Characterization of isothiocyanates, thioureas, and other lysine adduction products in carbon disulfide-treated peptides and protein

Carbon disulfide (CS2) is an industrial solvent used in rayon production and as an organic synthetic precursor. It is also a member of the class of neuropathy-inducing xenobiotics known as the "neurofilament (NF) neurotoxicants". Current hypotheses propose direct reaction of CS2 with NF lysine epsilon-amine moieties as a step in the mechanism of this neuropathy. In this study, covalent CS2 binding in a lysine-containing dipeptide and in bovine serum albumin (BSA) in vitro was characterized. Dipeptide and BSA, incubated with 14CS2, exhibited stable incorporation of radioactivity after removal of unbound CS2 and reincubation in physiological buffer for up to 10 days. In contrast, free thiol levels decreased from a maximum immediately following CS2 exposure to near-base-line levels after 10 days, consistent with time-dependent conversion of initially formed N-substituted dithiocarbamate adducts into secondary products. HPLC/thermospray-MS and HPLC/UV photodiode-array analysis of CS2-dipeptide adducts confirmed dithiocarbamate formation and demonstrated their conversion into N-alkylisothiocyanates and, ultimately, N,N'-disubstituted thioureas and ureas. The results of UV spectrophotometry of CS2-treated BSA were also consistent with loss of dithiocarbamate and appearance of thioureas. Similar time-dependent formation of these products, in addition to N,N'-disubstituted thiuram disulfides, was demonstrated in CS2-treated BSA by means of 13C-NMR spectroscopy. SDS-PAGE analysis of adducted protein revealed a discrete, higher mobility band, likely representing a specific intramolecular cross-link. In contrast, no evidence for intermolecular protein cross-linking was obtained. Identical results were obtained with cysteinyl-blocked BSA, indicating the lack of formation of N,S-dialkyldithiocarbamate (dithiourethane) cross-links in these preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of clinical laboratory measurements to issues of environmental health

Monitoring of biochemical constituents in serum is an important component in revealing potential toxicity in humans and experimental animals due to exposure to a variety of xenobiotic agents. The relative toxicity of pure compounds, usually at large doses, has helped elucidate the mode of action of these compounds and their relative risk. However, most actual cases of environmental exposure present an extensive range of components and the potential for synergistic or inhibitory interactions. In this paper we review two such environmental cases: The Love Canal chemical dump site in Niagara Falls, NY, and the transformer fire at the State Office Building in Binghamton, NY. We focus on the clinical laboratory measurements obtained in these studies (including serum glucose, triglycerides, aspartate aminotransferase, alanine aminotransferase, alkaline phosphatase, gamma-glutamyltransferase, lactate dehydrogenase, sodium and potassium), their usefulness, limitations, and application to such cases. Significant alterations in serum triglyceride and alanine aminotransferase levels were found in guinea pigs due to exposure to dioxins. These two tests were useful in estimating the 'equivalent' concentration of 2,3,7,8-tetrachlorodibenzo-p-dioxin in complex chemical mixtures.

Comparative neurotoxicity and pyrrole-forming potential of 2,5-hexanedione and perdeuterio-2,5-hexanedione in the rat

2,5-Hexanedione (2,5-HD), the neurotoxic metabolite of n-hexane, reacts with protein amines to form alkylpyrrole adducts. Pyrrolylation of neurofilament protein may be the initiating molecular event in 2,5-HD neuropathy. The present study compares the neurotoxic and pyrrole-forming potentials of 2,5-HD with those of perdeuterio-2,5-HD ([D10]-2,5-HD) in the rat. Due to a requirement for C-H bond breaking in the reaction mechanism, the latter derivative was expected to exhibit a primary isotope effect, thus forming the pyrrole at a slower rate. In vitro studies confirmed that [D10]-2,5-HD pyrrolylated protein at only one-third of the initial rate seen with native 2,5-HD. Prolonged incubation resulted in similar pyrrole concentrations with both derivatives. Adult, male Wistar rats were administered daily (5 days/week) ip doses of either 3.5 mmol 2,5-HD or [D10]-2,5-HD/kg/day for 17 days or 2.5 mmol/kg/day for 38 days. At termination, animals administered 2,5-HD and [D10]-2,5-HD exhibited 27 and 8% body weight loss, respectively. Moderate to severe hindlimb paralysis was present in the 2,5-HD groups while only mild effects were seen in [D10]-2,5-HD-dosed rats. Neuropathological changes were prominent in spinal cord sections from 2,5-HD-treated animals, while no effects were present in rats given the deuterated derivative. Pyrrole adduct concentrations in serum and axonal cytoskeletal proteins from 2,5-HD-treated animals were two- to threefold higher than in rats given equimolar doses of [D10]-2,5-HD. Levels of covalent crosslinking of axonal cytoskeletal proteins (assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) appeared to correlate with pyrrole concentrations. Tissue concentrations of each diketone isomer were not significantly different, indicating similar uptake of native and deuterated 2,5-HD. Mass spectrometry revealed rapid back exchange of the terminal (methyl) but not of the internal (methylene) deuteriums of [D10]-2,5-HD in vivo. These findings support an absolute requirement for pyrrole formation in gamma-diketone neurotoxicity.

