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 and toxicity in rainbow trout (O. mykiss) and coho salmon (O. kisutch). (R)-6PPD-Q was found to be rapidly metabolized in rainbow trout liver S9 with a half-life (t_1/2) of 11.4 min, which was 2.59 times faster than that of (S)-6PPD-Q. Similarly, (R)-6PPD-Q was preferentially metabolized in coho salmon liver S9. 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 a minimal metabolism of 6PPD-Q. (R)-6PPD-Q was found to strongly induce cytotoxicity in CSE-119 cells with a median effect concentration (EC₅₀) of 17.7 μg/L, which was 3.94 times stronger than that of (S)-6PPD-Q. Likewise, (R)-6PPD-Q was also the more toxic enantiomer in RTG-2 cells. In summary, this study reports the enantioselectivity of 6PPD-Q in both toxicity and metabolism.