Studies in this laboratory have developed the following hypothesis for the pathogenesis of the neurotoxicology of n-hexane, an environmental toxin of petroleum origin and a hazard of occupational exposure and inhalant drug abuse, viz., hexane is metabolized by the liver to the Gamma-diketone, 2,5-hexanedione, which reacts with amino groups of proteins resulting in an imine, which then cyclizes to form 2,5-dimethylpyrrole derivatives; autoxidation of pyrrole rings rendering them vulnerable to nucleophilic attack, resulting in covalent crosslinking of proteins; progressive crosslinking of neurofilaments during slow axonal transport generates aggregates too large to pass through nodes of Ranvier; the ensuing obstruction of slow axonal transport results in degeneration of the distal axon. This hypothesis is based on our observations that 2,5-hexanedione results in extensive pyrrole derivatization and protein crosslinking in vitro and in vivo. To test this hypothesis we shall (1) administer monoketones to rats to determine the relationships between diketone synthesis, diketone structure, clinical neurotoxicology, paranodal neurofilament accumulation and axonal degeneration; (2) characterize the kinetics of reaction of Gamma-diketones with primary amines and with lysyl groups of neurofilaments and other proteins; (3) establish proof of structure of the protein-protein crosslinks through model compound synthesis, (4) define whether reaction with neurofilaments results in aggregation through covalent crosslinking or via hydrophobic interactions; (5) seek morphologic, autoradiographic, and gel electrophoretic evidence for progressive neurofilament crosslinking by Gamma-diketones during slow axonal transport and obstruction of transport at nodes of Ranvier; and (6) determine whether additional environmental toxins which result in neurofilament accumulation do so via covalent crosslinking. Understanding the mechanism of neurofilament accumulation in hexane toxicity will direct prevention and therapy and allow generalizations to be made to other environmental toxins and exposure limited before human cases ever develop.
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