Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio)

6PPD impairs growth performance by inducing intestinal oxidative stress and ferroptosis in zebrafish

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a tire-derived pollutant, has gained increasing attention due to its potential toxicity to aquatic organisms. Although previous studies have revealed that 6PPD impacts early developmental stages of larval fish, its effects on adult fish, particularly on key organs, remain unclear. In this study, we observed that adult zebrafish exposed to 6PPD exhibited reduced growth performance and increased fecal output. Histological examination with hematoxylin and eosin (H&E) staining revealed damage to the intestinal villi and a reduction in goblet cell numbers, indicating that 6PPD impairs growth performance by disrupting the digestive system. Comparative transcriptomic analysis revealed that 6PPD caused significant changes in the expression of 727 genes in the intestine, of which 280 genes were up-regulated and 447 genes were down-regulated. These genes were primarily associated with nutrient digestion and absorption, energy metabolism, immune response, and redox regulation. Mechanistically, 6PPD induced oxidative stress and triggered ferroptosis in the intestine, leading to structural damage of the intestinal villi. Treatment with the antioxidant N-acetylcysteine (NAC) alleviated 6PPD-induced oxidative stress and ferroptosis, thereby improving intestinal villi structure and promoting fish growth. This study provides insights into the mechanisms by which 6PPD impairs growth in adult zebrafish and highlights NAC as a potential therapeutic strategy to mitigate its toxicity.

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.

Exploring the fate of 6PPD in zebrafish (Danio rerio): Understanding toxicokinetics, biotransformation mechanisms, and metabolomic profiling at environmentally relevant levels

In recent years, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) has attracted significant attention in environmental science, yet its behavior in biological systems remains poorly understood. This study involved a 28-day zebrafish exposure experiment at three concentrations (2, 20, and 200 μg/L), to investigate its physiologically based toxicokinetic (PBTK) properties, the formation of biotransformation products, and the metabolic characteristics of liver tissue. The results indicated that the liver and intestines are key organs for 6PPD accumulation, with tissue-specific distribution patterns. The biotransformation of 6PPD in the liver involves various phase I and phase II metabolic reactions, including hydroxylation, N-dealkylation, and sulfation processes. Furthermore, Metabolomics analysis revealed substantial changes in both the diversity and abundance of liver metabolites with increasing 6PPD concentrations, particularly in key biological processes such as lipid metabolism, amino acid metabolism, and redox balance. Notably, significant disruptions in sphingolipid and glycerophospholipid pathways suggest 6PPD may impair membrane fluidity and stability, potentially leading to membrane damage and dysfunction. Overall, this study provides crucial insights into the biological behavior of 6PPD in zebrafish, contributing essential knowledge for its ecotoxicological evaluation.

Analysis of 6PPD-Q in finfish, shellfish, and marine mammal tissues

6PPD-quinone (6PPD-Q), a transformation product of tire rubber anti-oxidant 6PPD, has been identified as the primary causal toxicant for the urban runoff mortality syndrome observed in coho salmon (Oncorhynchus kisutch) in the Pacific Northwest, USA. Several other fish species are also vulnerable to 6PPD-Q. However, monitoring efforts on 6PPD-Q have been focused on water, particulate matter, soils, and sediments, while that in tissues remains scarce. This study presents a workflow for extraction and quantitative analysis of 6PPD-Q in complex tissues from shellfish, finfish, and marine mammals. A multi-residue extraction protocol was developed for quantitative analysis of 6PPD-Q and persistent organic pollutants (PCBs, PBDEs, organochlorine pesticides) and PAHs in tissues in a single extraction. A GC-MS/MS based 6PPD-Q measurement was also developed. The protocol was evaluated in tissues including fish fillets, whole fish homogenates, mussels, and whale blubber. Limits of quantification of 6PPD-Q were between 0.03 and 0.12 ng/g ww and the surrogate (6PPD-Q-d5) recoveries were ∼60-100 % among matrices. We also conducted an initial biomonitoring study using caged mussels (Mytilus trossulus) and juvenile Chinook salmon (Oncorhynchus tshawytscha) from Puget Sound, WA. 6PPD-Q detection rates were at least 50 % but the concentrations were mostly <1 ng/g ww. Our protocol will aid 6PPD-Q biomonitoring in aquatic environments and also exposure assessments for improved understanding of 6PPD-Q bioaccumulation potential in these food webs.

Xenometabolomics in Ecotoxicology: Concepts and Applications

Nontargeted high-resolution mass spectrometry (HRMS) allows for the characterization of a large fraction of the exposome, i.e., the entirety of chemicals an organism is exposed to, and helps detect important exogenous chemical compounds that could be key drivers of toxicological impact. Along with these chemical compounds occur endogenous metabolites that are essential for the health of the host organism. Chemical compounds derived from the biotransformation of xenobiotics present in the exposome are referred to as the xenometabolome, while endogenous metabolites derived from the host organism are referred to as the endometabolome. Recent advancements in HRMS technology allow for the detection of chemical features of biological and ecological importance in the context of chemical safety assessments with unprecedented sensitivity and resolution. In this perspective, we highlight the application of HRMS-based metabolomics of organisms in the context of ecotoxicology, the complexity of comprehensively characterizing the endometabolome, and distinguishing chemical compounds of the xenometabolome.

Synthesis and Toxicity Evaluation of p-Phenylenediamine-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 potential replacement antiozonants for 6PPD that do not produce toxic quinones, seven PPD-quinones with distinct side chains were synthesized to investigate their structure-related toxicities in vivo using rainbow trout (Oncorhynchus mykiss). While 6PPD-Q exerted high toxicity (96 h LC50 = 0.35 μg/L), toxicity was not observed for six other PPD-quinones despite their similar structures. The fish tissue concentrations of 6PPD-Q after sublethal exposure (0.29 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 the alternate PPD-quinones, but two abundant OH-6PPD-Q isomers were detected for 6PPD-Q. MS2 spectra confirmed that hydroxylation occurred on the alkyl side chain for one isomer. The structurally selective toxicity of 6PPD-Q was also observed in a coho salmon (CSE-119) cell line, which further supports its intrinsic toxicity. This study reported the selective toxicity of 6PPD-Q and pinpointed the possibility for other PPDs to be applied as potential substitutes of 6PPD.

Rubber-derived chemicals in urban sewer networks and receiving waters: Fingerprints, driving factors and ecological impacts

Rubber-derived chemicals (RDCs), which include rubber additives (RAs) and their transformation products (TPs), can be released into aquatic environments when rubber products, such as vehicle tires, are in use or discarded. However, RDCs and associated ecological risks have not been thoroughly investigated inside urban sewer networks and their receiving water bodies. To address these issues, we investigated the RDCs in Hong Kong's municipal sewer networks, including sewage and stormwater, as well as their receiving waters, such as rivers and coastal water. Among 45 target RDCs, the vulcanizing agents and corrosion inhibitors were found to be predominant in the water samples, accounting for 26-66 % and 29-72 % of total concentrations of 45 RDCs (∑45RDC), respectively, while antioxidants and their TPs presented in smaller quantities, accounting for 0.21-26 % and 0-15 % of ∑45RDC, respectively. Ten RAs from five classes were additionally identified by suspect screening. An estimated mass load of ∑45RDC amounting to 1690 kg/month is discharged into the coastal marine environment of Hong Kong, with sewage effluent being the primary source. Population density and vehicle-related factors (e.g., traffic load) were the major drivers shaping the spatial distribution of RDCs in surface water. Based on the ecological risk assessment outcomes, 16 out of 45 RDCs exhibited medium to high risks, and lists of candidate contaminants for various water bodies were proposed to support future risk management in water quality. These findings suggest that RDCs in stormwater and rivers should be carefully monitored, and management strategies should be developed to mitigate their risks.

