Environmentally Relevant Concentrations of <i>S</i>-6PPD-Quinone Caused More Serious Hepatotoxicity Than <i>R</i>-Enantiomer and Racemate in <i>Oncorhynchus mykiss</i>

6PPD-quinone (6PPD-Q) is frequently detected in various environmental media, and the environmentally relevant concentrations can be fatal to <i>Oncorhynchus mykiss</i>. Notably, 6PPD-Q has two enantiomers (<i>S</i>-6PPD-Q and <i>R</i>-6PPD-Q). In this study, <i>O. mykiss</i> was separately exposed to each enantiomer and racemate of 6PPD-Q for 96 h at environmentally relevant concentrations, and livers were collected. Effects on the biochemical, pathological, and ultrastructural changes were assessed, and metabolomics was conducted to elucidate the potential hepatotoxicity mechanism. Compared with the control treatment, the levels of catalase (CAT, all treatments except for 0.1 μg/L <i>rac</i>-6PPD-Q), and glutathione-S-transferase (GST, all treatments) significantly declined. Hepatocyte space became smaller, nuclear morphology changed, and nucleolysis occurred. Mitochondrial malformation and vesicle-like structure dilation of the endoplasmic reticulum (ER) were observed in the hepatocytes, which was most serious after <i>S</i>-6PPD-Q exposure. Some amino acid metabolism, folate biosynthesis, taurine and hypotaurine metabolism and purine metabolism were disturbed, consistent with mitochondrial dysfunction and ER stress. The differential metabolites were in the order of <i>S</i>-6PPD-Q (216) > <i>rac</i>-6PPD-Q (88) > <i>R</i>-6PPD-Q (56). Thus, 6PPD-Q-induced hepatic mitochondrial dysfunction and ER stress, causing metabolic disturbance and oxidative stress might be the toxic mechanism of 6PPD-Q in <i>O. mykiss</i> liver, and <i>S</i>-6PPD-Q effects were the most serious.