Stable isotope-assisted mass spectrometry reveals in vivo distribution, metabolism, and excretion of tire rubber-derived 6PPD-quinone in mice

Comparative effects of 6PPD and 6PPD-Quinone at environmentally relevant concentrations on hepatotoxicity, glucolipid metabolism and ferroptotic response in adult zebrafish

The antioxidant 6PPD and its oxidized product 6PPD-Quinone (6PPDQ) have attracted considerable attention due to their various acute toxicities to aquatic organisms. However, the chronic toxicity of two compounds in aquatic animals is still unknown. Here, adult zebrafish were exposed to 6PPD and 6PPDQ at environmentally relevant concentrations (20 μg/L) for 28 days, and histological analysis showed that 6PPD caused more severe hepatic vacuolization than 6PPDQ. Meanwhile, 6PPD induced more serious lipid accumulation and a higher increase in triglyceride and total cholesterol levels than 6PPDQ, suggesting higher hepatotoxicity of 6PPD. Furthermore, transcriptomic analysis revealed that both compounds disturbed glucolipid metabolism to different degrees by altering the expression of different peroxisome proliferator-activated receptors (PPARs), in which 6PPD inhibited gene expressions in glucolipid metabolism possibly by PPARα, PPARβ and RXR, while 6PPDQ disrupted the expressions of partial genes in similar pathways probably via PPARγ. Additionally, 6PPD but not 6PPDQ increased Fe2+ content, decreased the protein levels of ferroportin 1, ferritin and glutathione peroxidase 4, accompanied with the increase of malondialdehyde level and the decrease of glutathione content, suggesting ferroptotic response by 6PPD. Overall, our data deepened the understanding of 6PPD- and 6PPDQ-induced hepatotoxicity association with glucolipid metabolism disorders and ferroptotic responses.

Environmental fate, toxicity, and mitigation of 6PPD and 6PPD-Quinone: Current understanding and future directions

N'-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a widely used antioxidant in the rubber industry, has garnered global attention due to the high toxicity and ecological-health risks posed by its environmental oxidation product, 6PPD-quinone (6PPD-Q). With the continuous release of tire wear particles (TWPs), 6PPD-Q is ubiquitously distributed in atmospheric, aquatic, and terrestrial environments, as well as within organisms, where it bioaccumulates through food chains. Notably, 6PPD-Q has been detected in human urine, serum, and cerebrospinal fluid, and its association with abnormal α-synuclein aggregation in the brains of Parkinson's patients further underscores its neurotoxic risks. This review systematically examines the environmental occurrence and migration patterns of 6PPD and 6PPD-Q, their multisystem toxicity, highly sensitive detection technologies, and pollution control strategies, while highlighting critical gaps in current research, such as chronic exposure mechanisms, combined pollution effects, and environmental safety thresholds. By synthesizing existing knowledge, this review provides a scientific foundation for elucidating the ecological and health risks of 6PPD-Q, offering critical insights to advance environmental regulatory policies, promote green transformation in the rubber industry, and safeguard global ecological security. Future research should prioritize long-term toxicity studies, refined detection techniques, and sustainable regulatory frameworks to mitigate the ecological and health risks posed by these emerging contaminants.

p-Phenylenediamines and their derived quinones: A review of their environmental fate, human exposure, and biological toxicity

p-Phenylenediamines (PPDs) are widely used as antioxidants in numerous rubber products to prevent or delay oxidation and corrosion. However, their derived quinones (PPD-Qs), generated through reactions with ozone, are ubiquitous in the environment and raise significant health and toxicity concerns. This review summarizes the current state of knowledge on environmental distribution and fate, human exposure, and biological toxicity of PPDs and PPD-Qs, and makes recommendations for future research directions. Although PPDs and PPD-Qs have been monitored in a variety of environmental matrices, studies on soil, sediment, and organisms remain limited. This shortcoming hinders our understanding of their distribution patterns and migration mechanisms in these specific environments. These contaminants can enter the human body through various exposure routes, but toxicological studies have not yielded sufficient results to derive risk thresholds for the assessment of human health. Most studies examining biological and toxicological effects have focused on acute exposure scenarios, which do not accurately reflect the long-term interactions that occur in natural settings. The toxic effects of PPDs and PPD-Qs on zebrafish, nematodes, and mammals include neurobehavioral changes, reproductive dysfunction, and digestive damage, which are linked to mitochondrial stress, DNA adduct formation, and disrupted lipid metabolism, respectively. However, the underlying toxicological mechanisms remain poorly understood. Future research should prioritize the investigation of the impacts of PPDs and PPD-Qs on various organizational levels within biota to provide a scientific basis for developing effective risk management measures.