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.

Bioactivity of the ubiquitous tire preservative 6PPD and degradant, 6PPD-quinone in fish- and mammalian-based assays

6PPD-quinone (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone), a transformation product of the antiozonant 6PPD (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine) is a likely causative agent of coho salmon (Oncorhynchus kisutch) pre-spawn mortality. Stormwater runoff transports 6PPD-quinone into freshwater streams, rapidly leading to neurobehavioral, respiratory distress, and rapid mortality in laboratory-exposed coho salmon, but causing no mortality in many laboratory-tested species. Given this identified hazard, and potential for environmental exposure, we evaluated a set of U.S. Environmental Protection Agency's high-throughput assays for their capability to detect the large potency difference between 6PPD and 6PPD-quinone observed in coho salmon and screen for bioactivities of concern. Assays included transcriptomics in larval fathead minnow (FHM), developmental and behavioral toxicity in larval zebrafish, phenotypic profiling in a rainbow trout gill cell line, acute and developmental neurotoxicity in mammalian cells, and reporter transcription factor activity in HepG2 cells. 6PPD was more consistently bioactive across assays, with distinct activity in the developmental neurotoxicity assay (mean 50th centile activity concentration = 0.91 µM). Although 6PPD-quinone was less potent in FHM and zebrafish, and displayed minimal neurotoxic activity in mammalian cells, it was highly potent in altering organelle morphology in RTgill-W1 cells (phenotype-altering concentration = 0.024 µM compared with 0.96 µM for 6PPD). Although in vitro sensitivity of RTgill-W1 cells may not be as sensitive as intact Coho salmon, the assay may be a promising approach to test chemicals for 6PPD-quinone-like activities. The other assays each identified unique bioactivities of 6PPD, with neurobehavioral and developmental neurotoxicity being most affected, indicating a need for further assessment of this chemical. Our results demonstrate that the common tire additive, 6PPD, is bioactive in a broader set of assays than the environmental transformation product 6PPD-quinone and that it may be a developmental neurotoxicant in mammals, whereas 6PPD-quinone was much more potent than 6PPD in altering the intracellular phenotype of rainbow trout gill cells. Application of the set of high-throughput and high-content bioassays to test the bioactivity of this emerging pollutant has provided data to inform both ecological and human health assessments.

Chronic toxicity and intergenerational effects of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) exposure alone and in combination with Zn2+ on Daphnia magna (Cladocera)

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and Zn2+, extensively used in the tire manufacturing process, are frequently detected in freshwater environments. However, the intergenerational effects of isolated 6PPD exposure and joint 6PPD and Zn2+ exposure at concentrations approximating environmental levels remain unknown. This study assessed the chronic toxicity and intergenerational effects of 6PPD (0.02-20 μg/L) and a mixture of 6PPD and Zn2+ (5 μg/L) over three generations in Daphnia magna bioassays. In the F0 generation, a dose-dependent decline in total offspring number was observed with 6PPD exposure alone, while co-exposure with Zn2+ exacerbated the reproductive toxicity of 6PPD. Across three generations, low-dose (0.02 µg/L) 6PPD alone and combined with Zn2+ induced a cumulative degenerative maternal effect. Conversely, high-dose (20 µg/L) 6PPD, both independently and in combination with Zn2+, exhibited an adaptive maternal effect. Notably, the grandmaternal effect emerged exclusively in the co-exposure group treated with 20 μg/L 6PPD and 5 μg/L Zn2+, with no such effect in the group exposed to 20 μg/L 6PPD alone, suggesting that Zn2+ may enhance the potential toxicity of 6PPD. Overall, this study provides novel insight into the intergenerational impacts of environmentally relevant levels of 6PPD alone and in combination with a heavy metal, elucidating the environmental risks posed by tire-derived chemicals through their synergistic effects on transgenerational toxicity.

Effects of tire particles on earthworm (Eisenia andrei) fitness and bioaccumulation of tire-related chemicals

Tire and Road Wear Particles (TRWP) are produced during the wear of tire rubber on the road pavement and contain various chemicals originating from the road environment and from the rubber. Toxic effects of TRWP and their associated chemicals on soil organisms remain poorly characterized. In a series of laboratory experiments, this study investigated the bioaccumulation kinetics of several common tire-related chemicals in the earthworm species Eisenia andrei using Cryogenically Milled Tire Tread (CMTT), as a surrogate for environmental TRWP. Effects on survival, growth, reproductive output and behaviour were determined. Average biota-soil accumulation factors ranged from 0.8 to 4.7 indicating low to moderate bioaccumulation of the tire-related chemicals. Toxicokinetics showed both high uptake (0.0-13.2 days-1) and elimination rates (0.0-6.3 days-1) in E.andrei. Still, the uptake of tire-related chemicals in earthworms' tissues and ingestion of tire particles could lead to trophic transfer to preys feeding on earthworms and requires further investigated. No significant effects on survival and growth were recorded after exposure to 0.05 and 5% CMTT. In the reproduction test, a slight increase of the reproductive output with increasing CMTT concentration and a slight decrease of the weight of the juveniles were observed. Moreover, a strong and significant avoidance behaviour was observed for worms exposed to 5% CMTT. This work highlights that soil highly contaminated with tire particles can negatively impact habitat function due to changes in texture and/or chemical stressors, lead to uptake of tire-related additives by earthworms and that high concentrations can impact organism's fitness.

Toxicity of tire rubber-derived pollutants 6PPD-quinone and 4-tert-octylphenol on marine plankton

The impacts of tire wear particles and their associated chemicals on the aquatic systems are a major environmental concern. In this study, we investigated the acute toxicity of two pollutants derived from tire rubber, 6PPD-quinone and 4-tert-octylphenol, on marine plankton. Specifically, we determined the acute effects of these pollutants on various organisms within the plankton food web: the microalgae Rhodomonas salina, the adult copepod Acartia tonsa, and the early life stages of the echinoderms Arbacia lixula and Paracentrotus lividus and the fish Sparus aurata. Exposure to 6PPD-quinone did not affect the microalgae growth, copepod survival, or fish embryo viability after 48 h of exposure at concentrations up to 1000 µgL-1. However, 6PPD-quinone significantly inhibited the growth of early developmental stages of both echinoderm species, with median effective concentrations of 7 and 8 µgL-1. Conversely, 4-tert-octylphenol was toxic to all studied organisms, with median lethal and effective concentrations ranging from 21 to 79 µgL-1 depending on the species and endpoints. The most sensitive planktonic organisms to 4-tert-octylphenol were echinoderm embryos and copepods, which exhibited negative effects at concentrations as low as 1 and 25 µgL-1, respectively. Our results demonstrate that acute exposure to 6PPD-quinone and 4-tert-octylphenol can cause harmful effects on key planktonic organisms at environmentally relevant concentrations. Overall, our findings highlight the need for develop ecologically safer tire rubber additives and reduce traffic-related tire particle emissions to mitigate their entry and potential impacts on aquatic ecosystems.

