Synthesis, characterization, and solvolysis of mono- and bis-S-(glutathionyl) adducts of methylene-bis-(phenylisocyanate) (MDI)

Absorption, distribution, metabolism, and excretion of methylene diphenyl diisocyanate and toluene diisocyanate: Many similarities and few differences

Methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) are high production volume chemicals used for the manufacture of polyurethanes. For both substances, the most relevant adverse health effects after overexposure in the workplace are isocyanate-induced asthma, lung function decrement and, to a much lesser extent, skin effects. Over the last two decades many articles have addressed the reactivity of MDI and TDI in biological media and the associated biochemistry, which increased the understanding of their biochemical and physiological behavior. In this review, these new insights with respect to similarities and differences concerning the adsorption, distribution, metabolism, and excretion (ADME) of these two diisocyanates and the implications on their toxicities are summarized. Both TDI and MDI show very similar behavior in reactivity to biological macromolecules, distribution, metabolism, and excretion. Evidence suggests that the isocyanate (NCO) group is scavenged at the portal-of-entry and is not systemically available in unbound reactive form. This explains the lack of other than portal-of-entry toxicity observed in repeated-dose inhalation tests.

Expanding the Scope of Asinger Chemistry towards Enantiomerically Pure Secondary Amines and β-Aminothiols through Chemoenzymatic Derivatization of 3-Thiazolines

A proof of concept for a novel approach towards enantiomerically highly enriched acyclic secondary amines and β-aminothiols as non-cyclic target molecules when starting from 3-thiazolines as heterocycles is presented. Starting from 2,2,4,5,5-pentamethyl-3-thiazoline, we demonstrated this chemoenzymatic pathway to both of these types of amine molecules, which were isolated as urea derivatives with a non-optimized yield of up to 20%. As a substrate, 2,2,4,5,5-pentamethyl-3-thiazolidine, which was obtained with an enantiomeric excess (ee) of 99% in a biotransformation from the corresponding 3-thiazoline according to a recently developed protocol, was used. For the reductive desulfurization of this substrate leading to a sulfur-free secondary amine, in situ formed Ni2B turned out to be a suitable reducing reagent. However, when using lithium aluminum hydride as a reducing agent, β-aminothiol was obtained.

Reaction products of hexamethylene diisocyanate vapors with "self" molecules in the airways of rabbits exposed via tracheostomy

1. Hexamethylenediisocyanate (HDI) is a widely used aliphatic diisocyanate and a well-recognized cause of occupational asthma. 2. "Self" molecules (peptides/proteins) in the lower airways, susceptible to chemical reactivity with HDI, have been hypothesized to play a role in asthma pathogenesis and/or chemical metabolism, but remain poorly characterized. 3. This study employed unique approaches to identify and characterize "self" targets of HDI reactivity in the lower airways. Anesthetized rabbits free breathed through a tracheostomy tube connected to chambers containing either, O2, or O2 plus ∼200 ppb HDI vapors. Following 60 minutes of exposure, the airways were lavaged and the fluid was analyzed by LC-MS and LC-MS/MS. 4. The low-molecular weight (<3 kDa) fraction of HDI exposed, but not control rabbit bronchoalveolar lavage (BAL) fluid identified 783.26 and 476.18 m/z [M+H]+ ions with high energy collision-induced dissociation (HCD) fragmentation patterns consistent with bis glutathione (GSH)-HDI and mono(GSH)-HDI. Proteomic analyses of the high molecular weight (>3 kDa) fraction of exposed rabbit BAL fluid identified HDI modification of specific lysines in uteroglobin (aka clara cell protein) and albumin. 5. In summary, this study utilized a unique approach to chemical vapor exposure in rabbits, to identify HDI reaction products with "self" molecules in the lower airways.