Mechanism of formation and quantitation of imines, pyrroles, and stable nonpyrrole adducts in 2,5-hexanedione-treated protein

The condensation of gamma-diketones with protein epsilon-amino moieties to yield alkylpyrrole adducts has been demonstrated in many in vitro and in vivo systems, although certain features of this reaction remain unclear. The present in vitro study was designed to examine additional aspects of gamma-diketone-protein interactions, including the possible formation of imine intermediates and stable nonpyrrole products, and the potential for conformational changes in pyrrolylated protein. Values for total, stable covalent binding were consistently higher than p-dimethylaminobenzaldehyde (DMAB)-detectable pyrrole adduct concentrations when bovine serum albumin (BSA) was incubated (24 hr, 37 degrees C) with [14C]-2,5-hexanedione (2,5-HD) at diketone:lysine ratios greater than or equal to 5:1 (at pH 9.5) or 1:1 (at pH 7.4). Treatment of pyrrolylated BSA with proteases before the DMAB assay decreased but did not eliminate the difference between these parameters. Quantitative amino acid analysis of pyrrolylated BSA revealed molar decreases in lysine content equivalent to DMAB-detectable pyrrole adduct concentrations; no other amino acids were significantly altered. Cleavage of disulfide bonds in pyrrolylated BSA by dithiothreitol resulted in an apparent decrease in DMAB-detectable pyrrole, which was reversible upon subsequent protease treatment. A similar decrease was not seen with pyrrolylated concanavalin A, a protein that lacks disulfide linkages. Samples of BSA were incubated with [14C]-2,5-hexanedione for 2-144 hr and a portion of each incubation mixture treated with NaCNBH3 to selectively reduce imines to stable amines. Substantial levels of an imine intermediate were detected at 2, 6, and 24 hr but not at 144 hr. The above findings support proposed mechanisms involving imine intermediates in the pyrrolylation reaction. In addition, evidence for the formation of stable nonpyrrole adducts at high diketone:amine molar ratios has been provided. Results consistent with potential conformational alterations in pyrrolylated protein have also been demonstrated.

Acute toxicity in the guinea pig and in vitro "dioxin-like" activity of the environmental contaminant 1,2,4,5,7,8-hexachloro (9H)xanthene

A number of sites in the state of Missouri have been contaminated with polychlorinated dibenzodioxins and dibenzofurans as a result of improper waste-oil application for dust control. In addition to these compounds, relatively high levels of 1,2,4,5,7,8-hexachloro(9H)xanthene (1,2,4,5,7,8-HCX), a by-product of hexachlorophene manufacture, were also detected. Unlike the dibenzodioxins and dibenzofurans, no animal toxicity data are available on the chlorinated xanthenes. In view of the potential importance of this novel class of environmental contaminants, studies were conducted to examine the acute oral toxicity in guinea pigs and in vitro "dioxin-like" activity of 1,2,4,5,7,8-HCX. Animals administered a single oral dose of 0.02, 0.1, 0.5 or 2.5 mg 1,2,4,5,7,8-HCX/kg in corn oil and sacrificed 42 d later exhibited no treatment-related pathology. Guinea pigs given 12.5 mg/kg displayed mild to moderate distension and histologically observable subserosal edema of the urinary bladder, in addition to mild fatty vacuolization of pancreatic acinar cells. The alterations were considered to be of minimal toxicological significance. No compound- or dose-related mortality, body weight loss, or organ weight changes were noted at any dose level. Results using an in vitro bioassay for "dioxin-like" activity confirmed preliminary data suggesting that 1,2,4,5,7,8-HCX is about 10(6) times less potent than 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) in this assay system. These findings indicate that 1,2,4,5,7,8-HCX may represent a relatively low environmental hazard compared to 2,3,7,8-TCDD.