Correlation between 6PPD-Q and immune along with metabolic dysregulation induced liver lesions in outdoor workers

Outdoor workers who are exposed to traffic-derived pollutants often suffer from a range of diseases, with liver disease being particularly notable. Recently, a rubber stabilizing additive antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its transformed-quinone product 6PPD-quinone (6PPD-Q) attracted attention. However, their implication for human health remains inadequately elucidated. In this study, outdoor and indoor workers were recruited to analyze 6PPD and 6PPD-Q distribution in their serum and urine. Simultaneously, blood cell counts, liver function, renal function, blood glucose level, and lipid profile were evaluated by 23 physiological parameters. For the first time, we found that the concentrations of 6PPD (0.54 - 1.66 μg L-1) and 6PPD-Q (0.58 - 4.04 μg L-1) in outdoor group serum were two- and three-fold in the indoor group, respectively. Compared with indoor workers, 18 biochemical parameters, notably total bilirubin and indirect bilirubin, were elevated in outdoor workers (p < 0.05). A computed tomography scan showed liver lesions in 60% of the outdoor group, whereas only 30% of the indoor group. The statistical analysis exhibited that significant positive correlations exist between the serum 6PPD-Q and immune cell counts, total bilirubin, indirect bilirubin, and triglycerides in human beings (p < 0.05). The logistic regression implied that for each 1 μg L-1 increase of 6PPD-Q in serum, the risk of human liver lesions increased by 2.31 times. Our results suggest that outdoor exposure is associated with increased concentrations of 6PPD-Q in serum, which could potentially influence glucose and lipid metabolism, immune cell regulation, and liver health.

Prediction of p-phenylenediamine antioxidant concentrations in human urine using machine learning models

p-phenylenediamine antioxidants (PPDs) are extensively used in rubber manufacturing for their potent antioxidative properties, but PPDs and 2-anilino-5-[(4-methylpentan-2yl)amino]cyclohexa-2,5-diene-1,4-dione (6PPDQ) pose potential environmental and health risks. Existing biomonitoring methods for assessing human exposure to PPDs are labor-intensive, costly, and provide limited data. Thus, there is a critical need to develop predictive models for evaluating PPDs and 6PPDQ exposure levels to facilitate health risk assessments. In this study, machine learning (ML) models were developed to predict the concentration of three PPDs and 6PPDQ in human urine samples. A total of 759 participants from three cities in Zhejiang Province, China, provided urine samples, which were analyzed for PPDs and 6PPDQ concentrations using liquid chromatography-tandem mass spectrometry. Eight ML models were employed to predict PPDs and 6PPDQ concentrations based on demographic and environmental exposure factors such as age, gender, body mass index (BMI), and occupation. N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) was the most frequently detected PPD (mean 3.03 ng/mL, range < LOD-18.65 ng/mL), followed by 6PPDQ (mean 2.76 ng/mL, range < LOD-20.85 ng/mL) and N-phenyl-N'-cyclohexyl-p-phenylenediamine (mean 2.04 ng/mL, range < LOD-10.22 ng/mL). Random forest model demonstrated the highest accuracy in predicting PPDs and 6PPDQ concentrations in human urine among the ML models evaluated. Through the application of these models, age, BMI, and occupation emerged as significant predictors of urinary PPDs and 6PPDQ concentrations. This research significantly contributes by using ML models to enhance exposure assessment accuracy and efficiency, providing a novel framework for future studies on environmental health risks related to PPDs and 6PPDQ.

Mass Spectrometry Imaging Unveils the Metabolic Effect of 6PPD-Quinone in Exposed Mice

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) has gained widespread attention as an emerging significant environmental contaminant, but its biochemical toxicity in mammals remains inadequately explored. In this study, the systemic toxicological effects of 6PPD-Q in mouse models exposed to both high and low doses were investigated. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) was applied to evaluate its effects on major organs, including the liver, kidneys, spleen, and testes. Notably, both high and low doses caused similar multiorgan metabolic disturbances, with significant changes in antioxidant levels, highlighting oxidative stress as a key factor in 6PPD-Q-induced damage. Further results revealed that 6PPD-Q disrupts critical metabolic pathways, including glutathione metabolism, ascorbic acid metabolism, energy metabolism, and amino acid metabolism, which are associated with systemic oxidative stress, immune dysfunction, and disruptions in liver, kidney and testis. This study provides important insights into the mechanisms of 6PPD-Q toxicity and underscores the need for further research to assess its potential health risks, which could guide future environmental policies and human health risk assessments.