Identification of novel reaction products of methylene-bis-phenylisocyanate ("MDI") with oxidized glutathione in aqueous solution and also during incubation of MDI with a murine hepatic S9 fraction

Methylene diphenyl diisocyanate (MDI) is an important industrial chemical and asthmagenic respiratory sensitizer, however its metabolism remains unclear. In this study we used LC-MS and LC-MS/MS to identify novel reaction products of MDI with oxidized glutathione (GSSG), including an 837m/z [M+H](+) ion corresponding to GSSG bound (via one of its N-termini) to partially hydrolyzed MDI, and an 863m/z [M+H](+) ion corresponding to GSSG cross-linked by MDI (via its two γ-glutamate N-termini) [corrected]. Further studies with heavy isotope labeled and native reduced glutathione (GSH) identified an [M+H](+) ion corresponding to previously described mono(GSH)-MDI, and evidence for "oligomeric" GSH-MDI conjugates. This study also investigated transformational changes in MDI after incubation with an S9 fraction prepared from murine liver. LC-MS analyses of the S9 reaction products revealed the formation of [M+H](+) ions with m/z's and retention times identical to the newly described GSSG-MDI (837 and 863) conjugates and the previously described mono(GSH)-MDI conjugate. Together the data identify novel biological transformations of MDI, which could have implications for exposure-related health effects, and may help target future in vivo studies of metabolism.

In vitro cleavage of diisocyanate-glutathione conjugates by human gamma-glutamyl transpeptidase-1

Isocyanates differ from many other xenobiotics in their ability to form S-linked conjugates with glutathione (GSH) through direct nucleophilic addition reactions (e.g. without enzymatic "preactivation" and/or transferase activity), potentially predisposing them to metabolism via the mercapturic acid pathway. In vivo, mono-isocyanates are metabolized via the mercapturic acid pathway and excreted as N-acetylated cysteine conjugates, however, the metabolism of di-isocyanates remains unclear. We assessed the ability of purified human gamma-glutamyl transpeptidase-1 (GGT-1), a primary enzyme of the mercapturic acid pathway, to cleave S-linked GSH conjugates of 4,4'-methylene diphenyl diisocyanate (MDI) and 1,6-hexamethylene diisocyanate (HDI), two widely used industrial chemicals. A combination of liquid chromatography (LC), tandem mass spectrometry (MS/MS) and hydrogen-deuterium exchange studies confirmed GGT-1 mediated formation of the 607.2 and 525.2 m/z (M + H)(+) ions corresponding to bis(cys-gly)-MDI and bis(cys-gly)-HDI, respectively, the cleavage products expected from the corresponding bis(GSH)-diisocyanate conjugates. Additional intermediate metabolites and mono(cys-gly)-conjugates with partially hydrolyzed diisocyanate were also observed. Consistent with GGT enzyme kinetics, metabolism proceeded more rapidly under conditions that favored transpeptidation versus hydrolytic mechanisms of cleavage. Together the data demonstrate the capacity of human GGT-1 to cleave GSH conjugates of both aromatic and aliphatic diisocyanates, suggesting a potential role in their metabolism.

Glutathione reaction products with a chemical allergen, methylene-diphenyl diisocyanate, stimulate alternative macrophage activation and eosinophilic airway inflammation

Isocyanates have been a leading chemical cause of occupational asthma since their utility for generating polyurethane was first recognized over 60 years ago, yet the mechanisms of isocyanate asthma pathogenesis remain unclear. The present study provides in vivo evidence that a GSH mediated pathway underlies asthma-like eosinophilic inflammatory responses to respiratory tract isocyanate exposure. In naïve mice, a mixture of GSH reaction products with the chemical allergen, methylene-diphenyl diisocyanate (MDI), induced innate immune responses, characterized by significantly increased airway levels of Chitinase YM-1 and IL-12/IL-23β (but not α) subunit. However, in mice immunologically sensitized to MDI via prior skin exposure, identical GSH-MDI doses induced substantially greater inflammatory responses, including significantly increased airway eosinophil numbers and mucus production, along with IL-12/IL-23β, chitinases, and other indicators of alternative macrophage activation. The "self"-protein albumin in mouse airway fluid was uniquely modified by GSH-MDI at position (414)K, a preferred site of MDI reactivity on human albumin. The (414)K-MDI conjugation appears to covalently cross-link GSH to albumin via GSH's NH2-terminus, a unique conformation possibly resulting from cyclized mono(GSH)-MDI or asymmetric (S,N'-linked) bis(GSH)-MDI conjugates. Together, the data support a possible thiol mediated transcarbamoylating mechanism linking MDI exposure to pathogenic eosinophilic inflammatory responses.

