Organic solvents are among the most common contaminants of soil and water at Superfund sites. We have made the important discovery that one type of solvent, an aromatic compound called 1,2-diethylbenzene (1,2-DEB), produces neurotoxicity in animals and may pose a health risk to humans. Rats metabolize 1,2-DEB to a reactive chemical that attaches to proteins, forming a chromophore that stains tissues and urine a bluish-purple color. We are characterizing the chemical structure and mechanism of formation of this chromophore and are defining the relationship between chromophore production and 1,2-DEB neurotoxicity. If the chromophore appears in tissues and is excreted in urine before neurological damage occurs, then its detection in urine could be useful as a biomarker of exposure to solvents containing 1,2-DEB.

Previous experiments have shown that rats repeatedly exposed to the 1,2-DEB active metabolite, a protein-reactive chemical called 1,2-diacetylbenzene (1,2-DAB), develop limb weakness due to injury of the spinal cord and nerve roots in the lower back. Under the microscope, injured nerves in this area contain swellings that are full of structural proteins known as neurofilaments (NF). We have shown that 1,2-DAB causes normal NF proteins to bind together inappropriately, thereby forming the swellings, disrupting nerve function and causing nerve injury. Our latest studies have shown that 1,2-DAB interacts differentially with each of the various NF proteins.

We have also been using molecular modeling techniques to determine the chemical structure of the chromophore that is formed in tissues by 1,2-DAB. Advanced computerized modeling by Dr. David Dixon, a member of the scientific team who works at the Pacific Northwest National Laboratory, has indicated that the bluish-purple coloration is most likely due to the presence of chemicals called isoindoles. Moreover, while single isoindole molecules (monomers) should not exhibit any coloration, molecular modeling suggests that complexes of two isoindole monomers (called dimers) would account for both the bluish-purple tissue staining and neurotoxicity of 1,2-DAB.

Similar work has been completed on a different class of neurotoxic and chromogenic solvent called 2,5-hexanedione (2,5-HD). This structural cousin of 1,2-DAB is actually the active metabolite of the industrial solvent n-hexane and about 1000 times less reactive than 1,2-DAB. Similar to 1,2-DAB, our results suggest that 2,5-HD reacts with proteins in tissues to form another type of chemical called a pyrrole. Furthermore, as with isoindoles formed by 1,2-DAB, molecular modeling suggests that both the chromogenicity and neurotoxicity of 2,5-HD are related to the formation of pyrrole dimers. These findings support the conclusion that the chromogenicity of 2,5-HD is closely related to its neurotoxicity and further predict that both the chromogenic and neurotoxic effects of 2,5-HD are associated with the same chemical reaction process. They also show that whether the solvent derivative forms a straight chain compound, as in 2,5-HD, or a ring structure, as in 1,2-DAB, the quality of the neurotoxic response is similar in laboratory species.

We are the first to demonstrate that (a) an aromatic hydrocarbon solvent metabolite (1,2-DAB) is able to cause nerve degeneration in the central and peripheral nervous system, (b) the neurotoxicity and chromogenicity of aromatic solvents are related phenomena, and (c) a common pattern of axonal degeneration is associated with repeated exposure to neurotoxic aliphatic (2,5-HD) and aromatic (1,2-DAB) solvent derivatives. These results raise the possibility that a number of other reportedly chromogenic organic solvents, including substances that are in very widespread commercial use, have the potential to cause nerve degeneration. Moreover, there may be the opportunity to measure chromophores as biological markers of exposure to aromatic hydrocarbon solvents inasmuch as the chromophore appears in tissues and urine before neurodegenerative changes appear. Since 2,5-HD is the ultimate metabolite of the neuropathy-producing aliphatic solvent n-hexane, and 1,2-DAB is the equivalent of the predictably much more potent neuropathy-producing aromatic solvent 1,2-diethylbenzene (1,2-DEB), permissible limits for human workplace exposure for 1,2-DEB need to be re-assessed.