)-mediated reaction involving oxidation of GSH to a GS.. Similar to the glutathione transferase-mediated reaction, EA-SG conjugate generated by SLO exhibited an absorbance maximum at 270 nm. The conjugate formation was dependent on the concentration of linoleic acid, EA, GSH, and SLO. The optimal assay conditions to observe a maximal rate of EA-SG formation required the presence of 0.4 mM linoleic acid, 1 mM GSH, 50 nM SLO, and 0.2 mM EA at pH 9.0. Classical inhibitors of lipoxygenase, e.g., nordihydroguaiaretic acid, gossypol, and 5,8,11-eicosatriynoic acid, significantly inhibited EA-SG conjugation. The SLO-generated EA-SG was isolated as a single peak by HPLC. Quantitation of EA-SG by HPLC-coupled radiometry using [3H]GSH yielded a rate of 16.5 mumol/min/mg SLO protein. This rate is up to 1650-fold greater than that reported for different purified isozymes of mammalian glutathione transferase. The structure of EA-SG isolated from HPLC column was confirmed by matrix-assisted laser desorption mass spectroscopy. These results suggest that lipoxygenase, which is primarily known for xenobiotic oxidation, may represent yet another important pathway for GSH conjugate formation that could lead to detoxification of certain chemicals.

Retinol binding proteinuria and phosphaturia: markers of paracetamol-induced nephrotoxicity

The occurrence of hypophosphataemia in paracetamol overdose suggests that nephrotoxicity is common, impaired renal tubular reabsorption of phosphate indicating renal damage. To investigate the potential nephrotoxicity of paracetamol, we studied 148 consecutive patients with paracetamol overdose. Serial clinical and biochemical measurements were made, and a fasting overnight urine collection was obtained for creatinine (Cr), phosphate and retinol-binding protein (RBP) determination. Renal threshold phosphate concentration (TmPO4/GFR) was determined from urinary parameters by an established nomogram. The degree of hypophosphataemia correlated with the severity of overdose, and with TmPO4/GFR. The median RBP/Cr ratio was higher in those patients exhibiting biochemical hepatotoxicity compared with those without hepatotoxicity, in whom median RBP/Cr was not significantly higher than controls. Within the group of patients showing biochemical hepatotoxicity, there was a correlation between log RBP/Cr and TmPO4/GFR. RBP/Cr ratio is a less sensitive marker of renal tubular toxicity than phosphaturia in these patients, and may indicate a different mechanism of toxicity.

Nephrotoxicity of 4-amino-3-S-glutathionylphenol and its modulation by metabolism or transport inhibitors

The nephrotoxicity of 4-amino-3-S-glutathionylphenol (PAP-GSH), a known metabolite of 4-amino-phenol (PAP), was determined in male Fischer 344 rats. Administration of a single dose of 40 or 60 mumol kg-1 caused a marked elevation in blood urea nitrogen and an increase in the urinary excretion of glucose, protein and gamma-glutamyltransferase (GGT). These changes were associated with histological alterations in the proximal tubule, where at the lower dose the lesion was restricted to the S3 region of the proximal tubule in the medullary rays, while at the higher dose the lesion extended to affect the S3 region in both the medullary rays and the outer stripe of the outer medulla. Studies with [35S]-PAP-GSH at 40 mumol kg-1 showed selective retention of radioactivity in the kidney, relative to other organs 24 h after dosing and that some radioactivity was covalently bound to renal proteins. Pretreatment of animals with probenecid, an inhibitor of renal organic anion transport, or aminooxyacetic acid, an inhibitor of cysteine conjugate beta-lyase, had little or no effect on the toxicity. In contrast, pretreatment of animals with acivicin, an inhibitor of gamma-glutamyltransferase, or co-administration of PAP-GSH with ascorbic acid almost completely protected against the nephrotoxicity. This protection was associated with a decreased concentration of radioactivity from [35S]-PAP-GSH in the kidneys and a decrease in the amount covalently bound to renal protein. Thus, the nephrotoxicity of PAP-GSH may be mediated by oxidation and further processing of the glutathione conjugate via gamma-glutamyltransferase.

Effect of ascorbic acid, acivicin and probenecid on the nephrotoxicity of 4-aminophenol in the Fischer 344 rat

4-Aminophenol (p-aminophenol, PAP) causes selective necrosis to the pars recta of the proximal tubule in Fischer 344 rats. The basis for this selective toxicity is not known but PAP can undergo oxidation in a variety of systems to form the 4-aminophenoxy free radical. Oxidation or disproportionation of this radical will form 1,4-benzoquinoneimine which can covalently bind to cellular macromolecules. We have recently reported that a glutathione conjugate of PAP, 4-amino-3-S-glutathionylphenol, is more toxic to the kidney than the parent compound itself. In this study we have examined the distribution and covalent binding of radiolabel from 4-[ring 3H]-aminophenol in the plasma, kidney and liver of rats 24 h after dosing and related these findings to the extent of nephrotoxicity. In addition, we have examined the effect of ascorbic acid which will slow the oxidation of PAP; acivicin, an inhibitor of gamma-glutamyltransferase and hence the processing of glutathione-derived conjugates; and probenecid, an inhibitor of organic anion transport on the nephrotoxicity produced by PAP. Administration of a single dose of PAP at 458 or 687 mumol kg-1 produced a dose-related alteration in renal function within 24 h which was associated with proximal tubular necrosis. The lesion at the lower dose was restricted to the S3 proximal tubules in the medullary rays, while at the higher dose it additionally affected the S3 tubules in the pars recta region of the cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the mechanism of 4-aminophenol-induced toxicity to renal proximal tubules

4-Aminophenol (PAP) is known to cause nephrotoxicity in the rat where it produces selective necrosis to renal proximal tubules. The aim of this work was to investigate the toxicity of PAP and its known nephrotoxic metabolite 4-amino-3-S-glutathionylphenol using a well defined suspension of rabbit renal proximal tubules. PAP at a concentration of 0.5 mM and 1 mM caused proximal tubule cell death (measured by lactate dehydrogenase release) in a time-dependent manner over a 4-h exposure. In contrast, 4-amino-3-S-glutathionylphenol at 1 mM produced no proximal tubule cell death over a similar 4-h exposure. At 2 h, 1 mM PAP inhibited proximal tubule respiration by 30% and decreased cellular adenosine triphosphate (ATP) levels by 60%. These events preceded cell death. The addition of PAP to proximal tubules led to a rapid depletion of cellular glutathione, exposure to 0.5 mM causing a 50% depletion within 1 h. The cytochrome P-450 inhibitors SKF525A (1 mM) and metyrapone (1 mM), the iron chelator deferoxamine (1 mM) and the antioxidant N,N'-phenyl-1,4-phenylenediamine (2 microM) had no effect on PAP-induced cell death. However ascorbic acid (0.1 mM), afforded a marked protection against the depletion of cellular glutathione and completely protected against the cell death produced by 1 mM-PAP. These results indicate that oxidation of PAP to generate a metabolite that can react with glutathione is an important step in the toxicity, while mitochondria appear to be a critical target for the reactive intermediate formed.

Paracetamol poisoning and the kidney

Nephrotoxicity is less common than hepatotoxicity in paracetamol overdose but renal tubular damage and acute renal failure occur even in the absence of hepatotoxicity. It is possible that antidotal therapy with agents such as N-acetylcysteine may not prevent renal toxicity and, indeed, on the basis of animal work, may actually potentiate tubular damage. Careful clinical attention should be paid to renal function in patients poisoned with paracetamol.