Connecting glutathione with immune responses to occupational methylene diphenyl diisocyanate exposure

Methylene diphenyl diisocyanate (MDI) is among the leading chemical causes of occupational asthma world-wide, however, the mechanisms of disease pathogenesis remain unclear. This study tests the hypothesis that glutathione (GSH) reacts with MDI to form quasi-stable conjugates, capable of mediating the formation of MDI-conjugated "self" protein antigens, which may participate in pathogenic inflammatory responses. To test this hypothesis, an occupationally relevant dose of MDI (0.1%w/v) was reacted with varying concentrations of GSH (10μM-10mM), and the reaction products were characterized with regard to mass/structure, and ability to carbamoylate human albumin, a major carrier protein for MDI in vivo. LC-MS/MS analysis of GSH-MDI reaction products identified products possessing the exact mass of previously described S-linked bis(GSH)-MDI and its partial hydrolysis product, as well as novel cyclized GSH-MDI structures. Upon co-incubation of GSH-MDI reaction products with human albumin, MDI was rapidly transferred to specific lysines of albumin, and the protein's native conformation/charge was altered, based on electrophoretic mobility. Three types of modification were observed, intra-molecular MDI cross-linking, addition of partially hydrolyzed MDI, and addition of "MDI-GSH", where MDI's 2nd NCO had reacted with GSH's "N-terminus". Importantly, human albumin carbamoylated by GSH-MDI was specifically recognized by serum IgG from MDI exposed workers, with binding dependent upon the starting GSH concentration, pH, and NaCl levels. Together, the data define a non-enzymatic, thiol-mediated transcarbamoylating mechanism by which GSH may promote immune responses to MDI exposure, and identify specific factors that might further modulate this process.

Synthetic approaches to obtain amino acid adducts of 4,4'-methylenediphenyl diisocyanate

4,4'-Methylenediphenyl diisocyanate (MDI) is the most important isocyanate used in the chemical industry. Lung sensitization and asthma are the main types of damage after exposure to MDI. Albumin adducts of MDI might be involved in the etiology of sensitization reactions. It is therefore necessary to have sensitive and specific biomarkers such as blood protein adducts to monitor people exposed to isocyanates. For the discovery of new isocyanate adducts with blood proteins present in vivo, new synthetic standards are needed. To achieve this, we developed five methods to obtain amino acid adducts of MDI. We synthesized and isolated MDI adducts of aspartic acid, glutamic acid, cysteine, and valine. The new adducts were characterized by LC-MS/MS and NMR. We synthesized the corresponding isotope-labeled MDI adducts to develop analytical methods using LC-MS/MS. Glutathione adducts of isocyanates are an important way of transportation of the reactive isocyanates to distant sites from the original site of exposure. Therefore, we used N-acetyl-cysteine adducts of MDI as reactants: N-acetyl-S-[[4-(4-aminobenzyl)phenyl]carbamoyl]-cysteine (MDI-AcCys) and N-acetyl-S-[[4-(4-acetylaminobenzyl)phenyl]carbamoyl]-cysteine (AcMDI-AcCys). MDI-AcCys or AcMDI-AcCys formed adducts with albumin, N(α)-acetyl lysine, and valine. Isotope-labeled albumin adducts (= d(4)-MDI-albumin) were synthesized from d(4)-MDI-AcCys and albumin. d(4)-MDI-albumin can be used as an internal standard to analyze biological samples. Such an internal standard will not correct only for the extraction recovery of the adducts but also for the potential variation of the enzymatic digestions used in the procedure to analyze albumin adducts of MDI. The synthetic procedures described in this manuscript will be applicable to the synthesis of amino acid adducts from other isocyanates.