Exploring potential targets and mechanisms of renal tissue damage caused by N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) through network toxicology and animal experiments: A case of chronic kidney disease

6-PPDQ is a new type of environmental contaminant contained in tire rubber. No studies have been reported on the potential targets and mechanisms of action of 6-PPDQ on renal tissue damage. In the present study, we used CKD as an example to explore the potential targets and biological mechanisms of renal injury caused by 6-PPDQ using Network toxicology and animal experiments. A total of 1361 6-PPDQ-related target genes were obtained from the CTD database. 17,296 CKD-related target genes were obtained through the GeneCards database. After intersecting the two, a total of 908 intersecting genes were obtained. Next, we constructed a PPI protein interaction network. Using different algorithms in Cytoscape software and "Logistic regression analysis", five key target genes were finally identified as NOTCH1, TP53, TNF, IL1B and IL6. We constructed a diagnostic model using five key target genes, and the ROC curves, calibration curves and DCA curves proved that the model has good diagnostic value. Molecular docking demonstrated high affinity between 6-PPDQ and five key target gene proteins. In animal experiments, repeated intraperitoneal injections of 6-PPDQ using different concentration gradients for 28 days revealed that the expression levels of five key target genes in renal tissues increased progressively with the increase of the concentration, and the damage to renal tissues was also aggravated. ssGSEA and animal experiments revealed a key role for activation of the MAPK signaling pathway. Finally, we also identified a significant correlation between five key target genes and the level of infiltration of multiple immune cells. In conclusion, these findings suggest that 6-PPDQ can cause damage to renal tissue and that the level of damage progressively increases with increasing concentration. Among them, NOTCH1, TP53, TNF, IL1B and IL6 may be its potential targets of action. Activation of the MAPK signaling pathway is a potential mechanism of action.

Long-Term Exposure to Tire-Derived 6-PPD Quinone Causes Neurotoxicity and Neuroinflammation via Inhibition of HTR2A in C57BL/6 Mice

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), a novel contaminant derived from tire wear, has raised concerns due to its potential neurotoxicity, yet its long-term effects on mammalian neurological health remain poorly understood. This study investigates the neurotoxic and neuroinflammatory impacts of prolonged 6-PPDQ exposure using male C57BL/6 mice. Behavioral assessments revealed significant cognitive deficits, while biochemical analyses demonstrated increased levels of reactive oxygen species, apoptosis, and blood-brain barrier (BBB) disruption. Elevated pro-inflammatory cytokines (TNF-α, IL-6, IL-1β) and activation of microglial cells were observed, indicating a robust neuroinflammatory response. Network pharmacology and molecular docking identified serotonin receptor HTR2A as a key target through which 6-PPDQ mediates its toxic effects. Activation of HTR2A by the agonist DOI (2,5-dimethoxy-4-iodoamphetamine) mitigated these effects, suggesting a potential therapeutic strategy. These findings provide the first evidence of 6-PPDQ-induced neurotoxicity in mammals, underscoring the need for preventive measures to protect neurological health.

6PPD induces cerebrovascular defects by triggering oxidative stress and ferroptosis in zebrafish

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), which is widely used as an antiozonant in rubber tires, has recently got much attention for its acute aquatic toxicity. However, the developmental toxicity of 6PPD in cerebrovascular network remains unknown. Here, we investigated the effects of 6PPD exposure in cerebral vascular using zebrafish. 6PPD would not affect the body length and shape of zebrafish larvae at the concentrations ranging from 20 μg/L to 1000 μg/L. 6PPD induced developmental defects in the brain in a concentration-dependent manner. The trunk vascular development would not be affected while the cerebrovascular network was disrupted upon 6PPD exposure. 6PPD would trigger excessive Reactive Oxygen Species (ROS) in the brain, indicating abnormal oxidative stress. Mechanistically, brain-specific transcriptome analysis showed that 6PPD could potentially cause the blockage of arachidonic acid (AA) metabolism-related genes and the upregulation of ferroptosis-related genes. Besides, treatment with ferroptosis inhibitor N-Acetyl-L-cysteine (NAC) attenuated oxidative damage and improved the construction of cerebrovascular network upon 6PPD exposure. Moreover, using a human vascular endothelial cell line, we further confirmed that 6PPD could trigger abnormal oxidative stress and defective expansion capacity, implying the conserved toxicity cross species. These findings are useful for the elucidation of toxicity underlying 6PPD in cerebrovascular systems of both zebrafish and humans.

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.

Transgenerational intestinal toxicity of 6-PPD quinone in causing ROS production, enhancement in intestinal permeability and suppression in innate immunity in C. elegans

Toxicity of 6-PPD quinone (6-PPDQ) on organisms at various aspects has been frequently observed at parental generation (P0-G). In contrast, we know little about its possible transgenerational toxicity and underlying mechanisms. In Caenorhabditis elegans, exposure to 6-PPDQ (0.1-10 μg/L) at P0-G induced transgenerational reactive oxygen species (ROS) production in intestine. Accompanied with this, transgenerational increase in intestinal permeability and decrease in expressions of genes governing intestinal function were observed. Exposure to 6-PPDQ (1 and 10 μg/L) at P0-G caused transgenerational suppression in expressions of antimicrobial genes (lys-7 and spp-1) and LYS-7::RFP. Meanwhile, intestinal ROS production could be enhanced by RNAi of acs-22, hmp-2, pkc-3, lys-7, and spp-1. Moreover, acs-22, hmp-2, and pkc-3 RNAi could inhibit innate immune response induced by 6-PPDQ. Additionally, lys-7 and spp-1 RNAi could strengthen intestinal permeability in 6-PPDQ exposed nematodes. Therefore, 6-PPDQ caused transgenerational intestinal toxicity, which was associated with both enhanced intestinal permeability and suppressed innate immunity.

Phytotoxicity of 6PPD and its uptake by Myriophyllum verticillatum: Oxidative stress and metabolic processes

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a widely utilized antioxidant in automobile tires and rubber goods, is frequently detected in aquatic ecosystems and poses a potential threat to aquatic organisms. However, research on the impact of 6PPD on aquatic plants is still scarce. Here, we investigated the bioaccumulation of 6PPD in Myriophyllum verticillatum (M. verticillatum) (watermilfoil), and its impacts on biochemical characteristics and metabolomics. 6PPD (10,100 mg/L) significantly inhibited the growth and photosynthetic pigment content of M. verticillatum. After 14 days of exposure to 100 μg/L 6PPD, accumulation levels of 6PPD and its metabolite 6PPDQ in M. verticillatum reached 0.52 mg/kg and 0.09 mg/kg, respectively. Moreover, 6PPD significantly induced the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) enzymes and glutathione (GSH), reducing the levels of reactive oxygen species (ROS) and malondialdehyde (MDA), thereby mitigating oxidative damage in M. verticillatum. Furthermore, metabolic pathway analysis revealed that 6PPD has remarkable effects on amino acid and sugar metabolism. This study provides data support for understanding the toxic effects of 6PPD on aquatic plants and evaluating its potential risks.