Metabolites in Serum Small Extracellular Vesicles Instead of Small Extracellular Vesicles-depleted Serum Have Better Diagnostic Value for Cancers at Early Stage

Serum is one of the most commonly used biofluids for biomarker exploration. Some studies examine serum directly, while others focus on specific components like small extracellular vesicles (sEVs), which are lipid-bilayer encapsulated particles carrying a variety of molecular cargos. However, the diagnostic value of serum sEVs versus sEVs-depleted fractions (EV-free serum) for early cancer detection are unclear. In the study, size exclusion chromatography (SEC) is employed to separate serum from prostate cancer (PCa) suspects into sEVs-enriched fractions (EV) and EV-free serum. Metabolic fingerprints are obtained using ferric nanoparticle-assisted laser ablation/ionization mass spectroscopy (FeNPALDI-MS), revealing heterogeneity in metabolic composition. Eleven key metabolites are identified in EV and two in EV-free serum that differentiate PCa from benign prostatic hyperplasia. The EV key metabolites showed higher diagnostic value in PCa patients with an area under the curve (AUC) of 0.76, p < 0.05 and improved diagnostic efficacy when combined with the prostate-specific antigen (PSA, AUC = 0.85).

Photochemical Formation of Trifluoroacetic Acid: Mechanistic Insights into a Fluoxetine-Related Aryl-CF3 Compound

Trifluoroacetic acid (TFA) is a ubiquitous environmental contaminant; however, its sources are poorly constrained. One understudied source is from the photochemical reactions of aromatic compounds containing -CF3 moieties (aryl-CF3) including many pharmaceuticals and agrochemicals. Here, we studied the aqueous photochemistry of 4-(trifluoromethyl)phenol (4-TFMP), a known transformation product of the pharmaceutical fluoxetine. When exposed to lamps centered at UV-B, 4-TFMP formed up to 9.2% TFA at a steady state under acidic conditions and 1.3% under alkaline conditions. TFA yields of fluoxetine were similar to 4-TFMP for acidic and neutral pH, but higher at alkaline pH, suggesting that fluoxetine may have a mechanism of TFA formation in addition to via the 4-TFMP intermediate. Use of an 13CF3 isotopologue of 4-TFMP allowed for the tracking of TFA formation, which formed via multiple oxidative additions prior to oxidative ring cleavage. The oxidation is mediated by reactive oxygen species (ROS) generated through self-sensitized photolysis, with singlet oxygen and hydroxyl radicals as the key ROS. In addition to the TFA formation mechanism, other photochemical reactions of 4-TFMP resulted in defluorination and dimerization. Overall, this work expands our understanding of how TFA forms from aryl-CF3 compounds to better understand the total global burden of TFA.

Tire rubber-derived contaminants 6PPD and 6PPD-quinone reduce attachment and outgrowth of trophoblast spheroids onto endometrial epithelial cells

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a synthetic additive widely used in the rubber industry, and its oxidized product 6PPD-quinone (6PPDQ), have garnered widespread attention as an emerging hazardous chemicals owing to their potential detrimental effects on aquatic ecosystem and human health. The effects of 6PPD and 6PPDq on the female reproductive tract, especially embryo implantation, remain unknown and were investigated in this study. We used the spheroid attachment and outgrowth models of BeWo trophoblastic spheroids and Ishikawa cells as surrogates for the human blastocyst and endometrial epithelium, respectively. Treatment with the chemicals for up to 48 h decreased the viability of the cells in a dose- and cell line-dependent manner (20-100 μM 6PPD and 10-100 μM 6PPDQ for both the cell lines). At a noncytotoxic concentration, exposure of Ishikawa cells to 1 and 10 μM 6PPD reduced the attachment of BeWo spheroids and further inhibited their invasion and outgrowth on the endometrial epithelial monolayer. A similar result was observed in 1 μM 6PPDQ-exposed groups. Gene expression profiling of 6PPD- and 6PPDQ-exposed endometrial epithelial cells revealed that both 6PPD and 6PPDQ differentially regulated a panel of transcript markers toward overall downregulation of receptivity and invasion. The study provides the first proof of the adverse effects of 6PPD and 6PPDQ on human endometrial receptivity and trophoblast invasion during the window of implantation, warranting the need for further in vivo and clinical studies.