Interrelating the acute and chronic mode of action of inhaled methylenediphenyl diisocyanate (MDI) in rats assisted by computational toxicology

Polymeric methylenediphenyl diisocyanate (MDI) is a high production volume chemical intermediate consisting of monomeric 4,4'-MDI, its 2,2'- and 2,4'-isomers, and higher oligomeric homologues. The toxicity of pMDI has systematically been investigated in previous regulatory and mechanistic studies. One cornerstone of toxicological risk assessment is to understand the critical Mode of Action (MoA) of inhaled MDI aerosol. This paper compares the no-observed-adverse effect levels (NOAELs) in rats from two published whole-body exposure chronic inhalation bioassays with the lung irritation-based point of departures (PODs) from acute and subacute nose-only inhalation studies. Acute irritation was related to elevated concentrations of protein in bronchoalveolar lavage fluid (short-term studies), whilst the chronic events were characterized by histopathology. In the chronic bioassay the exposure duration was either 6 or 18h/day while in all other studies a 6h/day regimens were applied. The major objective of this paper is to analyze the interrelationship of acute pulmonary irritation and the acute-on-chronic manifestations of pulmonary disease following recurrent chronic inhalation exposure. This included considerations on the most critical metrics of exposure with regard to the acute concentration×exposure duration per day (C×T(day)) and the chronic cumulative dose metrics. In summary, this analysis supports the conclusion that the C×T(day) relative to the acute pulmonary irritation threshold is more decisive for the chronic outcome than the concentration per se or the time-adjusted cumulative dose. For MDI aerosols, the acute threshold C×T(day) was remarkably close to the NOAELs of the chronic inhalation studies, independent on their differing exposure mode and regimens. This evidence is supportive of a simple, direct MoA at the site of initial deposition of aerosol. Accordingly, for chemicals reactive to the endogenous nucleophilic agents contained in the lining fluid of the lung, one unifying essential prerequisite for pulmonary injury appears to be a C×T(day) that exhausts the homeostatic pool of MDI-scavenging agents. In the case that threshold is exceeded, the secondary compensatory chronic response may then cause additional superimposed types of chronic pathologies.

Toluene diisocyanate reactivity with glutathione across a vapor/liquid interface and subsequent transcarbamoylation of human albumin

Glutathione has previously been identified as a reaction target for toluene diisocyanate (TDI) in vitro and in vivo, and has been suggested to contribute to toxic and allergic reactions to exposure. In this study, the reactivity of reduced glutathione (GSH) with TDI in vitro was further investigated using a mixed phase (vapor/liquid) exposure system to model the in vivo biophysics of exposure in the lower respiratory tract. HPLC/MS/MS was used to characterize the observed reaction products. Under the conditions tested, the major reaction products between TDI vapor and GSH were S-linked bis(GSH)-TDI and to a lesser extent mono(GSH)-TDI conjugates (with one N═C═O hydrolyzed). The vapor-phase-generated GSH-TDI conjugates were capable of transcarbamoylating human albumin in a pH-dependent manner, resulting in changes in the self-protein's conformation/charge, on the basis of electrophoretic mobility under native conditions. Specific sites of human albumin-TDI conjugation, mediated by GSH-TDI, were identified (Lys(73), Lys(159), Lys(190), Lys(199), Lys(212), Lys(351), Lys(136/137), Lys(413/414), and Lys(524/525)) along with overlap with those susceptible to direct conjugation by TDI. Together, the data extend the proof-of-principle for GSH to act as a "shuttle" for a reactive form of TDI, which could contribute to clinical responses to exposure.