Accurate and stable detection of p-phenylenediamine antioxidants and their transformation products in aquatic products using antioxidant protection - Analysis of actual aquatic products

Given the high toxicity of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) derivatives, such as 6PPD quinone (6PPDQ) to salmon, as well as their ubiquitous presence in the environment, the contaminant of aquatic food products has drawn significant attention. However, analytical methods for p-Phenylenediamines (PPDs) and their transformation products (TPs) in aquatic products remain underdeveloped. In particular, the degradation of some compounds and strong matrix effects complicate detection. In this study, we present a stable, rapid, and sensitive method combining salt-out assisted extraction, antioxidant protection, and multi-plug filtration clean-up (m-PFC) to detect two PPDs and five TPs in aquatic products. Crucially, the appropriate selection of antioxidants prevented the degradation of the easily oxidized target compounds. Further, the m-PFC method significantly enhanced the purification efficiency, achieving satisfactory recoveries (62.1-115 %), and method detection limits (MDLs) ranging from 0.00300 to 0.400 μg/kg. Subsequently, the method was applied to monitor PPDs and their TPs in aquatic products systematically, revealing the presence of 6PPD and N-isopropyl-N'-phenyl-1,4-phenylenediamine (IPPD) in white shrimp from aquafarms, whereas none of the seven target analytes were detected in fish and crab samples. These findings contribute to the detection of PPDs, their TPs and other unstable chemicals in aquatic products, thereby providing insights into their concentrations in these products.

Evidence for the formation of 6PPD-quinone from antioxidant 6PPD by cytochrome P450

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) as a rubber antioxidant has attracted global concern, since its ozone-oxidation product 6PPD-quinone (6PPDQ) was found to be the primary toxicant responsible for urban runoff mortality syndrome in coho salmon. However, the biotransformation fate and associated toxicological mechanism of 6PPD have not received much study yet. In this work, the in vitro assays showed 6PPD can be transformed into 6PPDQ by cytochromes P450 (CYP450) in human liver microsomes (HLMs) with 0.98 % production rate, and the adducts of 6PPDQ with calf thymus DNA and the N-N coupling product between 6PPD and 6PPDQ were further identified after 6PPD incubation in HLMs. Further evidence for the 6PPDQ formation can be obtained from the in vivo assays that the 6PPDQ-DNA adducts and 6PPD-N-N-6PPDQ dimer were detected in mice by oral gavage with 6PPD, and the latter dimer species was detected as well in 6PPD exposure to zebrafish larvae. Especially, the bioaccumulation property and high reactivity of 6PPDQ result in the continuous formation of the significant DNA adducts and 6PPD-N-N-6PPDQ dimer even in case of low production rate of biotransformation of 6PPD to 6PPDQ, which may provide potentially effective biomarkers for such process. DFT computations revealed the formation mechanism of 6PPDQ is the (N)H-abstraction of 6PPD by CYP450, followed by amino radical rebound at the nearby ortho-carbon, yielding a quinol intermediate due to spin delocalization, that might readily undergo further oxidation by CYP450 into 6PPDQ.

Stress of soil moisture and temperature exacerbates the toxicity of tire wear particles to soil fauna: Tracking the role of additives through host microbiota

Tire wear particles (TWPs) are considered as an emerging threat to soil fauna. However, how TWP toxicity to soil fauna responds to the stress of soil moisture and temperature remains unclear. We assessed the toxicity of environmentally relevant TWPs to the soil model species Enchytraeus crypticus under three soil moisture and two temperature gradients. Typical thermoplastic polypropylene (PP) was selected for comparison. Results showed that compared with PP, TWPs exerted stronger toxicity, including decreasing the worm growth, survival and reproduction rates, disturbing the soil and worm gut microbiota, and leaching more diverse and higher contents of additives. Stress of soil moisture and temperature exacerbated TWP toxicity mainly through affecting the leaching and transformation of additives. Fourteen mediated additives significantly contributed to the shift of the gut microbiota under soil moisture and temperature stress, among which 1,3-diphenylguanidine, N,N'-bis(methylphenyl)-1,4-benzenediamine quinone, N-tert-butyl-2-benzothiazolesulfenamide, and 2-aminobenzothiazole were identified as the main drivers. In addition, this study provided the first clear evidence that increased soil moisture and temperature promoted the transformation of additives in the soil. Our study revealed the non-negligible aggravated toxicity of TWPs to soil fauna under stress of soil moisture and temperature, providing novel insights into the environmental behavior of additives.

Development and application of diffusive gradients in thin-films for in-situ monitoring of 6PPD-Quinone in urban waters

The occurrence and risk of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), derived from the oxidation of the tire antidegradant 6PPD, has raised significant concern since it was found to cause acute mortality in coho salmon when exposed to urban runoff. Given the short half-life period and low solubility of 6PPD-Q, reliable in situ measurement techniques are required to accurately understand its occurrence and behaviour in aquatic environments. Here, using the diffusive gradients in thin-films (DGT) method with HLB as a binding agent, we developed a new methodology to measure 6PPD-Q in urban waters. 6PPD-Q was rapidly and strongly adsorbed on the HLB-binding gel and was efficiently extracted using organic solvents. The HLB-DGT accumulated 6PPD-Q linearly for >7 d and its performance was not significantly affected by pH (6.5-8.5), ionic strength (0.0001-0.5 M) or dissolved organic matter (0-20 mg L-1). Field evaluation of the DGT method demonstrated its effectiveness in urban runoff, detecting 6PPD-Q levels of 15.8-39.5 ng L-1 in rivers. In snowmelt, DGT detected 6PPD-Q levels of 210 ng L-1 which is two times higher than the value obtained by grab sampling. 6PPD-Q levels were much higher in snowmelt than those in rivers. This indicates that snowfall constitutes an important transport pathway for 6PPD-Q and that DGT effectively captured the fraction continuously released from dust particles in the snow samples. 6PPD-Q posed a substantial risk to migratory fish in urban waters, and its release from tire wear particles requires further investigation. This study is the first to develop a DGT-based method for 6PPD-Q determination in urban waters, and the method can ensure an accurate measurement of the release of 6PPD-Q to the environment, particularly in rainfall or snowmelt, important pathways for its entry into the aquatic environment.

Paeoniflorin alleviates toxicity and accumulation of 6-PPD quinone by activating ACS-22 in Caenorhabditis elegans

6-PPD quinone (6-PPDQ) is extensively existed in various environments. In Caenorhabditis elegans, exposure to 6-PPDQ could cause multiple toxic effects. In the current study, we further used C. elegans to investigate the effect of paeoniflorin (PF) treatment on 6-PPDQ toxicity and accumulation and the underlying mechanism. Treatment with PF (25-100 mg/L) inhibited 6-PPDQ toxicity on reproduction capacity and locomotion behavior and in inducing reactive oxygen species (ROS) production. Additionally, PF (25-100 mg/L) alleviated the dysregulation in expression of genes governing oxidative stress caused by 6-PPDQ exposure. Moreover, PF (25-100 mg/L) inhibited the enhancement in intestinal permeability caused by 6-PPDQ exposure and the accumulation of 6-PPDQ in the body of nematodes. In 6-PPDQ exposed nematodes, PF (25-100 mg/L) increased expression of acs-22 encoding a fatty acid transporter. RNAi of acs-22 could inhibit the beneficial effect of PF against 6-PPDQ toxicity in decreasing reproductive capacity and locomotion behavior, in inducing intestinal ROS production, and in enhancing intestinal permeability. RNAi of acs-22 could also suppress the PF beneficial effect against 6-PPDQ accumulation in the body of nematodes. Therefore, our results demonstrate the function of PF treatment against 6-PPDQ toxicity and accumulation in nematodes by activating the ACS-22.