Environmental occurrence, fate, human exposure, and human health risks of p-phenylenediamines and their quinones

P-phenylenediamine antioxidants (PPDs) are widely used in the rubber industry and their release and transformation in the environment has become one of the current environmental research hotspots. PPDs are readily oxidized in the environment to form quinone transformation products (PPD-Qs), some of which (e.g. 6PPD-Q) have been shown to be highly toxic and persistent in the environment, posing a potential threat to aquatic organisms and ecosystems. The present study provides an overview of the physicochemical properties, environmental distribution, and potential human exposure and toxicological effects of PPDs and PPD-Qs. PPDs and PPD-Qs are found in water, air, dust and soil around the world, and humans are inevitably exposed to them by inhaling, ingesting and through dermal contact. There is growing evidence indicates that PPDs and PPD-Qs are present in human body fluids and tissues, where they are subject to metabolic and transformational processes in the liver and blood. Furthermore, PPDs and PPD-Qs have the potential to induce adverse health effects, including digestive, respiratory, neurotoxic and reproductive toxicity. Nevertheless, there is a paucity of evidence concerning the direct effects of PPDs and PPD-Qs on human health. Consequently, future research should concentrate on this area in order to provide quantitative support for the assessment of the risk posed by PPDs and PPD-Qs to human health.

Network pharmacology and transcriptomics reveal androgen receptor as a potential protein target for 6PPD-quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone, or 6PPD-Q) has received increasing attention as an emerging hotspot contaminant. The occurrence of 6PPD-Q in dust and fine atmospheric particles indicates substantial human exposure to this toxicant but the hazards of 6PPD-Q to human health is unknown. We used in silico approaches to identify potential human protein targets of 6PPD-Q and conducted preliminary validation through an in vitro cell proliferation assay and an in vivo transcriptomic analysis of prostate tissues from 6PPD-Q-treated mice. Receptor-based reverse screening and network pharmacology identified four hub targets of 6PPD-Q that were closely related to prostate carcinogenesis. Among these four targets, 6PPD-Q exhibited a strong binding tendency to androgen receptor (AR) with a binding free energy of -23.04 kcal/mol. A support vector machine (SVM) model for predicting chemicals with AR agonism or AR-inactivity was established with good prediction performance (mean prediction accuracy: 0.92). SVM prediction and AR-mediated cell-based assays, with a known AR agonist and a proposed AR inactive agent as positive and negative controls, confirmed that 6PPD-Q displayed AR agonism. Upregulation of Ar mRNA expression (FC = 1.29, p = 0.0404) and its related prostate cancer pathway was observed in the prostate of mice exposed to environmentally realistic concentrations of 6PPD-Q, suggesting a potential role in promoting prostate carcinogenesis. These findings provide evidence that 6PPD-Q agonized AR to exert downstream gene transactivation and imply its prostate cancer risks to humans.

Potential human health risk of the emerging environmental contaminant 6-PPD quinone

The tire antioxidant 6-PPD has been widely used to enhance tire performance and extend tire lifespan. 6-PPD quinone (6-PPDQ), a quinone derivative derived from 6-PPD in the presence of ozone, has been recognized an emerging environmental contaminant. In addition to causing acute lethality to coho salmon, 6-PPDQ exhibits toxic effects on other aquatic species and mammals. Based on the existing evidence, we provide a critical overview on the human internal exposure, potential adverse effects on health, and prediction of human health risk of 6-PPDQ. 6-PPDQ could be detected in human samples, including human urine, blood, and cerebrospinal fluid. Human exposure to 6-PPDQ in the environment is inevitable and may lead to adverse health effects, including hepatotoxicity, enterotoxicity, pulmonary toxicity, neurotoxicity, reproductive toxicity, and cardiotoxicity. Additionally, potential human health risk to 6-PPDQ through exposure routes and human samples were predicted. This review is helpful to identify the existing knowledge gaps and future research directions regarding the human health effects of 6-PPDQ.

