Enantioselectivity in Metabolism and Toxicity of 6PPD-Quinone in Salmonids

The toxicity of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) in salmonids has been found to be sensitive to even minor structural changes on its alkyl side chain. Inspired by this, we herein isolated the enantiomers of 6PPD-Q and tested their in vitro metabolism in liver S9 of rainbow trout (O. mykiss), along with their toxicity in a coho salmon (O. kisutch) embryo (CSE-119) cell line. (R)-6PPD-Q was found to be rapidly metabolized in rainbow trout liver S9 with a half-life (t1/2) of 12.3 minutes, which was 2.92 times faster than that of (S)-6PPD-Q. This was further evidenced by the preferential formation of an (R)-aryl-OH-6PPD-Q metabolite. Supporting this, enantioselective accumulation of (S)-6PPD-Q was found in rainbow trout in vivo. To further distinguish between kinetics and intrinsic toxicity, we tested the toxicity of 6PPD-Q enantiomers in the CSE-119 cell line with minimal metabolism of 6PPD-Q. (R)-6PPD-Q was found to strongly induce cytotoxicity in CSE-119 cells with an IC50 of 17.7 µg/L, which was 3.94 times stronger than that of (S)-6PPD-Q. In summary, this study reported the enantioselectivity in both the toxicity and metabolism of 6PPD-Q, demonstrating that its toxicity should be mediated by specific protein binding.