A transcriptomics-based analysis of mechanisms involved in the neurobehavioral effects of 6PPD-quinone on early life stages of zebrafish

As an emerging pollutant frequently detected in aquatic ecosystems, the toxicity of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) on fish has been confirmed, but insight into the mechanisms underlying those adverse effects is still limited. Thus, we exposed zebrafish embryos to 6PPD-quinone at 0, 0.25, 2.5, and 25 μg/L until 120 h post-fertilization (hpf), and investigated the variations in their development, behavior, monoamine neurotransmitter levels, and transcriptional profile. Exposure to 6PPD-quinone notably elevated the heart rate of zebrafish at 48 hpf (at 2.5 and 25 μg/L) and 72 hpf (at 0.25, 2.5, and 25 μg/L). In the dark-light transition test, the locomotor activity of zebrafish larvae exposed to 6PPD-quinone significantly increased, especially in the dark periods. Exposure to 6PPD-quinone also altered the dopamine level and its turnover in zebrafish, which exhibited significant correlations to their locomotor activity. RNA sequencing identified 394 differentially expressed genes (DEGs), most of which have the molecular function of binding and catalytic activity. Five DEGs were predicted as the key driver genes in the protein-protein interaction networks associated with circadian rhythm (i.e., npas2), protein processing in endoplasmic reticulum (i.e., hsp90b1 and pdia4), and estrogen signaling pathway (i.e., hsp90aa1.1 and hsp90aa1.2). Our findings provide more insights into mechanisms underlying the toxicity of 6PPD-quinone to teleosts and highlight the necessity to assess its potential risks to aquatic ecosystems.

Typical Tire Additives in River Water: Leaching, Transformation, and Environmental Risk Assessment

Tire wear particles (TWPs) released during vehicle driving can enter water bodies, leading to leaching of tire additives (TAs) in aquatic environments. However, the transformation behavior and related ecological impacts of TAs and their transformation products (TPs) remain unclear. In this study, laboratory-based simulation experiments and field investigations were conducted to explore the transformation mechanisms and ecological risks of TAs. After being placed in river water for 24 h, about 7-95% of 12 investigated TAs in TWPs were leached. Forty-eight TPs from eight TAs were tentatively identified along with different transformation pathways via suspect screening by high-resolution mass spectrometry. Semiquantitative results indicated that TPs derived from N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylene-diamine (6PPD) were predominant in leachates, while aryl hydrolysis and quinone pathways were the main transformation pathways. Field investigations on urban surface water samples from 16 sites in Hong Kong revealed the occurrence of 17 TAs and 1 TP, with concentrations ranging from 13.9 to 2230 ng/L (median ± standard deviation: 226 ± 534 ng/L). Sixteen TPs from six TAs were additionally identified via suspect screening. It is estimated that 6PPD-quinone and seven TAs could pose medium to high ecological risk, while N-(1,3-dimethylbutyl)-N'-phenyl-p-quinonediimine, a frequently detected TP, was identified as a persistent-bioaccumulative-toxic substance.

Nontarget analysis and characterization of p-phenylenediamine-quinones and -phenols in tire rubbers by LC-HRMS and chemical species-specific algorithm

BACKGROUND

N,N'-disubstituted p-phenylenediamine-quinones (PPDQs) are oxidization derivatives of p-phenylenediamines (PPDs) and have raised extensive concerns recently, due to their toxicities and prevalence in the environment, particularly in water environment. PPDQs are derived from tire rubbers, in which other PPD oxidization products besides reported PPDQs may also exist, e.g., unknown PPDQs and PPD-phenols (PPDPs).

RESULTS

This study implemented nontarget analysis and profiling for PPDQ/Ps in aged tire rubbers using liquid chromatography-high-resolution mass spectrometry and a species-specific algorithm. The algorithm took into account the ionization behaviors of PPDQ/Ps in both positive and negative electrospray ionization, and their specific carbon isotopologue distributions. A total of 47 formulas of PPDQ/Ps were found and elucidated with tentative or accurate structures, including 25 PPDQs, 18 PPDPs and 4 PPD-hydroxy-quinones (PPDHQs). The semiquantified total concentrations of PPDQ/Ps were 14.08-30.62 μg/g, and the concentrations followed the order as: PPDPs (6.48-17.39) > PPDQs (5.86-12.14) > PPDHQs (0.16-1.35 μg/g).

SIGNIFICANCE

The high concentrations and potential toxicities indicate that these PPDQ/Ps could seriously threaten the eco-environment, as they may finally enter the environment, accordingly requiring further investigation. The analysis strategy and data-processing algorithm can be extended to nontarget analysis for other zwitterionic pollutants, and the analysis results provide new understandings on the environmental occurrence of PPDQ/Ps from source and overall perspectives.

Characterizing the Metabolism of Tire Rubber-Derived p-Phenylenediamine Quinones to Identify Potential Exposure Biomarkers in Humans

There is growing evidence of the frequent detection of tire rubber-derived contaminants p-phenylenediamine-derived quinones (PPD-Qs) (e.g., highly toxic N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q)) in the environment and biota and the adverse impact on organisms. Hence, a better understanding of their biotransformation/metabolism in humans is essential. However, relevant data are lacking owing to recent discoveries. Herein, the biotransformation patterns of 6PPD-Q and other five commonly detected PPD-Qs were characterized via combined in vitro assay and maternal cord blood screening monitoring. Rapid metabolism was found for each PPD-Q incubated with human liver S9 fraction and microsomes, resulting in the formation of abundant phase I and phase II metabolites. The subsequent screening for potential PPD-Q metabolites in blood samples showed the presence of suspect metabolites. Three detected metabolites were confirmed by matching the mass spectra and retention times of in vitro metabolites. N-Dealkylated, carboxy, carbonyl, and reductive metabolites and glucose, cysteine, and methionine conjugates were observed for the first time. The semiquantitative concentrations of metabolites were higher than those of the parent PPD-Qs, and several metabolites such as carboxy products were proposed as candidate biomarkers of PPD-Q exposure to humans. 6PPD-Q and N,N'-diphenyl-p-phenylenediamine quinone were detected in maternal and/or cord whole blood samples for the first time. This study holds great importance in elucidating the potential risks and health effects of 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.