Mechanisms of in vitro pyrrole adduct autoxidation in 2,5-hexanedione-treated protein

The neurotoxic gamma-diketone, 2,5-hexanedione, reacts with axonal protein amine residues to form 2,5-dimethylpyrrole adducts. Current evidence implicates this reaction as the potentially critical step in gamma-diketone neurotoxicity, although it is unclear whether pyrrole formation per se is sufficient to induce neuropathy or whether secondary autoxidative reactions are also required. The present in vitro study examines aspects of pyrrole formation and the secondary phenomena of chromophore development and covalent protein crosslinking in 2,5-hexanedione-treated protein. p-Dimethylaminobenzaldehyde (DMAB)-detectable pyrrole concentrations decreased linearly with time when pyrrolylated bovine serum albumin (pyrrole-BSA) was incubated under air, but remained unchanged following N2 incubation. The air-induced decrease was accompanied by the appearance of chromophores and crosslinked protein. Covalent crosslinking of pyrrole-BSA was pH-dependent, with relatively increased intermolecular bridging at pH 7.4 as compared to pH 9.5. Chromophore formation and the loss in DMAB-detectable pyrrole were also accelerated at the lower pH. Autoxidative parameters were inhibited in the presence of a free radical scavenger (ascorbic acid) but induced by free radical initiators (potassium persulfate and 2,2'-azobis[2-amidinopropane hydrochloride]). In vitro incubation followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of combinations of bovine serum albumin, ribonuclease, pyrrole-BSA, and pyrrolylated ribonuclease revealed that the intermolecular crosslinking pathway was mediated by pyrrole-pyrrole bridging. These findings demonstrate that the secondary autoxidative phenomena following pyrrole adduct formation in gamma-diketone-treated protein proceed via pH-dependent, free radical-mediated mechanisms. If similar mechanisms are present in vivo, the results also suggest that intermolecular covalent crosslinking of pyrrolylated axonal protein may be less widespread and more specific than previously thought.

Subchronic oral toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the guinea pig: comparisons with a PCB-containing transformer fluid pyrolysate

In contrast to the well-characterized acute toxicity of the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) in the guinea pig, the effects of prolonged po exposure in this species are unknown. The present report describes the results of administration to guinea pigs of 2,3,7,8-TCDD in the feed at levels of 0, 2, 10, 76, or 430 ppt for up to 90 days. Additional aims were to examine recovery following prolonged 2,3,7,8-TCDD exposure in the guinea pig and to generate data to facilitate comparison of the previously reported toxicity of a transformer fluid pyrolysate with that of pure 2,3,7,8-TCDD. Animals receiving 430 ppt 2,3,7,8-TCDD exhibited body weight loss, thymic atrophy, liver enlargement, and 60% mortality by Day 46 (males) and by Day 60 (females), when surviving animals in this group were sacrificed. Total 2,3,7,8-TCDD consumption was approximately 1.3 and 1.9 micrograms/kg, respectively. Animals receiving 76 ppt 2,3,7,8-TCDD for 90 days (total 0.44 microgram/kg) exhibited a decreased rate of body weight gain and increased relative (to body) liver weights. Male animals also displayed a reduction in relative thymus weights and elevated serum triglycerides, while females exhibited hepatocellular cytoplasmic inclusion bodies and lowered serum alanine aminotransferase activities. Toxic effects were generally similar to those observed after acute 2,3,7,8-TCDD administration. No dose-related alterations were seen in animals receiving either 10 ppt (total 0.06 micrograms/kg) or 2 ppt (total 0.01 micrograms/kg) for 90 days, establishing a no-observed-effect level of approximately 0.65 ng 2,3,7,8-TCDD/kg/day. In the recovery study, groups of guinea pigs were administered 430 ppt 2,3,7,8-TCDD for 11, 21, or 35 days and then allowed to recover for an additional 79, 69, or 55 days, respectively. Treatment-related mortality in each group was 0, 10, and 70%, respectively, by Day 90. An effective LD50 of 0.8 microgram 2,3,7,8-TCDD/kg for prolonged exposure was calculated on the basis of these results, a value lower than those previously reported from this laboratory for acute exposure. The results also suggested a possible lowering of the body weight "set point" following 2,3,7,8-TCDD exposure. Comparison of the present findings with those previously reported for a transformer fluid pyrolysate containing a mixture of polychlorinated aromatic species indicated both a greater variety of toxic effects and flatter dose-response relationships for the pyrolysate in the guinea pig.

