Hexamethylene diisocyanate (HDI) vapor reactivity with glutathione and subsequent transfer to human albumin

IntroductionAirway fluid glutathione (GSH) reactivity with inhaled vapors of diisocyanate, a common occupational allergen, is postulated to be a key step in exposure-induced asthma pathogenesis.MethodsA mixed (vapor/liquid) phase exposure system was used to model the in vivo reactivity of inhaled HDI vapor with GSH in the airway fluid. HDI–GSH reaction products, and their capacity to transfer HDI to human albumin, were characterized through mass spectrometry and serologic assays, using HDI-specific polyclonal rabbit serum.ResultsHDI vapor exposure of 10 mM GSH solutions resulted in primarily S-linked, bis(GSH)–HDI reaction products. In contrast, lower GSH concentrations (100 μM) resulted in mainly mono(GSH)–HDI conjugates, with varying degrees of HDI hydrolysis, dimerization and/or intra-molecular cyclization, depending upon the presence/absence of H₂PO₄^?/HPO₄^2? and Na⁺/Cl^? ions. The ion composition and GSH concentration of the fluid phase, during HDI vapor exposure, strongly influenced the transfer of HDI from GSH to albumin, as did the pH and duration of the carbamoylating reaction. When carbamoylation was performed overnight at pH 7, 25 of albumin’s lysines were identified as potential sites of conjugation with partially hydrolyzed HDI. When carbamoylation was performed at pH 9, more rapid (within 3 h) and extensive modification was observed, including additional lysine sites, intra-molecular cross-linkage with HDI, and novel HDI–GSH conjugation.ConclusionsThe data define potential mechanisms by which the levels of GSH, H₂PO₄^?/HPO₄^2?, and/or other ions (e.g. H⁺/OH^?, Na⁺, Cl^?) affect the reactivity of HDI vapor with self-molecules in solution (e.g. airway fluid), and thus, might influence the clinical response to HDI respiratory tract exposure.