P-phenylenediamine antioxidants and their quinone derivatives: A review of their environmental occurrence, accessibility, potential toxicity, and human exposure

Substituted p-phenylenediamines (PPDs), a class of antioxidants, have been widely used to extend the lifespan of rubber products, such as tires and pipes. During use, PPDs will generate their quinone derivatives (PPD-Qs). In recent years, PPDs and PPD-Qs have been detected in the global environment. Among them, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), the oxidation product of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), has been identified as highly toxic to coho salmon, with the lethal concentration of 50 % (LC50) being 95 ng/L, highlighting it as an emerging pollutant of great concern. This review summarizes the physicochemical properties, global environmental distribution, bioaccessibility, potential toxicity, human exposure risk, and green measures of PPDs and PPD-Qs. These chemicals exhibit lipophilicity, bioaccumulation potential, and poor aqueous stability. They have been found in water, air, dust, soil, and sediment worldwide, indicating their significance as emerging pollutants. Notably, current studies have identified electronic waste (e-waste), such as discarded wires and cables, as a non-negligible source of PPDs and PPD-Qs, in addition to tire wear. PPDs and PPD-Qs exhibit strong bioaccumulation in aquatic organisms and mammals, with a tendency for biomagnification within the food web, posing health threats to humans. Available toxicity data indicate that PPDs and PPD-Qs have negative effects on aquatic organisms, mammals, and invertebrates. Acute exposure leads to death and acute damage, and long-term exposure can cause a series of adverse effects, including growth and development toxicity, reproductive toxicity, neurotoxicity, intestinal toxicity, and multi-organ damage. This paper discusses current research gaps and offers recommendations to understand better the occurrence, behavior, toxicity, and environmental exposure risks of PPDs and PPD-Qs.

Protective role of ghrelin against 6PPD-quinone-induced neurotoxicity in zebrafish larvae (Danio rerio) via the GHSR pathway

The toxicity mechanisms of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), an antioxidant derivative of 6PPD via ozone reaction commonly used in rubber and tire industries, were investigated in zebrafish larvae with concentrations ranging from 0 to 50 μg/L. Despite normal hatchability, 6PPD-Q exposure led to reduced body length and swimming distance in 120 hours post-fertilization (hpf) larvae. At the highest concentration (50 μg/L), 6PPD-Q significantly impaired dopaminergic neuron development and neurotransmitter levels, including dopamine, 5-hydroxytryptamine, and glutamate. Transcriptome profiling unveiled perturbations in growth and developmental gene expression, such as upregulation of runx2a, runx2b, and ghrl (ghrelin and obestatin prepropeptide), and downregulation of stat1b, auto1, and cidea. Notably, anamorelin, a growth hormone secretagogue receptor (GHSR) agonist, recovered the behavioral deficits induced by 6PPD-Q, implying a neuroprotective role of ghrelin possibly mediated via the ghrelin/GHSR pathway. Collectively, our findings indicate that ghrelin upregulation may counteract 6PPD-Q toxicity in zebrafish larvae, shedding light on potential therapeutic avenues for mitigating the adverse effects of this antioxidant byproduct.

Distinct Species-Specific and Toxigenic Metabolic Profiles for 6PPD and 6PPD Quinone by P450 Enzymes: Insights from In Vitro and In Silico Studies

The tire rubber antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone product (6PPDQ) are prevalent emerging contaminants, yet their biotransformation profiles remain poorly understood, hampering the assessment of environmental and health risks. This study investigated the phase-I metabolism of 6PPD and 6PPDQ across aquatic and mammalian species through in vitro liver microsome (LM) incubations and in silico simulations. A total of 40 metabolites from seven pathways were identified using the highly sensitive nano-electrospray ionization mass spectrometry. Notably, 6PPDQ was consistently detected as a 6PPD metabolite with an approximate 2% yield, highlighting biotransformation as a neglected indirect exposure pathway for 6PPDQ in organisms. 6PPDQ was calculated to form through a facile two-step phenyl hydroxylation of 6PPD, catalyzed by cytochrome P450 enzymes. Distinct species-specific metabolic kinetics were observed, with fish LM demonstrating retarded biotransformation rates for 6PPD and 6PPDQ compared to mammalian LM, suggesting the vulnerability of aquatic vertebrates to these contaminants. Intriguingly, two novel coupled metabolites were identified for 6PPD, which were predicted to exhibit elevated toxicity compared to 6PPDQ and result from C-N oxidative coupling by P450s. These unveiled metabolic profiles offer valuable insights for the risk assessment of 6PPD and 6PPDQ, which may inform future studies and regulatory actions.

