Synthesis and Toxicity Evaluation of Tire Rubber-Derived Quinones

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), the tire rubber-derived transformation product of 6PPD, was recently discovered to cause the acute mortality of coho salmon (Oncorhynchus kisutch). Aiming to identify a potential nontoxic replacement antioxidant for 6PPD, we herein synthesized seven PPD-quinones with distinct side chains to investigate their structure-related toxicities in rainbow trout (Oncorhynchus mykiss). While 6PPD-Q exerted strong toxicity (96 h LC50 = 0.64 µg/L), toxicity was not observed for six other PPD-quinones despite their similar structures. The fish tissue concentrations of 6PPD-Q after exposure (0.8 µg/L) were comparable to the other PPD-quinones, which indicated that bioaccumulation levels were not the reason for the selective toxicity of 6PPD-Q. Hydroxylated PPD-quinones were detected as the predominant metabolites in fish tissue. Interestingly, a single major aromatic hydroxylation metabolite was detected for nontoxic PPD-quinones, but two abundant OH-6PPD-Q isomers were detected. MS2 spectra confirmed that hydroxylation occurred on the alkyl side chain for one isomer. Based on this fact, we suggested a ‘dual-action’ model wherein OH-6PPD-Q was generated by an enzyme with a high regioselectivity, which further attacks an unknown protein to cause lethality. This study reported the selective toxicity of 6PPD-Q and pinpointed the possibility for other PPDs to be applied as safe replacements of 6PPD.