Presence of N, N'-Substituted p-Phenylenediamine-Derived Quinones in Human Urine

Human exposure to various N,N'-substituted p-phenylenediamine-derived quinones (PPDQs) has been of increasing concern. Recent studies have examined N-phenyl-N'-(1,3-dimethylbutyl)-p-phenylenediamine-derived quinone (6PPDQ) in human urine to evaluate human exposure. However, other PPDQs in human urine have not been thoroughly investigated. This study analyzed six PPDQs in urine collected from 149 healthy individuals in Taizhou, China. All target PPDQs were detected, with 6PPDQ (mean 2.4 ng/mL, Collapse

Key Words

• 6PPDQ
• CPPDQ
• human exposure
• human urine
• p-phenylenediamine-derived quinones

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MESH Headings Collapse

Grants

• LY21B070006 Natural Science Foundation of Zhejiang Province
• LY21H160008 Natural Science Foundation of Zhejiang Province
• LBY23H200003 Natural Science Foundation of Zhejiang Province

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Affiliation(s)

Juxiu Huang Taizhou Central Hospital (Taizhou University Hospital), School of Medicine, Taizhou University, Taizhou 318000, China; (J.H.); (L.Z.)
Hangbiao Jin Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou 310032, China; (H.J.); (R.G.)
Yingying Zhu School of Life Sciences, Taizhou University, Taizhou 318000, China;
Ruyue Guo Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou 310032, China; (H.J.); (R.G.)
Lisha Zhou Taizhou Central Hospital (Taizhou University Hospital), School of Medicine, Taizhou University, Taizhou 318000, China; (J.H.); (L.Z.)
Xiaoyu Wu Taizhou Central Hospital (Taizhou University Hospital), School of Medicine, Taizhou University, Taizhou 318000, China; (J.H.); (L.Z.)

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Impact of Biotransformation on Internal Concentrations and Specificity Classification of Organic Chemicals in the Zebrafish Embryo (Danio rerio)

Internal concentrations (ICs) are crucial for linking exposure to effects in the development of New Approach Methodologies. ICs of chemicals in aquatic organisms are primarily driven by hydrophobicity and modulated by biotransformation and efflux. Comparing the predicted baseline to observed toxicity enables the estimation of effect specificity, but biological processes can lead to overestimating ICs and bias the specificity assessment. To evaluate the prediction of a mass balance model (MBM) and the impact of biotransformation on ICs, experimental ICs of 63 chemicals in zebrafish embryos were compared to predictions with physicochemical properties as input parameters. Experimental ICs of 79% (50 of 63) of the chemicals deviated less than 10-fold from predictions, and the remaining 13 deviated up to a factor of 90. Using experimental ICs changed the classification for 19 chemicals, with ICs 5 to 90 times lower than predicted, showing the bias of specificity classification. Uptake kinetics of pirinixic acid, genistein, dexamethasone, ethoprophos, atorvastatin, and niflumic acid were studied over a 96 h exposure period, and transformation products (TPs) were elucidated using suspect- and nontarget screening with UPLC-HRMS. 35 TPs (5 to 8 TPs per compound) were tentatively identified and semiquantified based on peak areas, suggesting that biotransformation may partly account for the overpredictions of ICs.

Automated Image-Based Fluorescence Screening of Mitochondrial Membrane Potential in Daphnia magna: An Advanced Ecotoxicological Testing Tool

This study demonstrated the strengths of in vivo molecular staining coupled with automated imaging analysis in Daphnia magna. A multiwell plate protocol was developed to assess mitochondrial membrane potential using the JC-1 dye. The suitability of five common anesthetics was initially tested, and 5% ethanol performed best in terms of anesthetic effects and healthy recovery. The staining conditions were optimized to 30 min staining with 2 μM JC-1 for best J-aggregate formation. The protocol was validated with the model compound carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and used to measure the effect of four environmental contaminants, 2,4-dinitrophenol, triclosan, n-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), and ibuprofen, on mitochondrial health. Test organisms were imaged using an automated confocal microscope, and fluorescence intensities were automatically quantified. The effect concentrations for CCCP were lower by a factor of 30 compared with the traditional OECD 202 acute toxicity test. Mitochondrial effects were also detected at lower concentrations for all tested environmental contaminants compared to the OCED 202 test. For 2,4-dinitrophenol, mitochondria effects were detectable after 2 h exposure to environmentally relevant concentrations and predicted organism death was observed after 24 h. The high sensitivity and time efficiency of this novel automated imaging method make it a valuable tool for advancing ecotoxicological testing.

Neurotoxicity from long-term exposure to 6-PPDQ: Recent advances

The recent acceleration of industrialization and urbanization has brought significant attention to N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), an emerging environmental pollutant from tire wear, due to its long-term effects on the environment and organisms. Recent studies suggest that 6-PPDQ can disrupt neurotransmitter synthesis and release, impact receptor function, and alter signaling pathways, potentially causing oxidative stress, inflammation, and apoptosis. This review investigates the potential neurotoxic effects of prolonged 6-PPDQ exposure, the mechanisms underlying its cytotoxicity, and the associated health risks. We emphasize the need for future research, including precise exposure assessments, identification of individual differences, and development of risk assessments and intervention strategies. This article provides a comprehensive overview of 6-PPDQ's behavior, impact, and neurotoxicity in the environment, highlighting key areas and challenges for future research.

Tire-derived contaminants 6PPD and 6PPD-Q: Analysis, sample handling, and reconnaissance of United States stream exposures

The environmental ubiquity of tire and road wear particles (TRWP) underscores the need to understand the occurrence, persistence, and environmental effects of tire-related chemicals in aquatic ecosystems. One such chemical is 6PPD-quinone (6PPD-Q), a transformation product of the tire antioxidant 6PPD. In urban stormwater runoff 6PPD-Q can exceed acute toxicity thresholds for several salmonid species and is being implicated in significant coho salmon losses in the Pacific Northwest. There is a critical need to understand the prevalence of 6PPD-Q across watersheds to identify habitats heavily affected by TRWPs. We conducted a reconnaissance of 6PPD and 6PPD-Q in surface waters across the United States from sites (N = 94) with varying land use (urban, agricultural, and forested) and streamflow to better understand stream exposures. A rapid, low-volume direct-inject, liquid chromatography mass spectrometry method was developed for the quantitation of 6PPD-Q and screening for 6PPD. Laboratory holding times, bottle material, headspace, and filter materials were investigated to inform best practices for 6PPD-Q sampling and analysis. Glass bottles with PTFE-lined caps minimized sorption and borosilicate glass fiber filters provided the highest recovery. 6PPD-Q was stable for at least 5 months in pure laboratory solutions and for 75 days at 5 °C with minimal headspace in the investigated surface water and stormwaters. Results also indicated samples can be frozen to extend holding times. 6PPD was not detected in any of the 526 analyzed samples and there were no detections of 6PPD-Q at agricultural or forested sites. 6PPD-Q was frequently detected in stormwater (57%, N = 90) and from urban impacted sites (45%, N = 276) with concentrations ranging from 0.002 to 0.29 μg/L. The highest concentrations, above the lethal level for coho salmon, occurred during stormwater runoff events. This highlights the importance of capturing episodic runoff events in urban areas near ecologically relevant habitat or nursery grounds for sensitive species.