Molecular mechanisms of diketone neurotoxicity

The important industrial and commercial solvents n-hexane and methyl n-butyl ketone undergo metabolic conversion in experimental animals and man to the neurotoxic gamma-diketone 2,5-hexanedione. Several molecular mechanisms of action have been proposed to explain the pathogenesis of gamma-diketone neuropathy. Such a mechanism must account for the target organ specificity, neurofilament accumulation, structure/activity relationships, in vivo covalent binding, and apparent direct axonal toxicity encountered in this syndrome. It has been proposed that the gamma-diketones exert their effects by reaction with sulfhydryl moieties of energy-producing axonal glycolytic enzymes, with resultant disruption of axoplasmic transport. Others have suggested that reaction instead occurs with lysine moieties of axonal cytoskeletal proteins to form alkyl pyrrole adducts, leading to damaging physicochemical changes in these proteins. Additional hypotheses involve inhibition of axonal sterologenesis, alterations in nerve membrane properties, and reduced neurofilament proteolysis within the nerve terminal. Although a comprehensive mechanism of action for the gamma-diketones remains to be demonstrated, much progress has been made toward this goal. Ultimate success awaits elucidation of the interactions of the neurotoxic diketones with axonal components at the molecular level. Previous reviews have addressed the historical, pharmacokinetic, and neuropathological aspects of this neuropathy. The present critique will examine proposed molecular mechanisms for the gamma-diketones with regard to theoretical considerations and experimental evidence.

Chemical and biological investigations of a transformer accident at Binghamton, NY

A transformer fire occurred in a state office building in Binghamton, NY on February 5, 1981. Particulates from inside surfaces of ceiling panels on 16 of the 17 floors had concentrations of polychlorinated dibenzofurans (PCDFs) ranging from less than 1 part per million (ppm) to 1200 ppm while polychlorinated biphenyl (PCB) concentrations varied from 28 ppm to 23,000 ppm. In spite of the wide variations in contaminant concentrations, complete analytical data from 11 floors showed that there was a consistent PCDF/PCB ratio (0.067 +/- 0.026) and also consistent PCDF isomer group distributions (tetra-CDFs, 33 +/- 5%; penta-CDFs, 40 +/- 3%; hexa-CDFs, 18 +/- 7%; hepta-CDFs, 6 +/- 3%). It was found that the particulate samples could be successfully ranked in order of their degree of chemical contamination by an in vitro bioassay. The bioassay was based on induction of keratinization or changes in morphology in mouse epithelial cells. Animal toxicology experiments were carried out with a soot sample containing a PCDF concentration which approximated the mean value found on the ceiling particulates. The single dose oral LD values of the soot and its benzene extract equivalent, each administered to female guinea pigs in 0.75% methyl cellulose, were 410 and 327 mg/kg, respectively. These results demonstrated that the soot matrix had virtually no effect on the toxicity of the chemical contaminants in the soot. Morphological alterations in liver tissues from animals receiving the soot were found after examination by electron and light microscopy.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in rat axonal cytoskeletal proteins induced by in vitro and in vivo 2,5-hexanedione exposure

The neurotoxic gamma-diketone 2,5-hexanedione (2,5-HD) reacts in vitro and in vivo with protein lysine epsilon-amine moieties to yield 2,5-dimethylpyrrole adducts. It has been hypothesized that pyrrole adduct formation in neurofilament (NF) or other axonal proteins may lead to increased hydrophobicity, secondary autoxidative crosslinking, or the loss of essential lysine amine groups, and that pyrrolylation therefore represents the critical initiating event in gamma-diketone neuropathy. The present investigation was designed to evaluate pyrrole levels and other changes in brain stem and spinal cord axonal cytoskeletal proteins from rats receiving 0.5% 2,5-HD in the drinking water for up to 8 weeks and following recovery. Clinical signs of neuropathy were apparent in rats after 5 weeks exposure, while no histopathological effects were seen until 8 weeks. Cessation of dosing resulted in some recovery from clinical neuropathy but virtually no change in histopathologically demonstrable CNS damage. 2,5-Dimethylpyrrole adduct was detected in serum and axonal cytoskeletal proteins from animals in all exposure groups and its formation appeared to reach a plateau in both serum and axonal protein. Assay of total protein lysine vs pyrrole content demonstrated an average conversion of less than 1% of epsilon-amine groups into pyrrole adducts in axonal protein after 2 weeks exposure. Gel electrophoresis revealed discrete new protein bands in brain stem and spinal cord axonal protein preparations from treated animals, along with high-molecular-weight, nonmigrating proteinaceous material. Concentration of the nonmigrating material appeared to increase in a time-dependent fashion. A concurrent decrease in the relative amounts of native NF subunit proteins was observed in brain stem but not spinal cord. Reversal of these changes was observed 9 weeks after cessation of dosing, although residual nonmigrating protein and pyrrole adduct were present. In vitro incubation of axonal cytoskeletal protein preparations (pH 7.2, 37 degrees C) with 2,5-HD resulted in the formation of high-molecular-weight bands identical to those seen in vivo. These findings provide evidence for pyrrole adduct formation and secondary covalent crosslinking in CNS axonal cytoskeletal proteins from 2,5-HD-treated animals.