Nuclear magnetic resonance studies on covalent modification of amino acids thiol and amino residues by monofunctional aryl 13C-isocyanates, models of skin and respiratory sensitizers: transformation of thiocarbamates into urea adducts

Exposure to aryl isocyanates, intermediates in the manufacture of polyurethanes, provokes lung sensitization and asthma but also occupational allergic contact dermatitis, sensitization occurring from a single accidental exposure. The initial step in the sensitization process is believed to be the covalent binding of the -N triple bond C triple bond O group with nucleophilic residues on proteins. While a wide knowledge exists on the reactivity of skin sensitizers toward amino acids, little is known about respiratory sensitizers such as aryl isocyanates. (13)C-Labeled monofunctional aryl isocyanates were synthesized, and their reactivities toward nucleophilic amino acids, GSH, and a model peptide were studied by (13)C and [(1)H-(13)C] NMR spectroscopy. An acetonitrile/buffer solution was used as a solvent to avoid the hampering of the follow up of the reactivity by the isocyanate hydrolysis competing reaction. The compounds reacted with thiol groups, through the formation of thiocarbamate bonds and with amino groups to form urea derivatives. The reactivity was confirmed with GSH, containing both free amino and thiol groups, and with a model peptide, particularly in the case of the reaction with lysine. The use of (13)C NMR to follow the aryl isocyanates reversible conjugation with thiol groups is also reported. Particularly, it is shown that thiocarbamate adducts can be converted into adducts of the urea kind by reaction with amino groups. These results confirmed the hypothesis by which thiol-containing peptides/proteins may act as carriers of isocyanates for possible reaction at a later time and/or place with other nucleophiles and confirmed the role of lysine as a good competing nucleophilic amino acid. The reactivity of aryl isocyanates with thiol and amino groups needs thus to be considered in their assigned sensitization processes.

Analysis of biomarkers in rats and dogs exposed to polymeric methylenediphenyl diisocyanate (pMDI) and its glutathione adduct

Hemoglobin adducts (Hb-MDX) of monomeric methylenediphenyl diisocyanate (MDI) are often interpreted as indirect evidence of hydrolysis of the diisocyanate moiety to the respective amine (diphenylmethane-4,4'-diamine, 4,4'-MDA) which constitutes the rationale of using this biomarker as an internal dosimeter of exposure to putatively formed MDA. In contrast, more recently published data suggest that following inhalation the high concentration of glutathione (GSH) present in lungs favor an adduct formation with GSH and/or peptides/proteins rather than hydrolysis. The focus of this study was to test this alternate hypothesis, viz. whether Hb-MDX can also be formed by the GSH bis-adduct of monomeric MDI. The synthesized mMDI-GSH bis-adduct was administered to rats by single intratracheal instillation. Additional groups were dosed by gavage and intraperitoneal injection. Biomarkers of exposure were determined in blood (plasma protein and hemoglobin adducts) and urine after harsh alkaline and acid hydrolysis, respectively. Data from previous single inhalation exposure studies with aerosols of MDI and 4,4'-MDA in rats served as reference. As to whether N-acetylation plays any modifying role to yield these mMDI-specific biomarkers was addressed in similarly head-only exposed dogs, a species with no appreciable N-acetylation capacity whereas rats are strong N-acetylators. The results obtained suggest that biomarkers in blood from controlled exposures above current workplace standards of mMDI appear not to be suitable for reliable assessments of past exposures. The biomarkers typically used to assess past exposures to MDI were also identified following exposure to the MDI-GSH bis-adduct. Their yield was low but quite similar for MDI aerosol and the MDI-GSH bis-adduct, whilst that of MDA was distinctively higher. The findings of this study are supportive of a conceptual pathway that the MDI-derived biomarkers of exposure are formed through MDI-GSH adducts rather than MDA. Data from dogs support the findings from rats and show that N-acetylation does not appear to be an essential modifying factor. It is concluded that the yield of MDI-related markers of exposure is relatively low and dependent on the exposure dose (and route). MDA originating from hydrolyzed serum protein or hemoglobin appear to be confounded by false-positive background levels which are surmised to be associated with the method of hydrolysis. The determination of urinary biomarkers might be a useful tool to identify recent exposures (by any route). Due methodological uncertainties associated with the harsh hydrolysis of biological specimens may be reduced substantially when using incremental pre- to post-shift changes rather than relying solely on absolute data.