Environmental fate of tire-rubber related pollutants 6PPD and 6PPD-Q: A Review

To enhance tire durability, the antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is used in rubber, but it converts into the toxic 6PPD quinone (6PPD-Q) when exposed to oxidants like ozone (O3), causing ecological concerns. This review synthesizes the existing data to assess the transformation, bioavailability, and potential hazards of two tire-derived pollutants 6PPD and 6PPD-Q. The comparative analysis of different thermal methods utilized in repurposing waste materials like tires and plastics into valuable products are analyzed. These methods shed light on the aspects of pyrolysis and catalytic conversion processes, providing valuable perspectives into optimizing the waste valorization and mitigating environmental impacts. Furthermore, we have examined the bioavailability and potential hazards of chemicals used in tire manufacturing, based on the literature included in this review. The bioavailability of these chemicals, particularly the transformation of 6PPD to 6PPD-Q, poses significant ecological risks. 6PPD-Q is highly bioavailable in aquatic environments, indicating its potential for widespread ecological harm. The persistence and mobility of 6PPD-Q in the environment, along with its toxicological effects, highlight the critical need for ongoing monitoring and the development of effective mitigation strategies to reduce its impact on both human health and ecosystem. Future research should focus on understanding the chronic effects of low-level exposure to these compounds on both terrestrial and aquatic ecosystems, as well as the potential for bioaccumulation in the food chain. Additionally, this review outlines the knowledge gaps, recommending further research into the toxicity of tire-derived pollutants in organisms and the health implications for humans and ecosystems.

Tissue Accumulation and Biotransformation of 6PPD-Quinone in Adult Zebrafish and Its Effects on the Intestinal Microbial Community

The pronounced lethality of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone or 6PPDQ) toward specific salmonids, while sparing other fish species, has received considerable attention. However, the underlying cause of this species-specific toxicity remains unresolved. This study explored 6PPDQ toxicokinetics and intestinal microbiota composition in adult zebrafish during a 14-day exposure to environmentally realistic concentrations, followed by a 7-day recovery phase. Predominant accumulation occurred in the brain, intestine, and eyes, with the lowest levels in the liver. Six metabolites were found to undergo hydroxylation, with two additionally undergoing O-sulfonation. Semiquantitative analyses revealed that the predominant metabolite featured a hydroxy group situated on the phenyl ring adjacent to the quinone. This was further validated by assessing enzyme activity and determining in silico binding interactions. Notably, the binding affinity between 6PPDQ and zebrafish phase I and II enzymes exceeded that with the corresponding coho salmon enzymes by 1.04-1.53 times, suggesting a higher potential for 6PPDQ detoxification in tolerant species. Whole-genome sequencing revealed significant increases in the genera Nocardioides and Rhodococcus after exposure to 6PPDQ. Functional annotation and pathway enrichment analyses predicted that these two genera would be responsible for the biodegradation and metabolism of xenobiotics. These findings offer crucial data for comprehending 6PPDQ-induced species-specific toxicity.

Chronic exposure to tire rubber-derived contaminant 6PPD-quinone impairs sperm quality and induces the damage of reproductive capacity in male mice

It has been reported that N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), a derivative of the tire antioxidant, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), exhibits acute toxicity towards organisms. However, the possible reproductive toxicity of 6PPD-Q in mammals has rarely been reported. In this study, the effects of 6PPD-Q on the reproductive toxicity of C57Bl/6 male mice were assessed after exposure to 6PPD-Q for 40 days at 4 mg/kg body weight (bw). Exposure to 6PPD-Q not only led to a decrease in testosterone levels but also adversely affected semen quality and in vitro fertilization (IVF) outcomes, thereby indicating impaired male fertility resulting from 6PPD-Q exposure. Additionally, transcriptomic and metabolomic analyses revealed that 6PPD-Q elicited differential expression of genes and metabolites primarily enriched in spermatogenesis, apoptosis, arginine biosynthesis, and sphingolipid metabolism in the testes of mice. In conclusion, our study reveals the toxicity of 6PPD-Q on the reproductive capacity concerning baseline endocrine disorders, sperm quality, germ cell apoptosis, and the sphingolipid signaling pathway in mice. These findings contribute to an enhanced understanding of the health hazards posed by 6PPD-Q to mammals, thereby facilitating the development of more robust safety regulations governing the utilization and disposal of rubber products.