Synthesis of a cysteine functional covalent organic framework via facile click reaction for the efficient solid phase extraction of substituted p-phenylenediamine-derived quinones

N,N'-Substituted p-phenylenediamine quinones (PPD-Qs) are the emerging toxicant, which transform from the rubber tire antioxidant N,N'-substituted p-phenylenediamines (PPDs). Because of their potential toxic and widespread occurrence in the environment, PPD-Qs have received great attention. However, efficiently extracting PPD-Qs from complex samples is still a challenge. Herein, a cysteine functional covalent organic framework (Cys-COF) designed according to the "donor-acceptor" sites of hydrogen bonding of PPD-Qs was synthesized via click reaction and then used as solid-phase extraction (SPE) adsorbent. Cys-COF can form the seven-member ring adsorption structure with PPD-Qs via hydrogen bonding. The adsorption mechanism was tentatively revealed by density functional theory (DFT). After optimizing the Cys-COF-SPE parameters, PPD-Qs were efficiently extracted from water, soil, sediment, and fish, followed by detection using ultra-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). The Cys-COF-SPE-UHPLC-MS/MS method exhibited ideal linearity (R2 ≥ 0.9932), high relative recoveries (80.4-111 %), and low limits of detection (0.0001-0.0013 ng mL-1). In addition, the bioconcentration kinetics in goldfish provides a feasible platform to investigate the toxicity and accumulated ability of PPD-Qs.

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.

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.

Navigating the environmental dynamics, toxicity to aquatic organisms and human associated risks of an emerging tire wear contaminant 6PPD quinone

N-1,3-Dimethylbutyl-N'-phenyl-p-quinone diamine (6PPDQ) is a derivative of 6PPD, a synthetic antioxidant used in tire manufacturing to control the degradation caused by oxidation and heat aging. Its discovery in 2020 has raised important environmental concern, particularly regarding its association with acute mortality in coho salmon, prompting surge in research on its occurrence, fate, and transport in aquatic ecosystems. Despite this attention, there remain notable gaps in grasping the knowledge, demanding an in depth overview. Thus, this review consolidates recent studies to offer a thorough investigation of 6PPDQ's environmental dynamics, pathways into aquatic ecosystems, toxicity to aquatic organisms, and human health implications. Various aquatic species exhibit differential susceptibility to 6PPDQ toxicity, manifesting in acute mortalities, disruption of metabolic pathways, oxidative stress, behavioral responses, and developmental abnormalities. Whereas, understanding the species-specific responses, molecular mechanisms, and broader ecological implications requires further investigation across disciplines such as ecotoxicology, molecular biology, and environmental chemistry. Integration of findings emphasizes the complexity of 6PPDQ toxicity and its potential risks to human health. However, urgent priorities should be given to the measures like long-term monitoring studies to evaluate the chronic effects on aquatic ecosystems and the establishment of standardized toxicity testing protocols to ensure the result comparability and reproducibility. This review serves as a vital resource for researchers, policymakers, and environmental professionals seeking appraisals into the impacts of 6PPDQ contamination on aquatic ecosystems and human health.

Comparison of intestinal toxicity in enhancing intestinal permeability and in causing ROS production of six PPD quinones in Caenorhabditis elegans

As the derivatives of p-phenylenediamines (PPDs), PPD quinones (PPDQs) have received increasing attention due to their possible exposure risk. We compared the intestinal toxicity of six PPDQs (6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ and IPPDQ) in Caenorhabditis elegans. In the range of 0.01-10 μg/L, only 77PDQ (10 μg/L) moderately induced the lethality. All the examined PPDQs at 0.01-10 μg/L did not affect intestinal morphology. Different from this, exposure to 6-PPDQ (1-10 μg/L), 77PDQ (0.1-10 μg/L), CPPDQ (1-10 μg/L), DPPDQ (1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (10 μg/L) enhanced intestinal permeability to different degrees. Meanwhile, exposure to 6-PPDQ (0.1-10 μg/L), 77PDQ (0.01-10 μg/L), CPPDQ (0.1-10 μg/L), DPPDQ (0.1-10 μg/L), DTPDQ (1-10 μg/L), and IPPDQ (1-10 μg/L) resulted in intestinal reactive oxygen species (ROS) production and activation of both SOD-3::GFP and GST-4::GFP. In 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ exposed nematodes, the ROS production was strengthened by RNAi of genes (acs-22, erm-1, hmp-2, and pkc-3) governing functional state of intestinal barrier. Additionally, expressions of acs-22, erm-1, hmp-2, and pkc-3 were negatively correlated with intestinal ROS production in nematodes exposed to 6-PPDQ, 77PDQ, CPPDQ, DPPDQ, DTPDQ, and/or IPPDQ. Therefore, exposure to different PPDQs differentially induced the intestinal toxicity on nematodes. Our data highlighted potential exposure risk of PPDQs at low concentrations to organisms by inducing intestinal toxicity.

In Vitro and In Vivo Biotransformation Profiling of 6PPD-Quinone toward Their Detection in Human Urine

The antioxidant N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its oxidized quinone product 6PPD-quinone (6PPD-Q) in rubber have attracted attention due to the ecological risk that they pose. Both 6PPD and 6PPD-Q have been detected in various environments that humans cohabit. However, to date, a clear understanding of the biotransformation of 6PPD-Q and a potential biomarker for exposure in humans are lacking. To address this issue, this study presents a comprehensive analysis of the extensive biotransformation of 6PPD-Q across species, encompassing both in vitro and in vivo models. We have tentatively identified 17 biotransformation metabolites in vitro, 15 in mice in vivo, and confirmed the presence of two metabolites in human urine samples. Interestingly, different biotransformation patterns were observed across species. Through semiquantitative analysis based on peak areas, we found that almost all 6PPD-Q underwent biotransformation within 24 h of exposure in mice, primarily via hydroxylation and subsequent glucuronidation. This suggests a rapid metabolic processing of 6PPD-Q in mammals, underscoring the importance of identifying effective biomarkers for exposure. Notably, monohydroxy 6PPD-Q and 6PPD-Q-O-glucuronide were consistently the most predominant metabolites across our studies, highlighting monohydroxy 6PPD-Q as a potential key biomarker for epidemiological research. These findings represent the first comprehensive data set on 6PPD-Q biotransformation in mammalian systems, offering insights into the metabolic pathways involved and possible exposure biomarkers.

Environmental profiles, hazard identification, and toxicological hallmarks of emerging tire rubber-related contaminants 6PPD and 6PPD-quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is commonly used in rubber compounds as antioxidants to protect against degradation from heat, oxygen, and ozone exposure. This practice extends the lifespan of rubber products, including tires, by preventing cracking, aging, and deterioration. However, the environmental consequences of waste generated during rubber product use, particularly the formation of 6PPD-quinone (6PPD-Q) through the reaction of 6PPD with ozone, have raised significant concerns due to their detrimental effects on ecosystems. Extensive research has revealed the widespread occurrence of 6PPD and its derivate 6PPD-Q in various environmental compartments, including air, water, and soil. The emerging substance of 6PPD-Q has been shown to pose acute mortality and long-term hazards to aquatic and terrestrial organisms at concentrations below environmentally relevant levels. Studies have demonstrated toxic effects of 6PPD-Q on a range of organisms, including zebrafish, nematodes, and mammals. These effects include neurobehavioral changes, reproductive dysfunction, and digestive damage through various exposure pathways. Mechanistic insights suggest that mitochondrial stress, DNA adduct formation, and disruption of lipid metabolism contribute to the toxicity induced by 6PPD-Q. Recent findings of 6PPD-Q in human samples, such as blood, urine, and cerebrospinal fluid, underscore the importance of further research on the public health and toxicological implications of these compounds. The distribution, fate, biological effects, and underlying mechanisms of 6PPD-Q in the environment highlight the urgent need for additional research to understand and address the environmental and health impacts of these compounds.