Absorption, distribution, metabolism and excretion of an inhalation dose of [14C] 4,4'-methylenediphenyl diisocyanate in the male rat

The received dose, tissue distribution, metabolism, routes and rates of excretion of [(14)C]-4, 4(')-methylenediphenyl diisocyanate (MDI) were investigated in the male rat following a 6-h inhalation exposure to [(14)C]-MDI at a target concentration of 2 mg m(-3). The mean dose received was equivalent to 0.078 mg MDI per animal, of this between 25 and 32% of radiolabelled material was available systemically. Radioactivity was distributed to all tissues examined with the highest proportions present in the respiratory and gastrointestinal tracts, suggesting that both oral ingestion and pulmonary absorption contributed to the systemic dose of [(14)C]-MDI derived material, with the oral ingestion and the majority of the internal dose resulting from ingestion of radiolabelled material by grooming the pelt after exposure. Radioactivity was excreted mainly via faeces (about 80% of the received dose). Excretion in bile and urine each accounted for less than 15% of the dose. MDI was extensively metabolized after uptake, with two routes of transformation evident; the proposed spontaneous formation of mixed molecular weight polyureas and the enzyme catalysed metabolism of systemically available MDI or MDI derivatives to give N-acetylated and N-acetylated hydroxylated products. No free MDA was detected in any of the biomatrices (urine, faeces, bile) investigated.

Glutathione protects human airway proteins and epithelial cells from isocyanates

BACKGROUND

Glutathione (GSH), one of the major anti-oxidants of the lung, has been linked to the human response to isocyanate exposure. However, the ability of GSH to modulate key chemical reactions, thought to be central to the development of human isocyanate allergy, has not been directly analyzed under biologic exposure conditions.

OBJECTIVE

To better understand the potential role of GSH in the response to occupational isocyanate exposure, we evaluated its effects on two processes thought to be involved in the development of isocyanate allergy, isocyanate-protein conjugation and epithelial cell toxicity.

METHODS

The effects of GSH on (1) isocyanate conjugation with albumin, its major target in the airway fluid and (2) isocyanate-induced toxicity to human airway epithelial cell lines, A549 and NCI-H292, were tested using two different in vitro models. For protein conjugation studies, a newly described vapour exposure system was used to model the air/liquid interface at the surface of the epithelial fluid in the airways. Epithelial cell exposures were performed in fluid phase to mimic the in vivo exposure of airway cells covered by epithelial lining fluid.

RESULTS

Reduced GSH prevented hexamethylene diisocyanate (HDI) conjugation to albumin in a dose-dependent manner, while oxidized GSH (GSSG) conversely increased conjugation rates. GSH levels equivalent to those found in normal human airway fluid (100 microm) provided >90% protection against HDI-protein conjugation when albumin was exposed to HDI vapour levels 10-fold above permissible occupational limits. Physiologic levels of GSH, but not GSSG, also reduced HDI toxicity to human airway epithelial cells in a dose-dependent manner, when present extracellularly, however, drugs that modulate intra-cellular GSH levels did not significantly alter isocyanate toxicity.

CONCLUSIONS

Together with previously reported genetic and toxicity studies, the data suggest that airway GSH plays an important role in protection against HDI exposure and may help prevent the development of allergic sensitization and asthma.