Comparative neurotoxicity and pyrrole-forming potential of 2,5-hexanedione and perdeuterio-2,5-hexanedione in the rat.
An investigation was conducted to compare the in-vitro and in-vivo pyrrole forming potential of 2,5-hexanedione (110134) (2,5-HD) and perdeuterio-2,5-hexanedione (P-2,5-HD) and to examine the neurotoxic potency of each isomer in rats. Male Sprague-Dawley-rats were injected intraperitoneally with 2,5-HD or P-2,5-HD each day, 5 days per week, at a dose of 3.5 millimoles per kilogram (mmol/kg) per day for 17 days, or 0.5mmol/kg per day for 38 days. In a separate study of tissue distribution after uptake of the two diketones, rats were given single intraperitoneal doses of 7.5mmol/kg of 2,5-HD or P-2,5- HD and then terminated 1, 2, 6 or 18 hours after dosing. Pyrrolylation of bovine serum albumin (BSA) in-vitro proceeded in a linear fashion for both isomers for incubation times of between 0.5 and 5 hours. Rats given 2,5-HD at either dose progressed to moderate or marked levels of clinical neuropathy at the time of termination. Numerous swollen axons were seen in sections of thoracic and lumbar spinal cord from 2,5-HD treated rats. Testicular alterations were seen in the 2,5-HD, but not P-2,5-HD, treated rats. In general, pyrrole adduct concentrations were two to three times higher in serum and in axonal cytoskeletal proteins from the 2,5-HD treated rats than in proteins from the P-2,5-HD treated rats. Levels of covalently crosslinked, high molecular weight protein were generally significantly lower in preparations from P- 2,5-HD treated rats. Tissue distribution analysis showed no significant differences between tissue concentrations of the two isomers. In experiments on deuterium/hydrogen exchange rates in- vivo, essentially all the exchange detected in the whole P-2,5-HD molecule was attributable to loss of deuterium from the terminal methyl groups. The authors conclude that pyrrole formation is required for diketone neurotoxicity.