Mixture toxicity of 6PPD-quinone and polystyrene nanoplastics in zebrafish

Plastic pollution, including micro- and nanoplastics, is a growing concern. Tyre-wear particles (TWPs) are the second largest source of microplastics in the ocean following abrasion of synthetic fibres. In addition to the particles themselves, TWPs contain many harmful chemicals, including 6PPD. This chemical reacts with atmospheric ozone and forms the toxic compound 6PPD-quinone (6PPDq), which poses a danger to aquatic life. There is a knowledge gap in understanding risks associated with the combined toxicity of nanoplastics (NPs) and 6PPDq. The present study aimed to investigate the toxicity of NPs and 6PPDq on adult zebrafish using phenotypic (behaviour, histology) and transcriptomic endpoints. Zebrafish were exposed to four treatments: control (contaminant-free), 50 μg/L 6PPDq, 3 mg/L polystyrene (PS)-NPs, and a combination of 50 μg/L 6PPDq and 3 mg/L PS-NPs. We did not observe locomotory dysregulation in zebrafish exposed to NPs. However, we found significant hyperlocomotion in zebrafish exposed to 6PPDq and this effect was even more substantial after co-exposure with PS-NPs. This study explores the molecular mechanisms behind these effects, identifying genes associated with neurotransmitters and fatty acid metabolism that were dysregulated by the co-exposure. Transcriptomic analysis further showed that both 6PPDq and PS-NPs impacted cellular processes associated with sterol biosynthesis, cholesterol metabolism, and muscle tissue development. The effects on these mechanisms were stronger in co-exposed zebrafish, indicating a heightened risk to cellular integrity and mitochondrial dysfunction. These results highlight the significance of mixture toxicity when studying the effects of NPs and associated chemicals like 6PPDq.

Fragmentation Pattern-Based Screening Strategy Combining Diagnostic Ion and Neutral Loss Uncovered Novel para-Phenylenediamine Quinone Contaminants in the Environment

Identifying transformed emerging contaminants in complex environmental compartments is a challenging but meaningful task. Substituted para-phenylenediamine quinones (PPD-quinones) are emerging contaminants originating from rubber antioxidants and have been proven to be toxic to the aquatic species, especially salmonids. The emergence of multiple PPD-quinones in various environmental matrices and evidence of their specific hazards underscore the need to understand their environmental occurrences. Here, we introduce a fragmentation pattern-based nontargeted screening strategy combining full MS/All ion fragmentation/neutral loss-ddMS2 scans to identify potential unknown PPD-quinones in different environmental matrices. Using diagnostic fragments of m/z 170.0600, 139.0502, and characteristic neutral losses of 199.0633, 138.0429 Da, six known and three novel PPD-quinones were recognized in air particulates, surface soil, and tire tissue. Their specific structures were confirmed, and their environmental concentration and composition profiles were clarified with self-synthesized standards. N-(1-methylheptyl)-N'-phenyl-1,4-benzenediamine quinone (8PPD-Q) and N,N'-di(1,3-dimethylbutyl)-p-phenylenediamine quinone (66PD-Q) were identified and quantified for the first time, with their median concentrations found to be 0.02-0.21 μg·g-1 in tire tissue, 0.40-2.76 pg·m-3 in air particles, and 0.23-1.02 ng·g-1 in surface soil. This work provides new evidence for the presence of unknown PPD-quinones in the environment, showcasing a potential strategy for screening emerging transformed contaminants in the environment.

Occurrence of p-phenylenediamine antioxidants in human urine

General populations are widely exposed to various p-phenylenediamine antioxidants (PPDs). N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), a typical p-phenylenediamine antioxidant, has been detected in human urine samples. However, the occurrence of other widely used PPDs in human urine is still unclear. This study comprehensively characterized the occurrence of 9 PPDs in human urine from 151 Chinese adults. Our results showed that all target PPDs were detected in human urine samples, with the total concentrations of PPDs ranging from 0.41 to 38 ng/mL. PPDs in human urine was dominated by 6PPD (mean 1.2 ng/mL, range < LOD - 3.8 ng/mL), followed by N-phenyl-N'-cyclohexyl-p-phenylenediamine (CPPD; 0.85 ng/mL, Collapse

Key Words

• 6PPD
• Human exposure
• Human urine
• PPDs
• Synthetic antioxidants

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MESH Headings

• Adult
• Humans
• Male
• Female
• Antioxidants
• Phenylenediamines
• Nitro Compounds

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Affiliation(s)

Weili Mao Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China
Hangbiao Jin Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, PR China; Innovation Research Center of Advanced Environmental Technology, Eco-Industrial Innovation Institute ZJUT, Quzhou, Zhejiang 324400, PR China
Ruyue Guo Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang Province, College of Environment, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, PR China
Ping Chen Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China
Songyang Zhong Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China
Xilin Wu Department of Pharmacy, Quzhou Affiliated Hospital of Wenzhou Medical University, Quzhou People's Hospital, Quzhou, Zhejiang 324000, PR China.

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

6PPD-quinone (6PPD-Q) has been identified as a ubiquitous contaminant in the surrounding locality, including air particles, roadside soils, dust, and water. Recently, the prevalence of 6PPD-Q in human urine has accentuated the urgency for investigating its biological fate. To address this, we conducted a stable isotope-assisted high-resolution mass spectrometry (HRMS) assay to unveil the distribution, metabolism, excretion, and toxicokinetic properties of this contaminant in a mouse model. Mice were fed with a single dose of deuterated 6PPD-Q-d5 at human-relevant exposure levels. Results indicated that 6PPD-Q was quickly assimilated and distributed into bloodstream and main organs of mice, with the concentrations reaching peaks under 1 h following administration. Notably, 6PPD-Q was primarily distributed in the adipose tissue, marked by a significant Cmax (p < 0.05), followed by the kidney, lung, testis, liver, spleen, heart, and muscle. In addition, our measurement demonstrated that 6PPD-Q can penetrate the blood-brain barrier of mice within 0.5 h after exposure. The half-lives (t1/2) of 6PPD-Q in serum, lung, kidney, and spleen of mice were measured at 12.7 ± 0.3 h, 20.7 ± 1.4 h, 21.6 ± 5.3 h, and 20.6 ± 2.8 h, respectively. Using HRMS combined with isotope tracing techniques, two novel hydroxylated metabolites of 6PPD-Q in the mice liver were identified for the first time, which provides new insights into its rapid elimination in-vivo. Meanwhile, fecal excretion was identified as the main excretory pathway for 6PPD-Q and its hydroxylated metabolites. Collectively, our findings extend the current knowledge on the biological fate and exposure status of 6PPD-Q in a mouse model, which has the potential to be extrapolated to humans.