Neurotoxicity and protein binding of 2,5-hexanedione in the hen

Previous studies in this laboratory have demonstrated 2,5-dimethylpyrrole adduct formation during in vitro exposure of protein amino groups to the neurotoxic n-hexane metabolite 2,5-hexanedione (2,5-HD). The present investigation reports in vivo pyrrole adduct formation in neural and nonneural protein from 2,5-HD-treated animals. Adult, White-Leghorn hens were given daily doses of either 200 or 70 mg 2,5-HD/kg, po, for up to 55 or 135 days, respectively. Additional animals were given 70 mg/kg for 63 days and then allowed to recover for 72 more days. Protein separation by gel electrophoresis followed by staining with a pyrrole-specific reagent yielded evidence of widespread adduct formation in protein from serum, liver, kidney, brain, and purified myelin. Binding was particularly strong in serum albumin nd myelin basic protein. Quantitation of the adduct in these tissues revealed that its formation reached peak levels at 20 days in high dose and 30 days in low-dose animals. Levels subsequently declined, suggesting the presence of a clearance mechanism capable of removing altered protein during continuing 2,5-HD exposure. Protein from animals on the recovery regimen contained no detectable pyrrole adduct. Pyrrole adduct formation was also detected in neurofilament protein preparations, although protein yields were too low to allow assessment of clearance. Hens at both dosages displayed clinical signs indicative of CNS and PNS neuropathy. Histologic findings included axonal swelling and degeneration in peripheral nerve and some spinal cord nerve tracts. A hypothesis is proposed involving differential clearance of pyrrole adduct from neural vs nonneural tissue to explain the mechanism of action and target organ specificity of 2,5-hexanedione.

Subchronic oral toxicity in guinea pigs of soot from a polychlorinated biphenyl-containing transformer fire

We have previously described the acute po toxicity in guinea pigs of soot from a transformer fire at the State Office Building in Binghamton, New York. The soot was determined to contain polychlorinated biphenyls, biphenylenes, dibenzodioxins, and dibenzofurans. The present study evaluates soot toxicity in guinea pigs receiving 0, 0.2, 1.9, 9.3, or 46.3 ppm soot in the feed for 90 days or 231.5 ppm for 32 days. At 231.5 ppm, body weight loss, thymic atrophy, bone marrow depletion, skeletal muscle and gastrointestinal tract epithelial degeneration, and fatty infiltration of hepatocytes were observed. Mortality had reached 35% by Day 32 (when survivors were killed), with total soot consumption of approximately 400 mg/kg. At 46.3 or 9.3 ppm soot, a reduced rate of body weight gain was observed, and at 46.3 ppm, the mortality by Day 90 was 30%. Relative (to body) thymus weights were decreased in both groups, while relative spleen weights were increased at 46.3 ppm soot only. Salivary gland interlobular duct squamous metaplasia and focal lacrimal gland adenitis were detected histopathologically, while bone marrow depletion was noted only in females at the higher dose. Diminished serum alanine aminotransferase (ALT) activity in both sexes and decreased serum sodium levels in male and potassium levels in female animals were detected at both dose levels. decreased gamma-glutamyl transferase activity and red blood cell count and elevated serum creatinine and triglycerides were observed only in animals fed 46.3 ppm soot. At 1.9 ppm soot, salivary gland duct metaplasia was observed in both sexes, along with decreased relative thymus weights, ALT activity, and serum sodium levels in male animals only. No effects attributable to soot exposure were noted in animals receiving 0.2 ppm soot for 90 days. Total average soot consumption for male and female animals in the 0.2, 1.9, 9.3, and 46.3 ppm dosage groups was 1.2, 12, 55, and 275 mg/kg, respectively. Although many of the observed effects were typical of acute exposure of guinea pigs to the Binghamton soot or to polychlorinated aromatic hydrocarbons in general, salivary gland duct metaplasia has not been previously reported. Toxic effects of this subchronic exposure were observed at lower total doses than with acute exposure, although variations in absorption due to the effects of different vehicles (aqueous in the acute study versus the feed in this study) could account for some or all of this difference.