Comparative in vitro metabolism of short chain chlorinated paraffins (SCCPs) by human and chicken liver microsomes: First insight into heptachlorodecanes

Identification of Hydroxylated Chlorinated Paraffins in Human Serum and Their Potential Metabolic Pathways

Short- and medium-chain chlorinated paraffins (SCCPs and MCCPs) are frequently detected in humans. However, information regarding their metabolites is still very limited. Herein, target analysis and halogenation-guided nontarget and suspect screening were conducted on serum samples using UHPLC-Orbitrap-HRMS. The median concentrations of SCCPs and MCCPs were 7.76 and 4.31 ng/mL, respectively. A series of hydroxylated chlorinated paraffins (OH-CPs) were tentatively identified with an estimated average concentration of 1.80 ng/mL, which was approximately 9.9% of the total SCCPs and MCCPs. A chlorine distribution shift was observed from chlorinated paraffins (CPs) dominated by Cl6 and Cl7 to OH-CPs dominated by Cl5, Cl6, and Cl4. In human liver cytochrome P450 (CYP) enzyme incubation assays, the CPs in commercial mixtures were mainly metabolized into OH-CPs with various carbon lengths and chlorine substituents. The results obtained from human serum and in vitro experiments suggested the oxidative metabolism of SCCPs and MCCPs in humans. The metabolic pathways were then comprehensively explored using a CP monomer (1,1,1,3,10,11-hexachloroundecane) incubated with the same CYP enzymes, demonstrating that CPs can be metabolized through successive oxidative dechlorination and direct hydroxylation, with subsequent oxidation to carboxylic acids. Further studies should focus on the long-term toxicity of OH-CPs.

A comprehensive characterization biotransformation of chlorinated paraffin by human and carp liver microsomes via liquid chromatography-high-resolution mass spectrometry and screening algorithm

The chlorinated paraffin (CP) monomer 1,2,5,6,9,10-Hexachlorodecane (CP-4) was subjected to in vitro biotransformation using human and carp liver microsomes. Five types of CP-4 metabolites (OH-, keto-, enol-, aldehyde- and carboxy-CP-4) were identified in human liver microsomer while only mono-OH-CP-4 was found in the carp liver microsomes. Kinetic studies revealed that the formation of mono-, di-, tri-hydroxylated CP-4, keto-, enol-, and aldehyde-CP-4 in human liver microsomes was best described by substrate inhibition models, whereas the formation of carboxylated CP-4 metabolites best fit the Michaelis-Menten model. Notably, keto-CP-4, enol-CP-4 and aldehyde-CP-4 were the predominant metabolites. The estimated Vmax values for these metabolites were significantly higher in the human liver microsomes than in the carp liver microsomes. The intrinsic hepatic clearance (CLint) of CP-4 was higher in humans than in carp, indicating species-specific differences in its metabolism. This study also highlighted potential toxicity concerns, with computational predictions showing varying degrees of acute oral toxicity for CP-4 and its metabolites. These findings indicate significant species-specific differences in the biotransformation of CP-4, emphasizing the potential health and environmental risks associated with chlorinated paraffins and their metabolites, and underscore the need for further research to address these concerns.

Interspecies comparison of metabolism of two novel prototype PFAS

As a result of proposed global restrictions and regulations on current-use per-and polyfluoroalkyl substances (PFAS), research on possible alternatives is highly required. In this study, phase I in vitro metabolism of two novel prototype PFAS in human and rat was investigated. These prototype chemicals are intended to be safer-by-design and expected to mineralize completely, and thus be less persistent in the environment compared to the PFAS available on the market. Following incubation with rat liver S9 (RL-S9) fractions, two main metabolites per initial substance were produced, namely an alcohol and a short-chain carboxylic acid. While with human liver S9 (HL-S9) fractions, only the short-chain carboxylic acid was detected. Beyond these major metabolites, two and five additional metabolites were identified at very low levels by non-targeted screening for the ether- and thioether-linked prototype chemicals, respectively. Overall, complete mineralization during the in vitro hepatic metabolism of these novel PFAS by HL-S9 and RL-S9 fractions was not observed. The reaction kinetics of the surfactants was determined by using the metabolite formation, rather than the substrate depletion approach. With rat liver enzymes, the formation rates of primary metabolite alcohols were at least two orders of magnitude higher than those of secondary metabolite carboxylic acids. When incubating with human liver enzymes, the formation rates of single metabolite carboxylic acids, were similar or smaller than those experienced in rat. It also indicates that the overall metabolic rate and clearance of surfactants are significantly higher in rat liver than in human liver. The maximum formation rate of the thioether congener exceeded 10-fold that of the ether in humans but were similar in rats. Overall, the results suggest that metabolism of the prototype chemicals followed a similar trend to those reported in studies of fluorotelomer alcohols.

Identification of Sulfur-Containing Chlorinated Paraffin Structural Analogues in Human Serum: Origination from Biotransformation or Bioaccumulation?

Chlorinated paraffins (CPs) are manufactured and used in high quantities and have diverse structural analogues. It is generally recognized that sulfur-containing structural analogues of CPs are mainly derived from sulfate-conjugated phase II metabolism. In this study, we non-targeted identified three classes of sulfur-containing CP structural analogues (CPs-S) in human serum, including 44 CP sulfates (CPs-SO4H/CPs-SO4H-OH), 14 chlorinated benzene sulfates (CBs-SO4H), and 19 CP sulfite esters (CPs-SO3/CPs-S2O6), which were generated during the production of commercial mixtures of CPs and, thus, bioaccumulated via environmental exposures. We first wrote a program to screen CPs-S, which were baseline-separated from CPs according to their polar functional groups. Then, mass spectral analyses of alkalization-acidification liquid-liquid extracts of serum samples and Orbitrap mass spectrometry analyses in the presence and absence of tetraphenylphosphonium chloride (Ph4PCl), respectively, were performed to determine the ionization forms ([M + Cl]- or [M - H]-) of CPs-S. The presence of fragment ions (SO4H-, SO3-, SO2Cl-, and HSO3-) revealed the structures of CPs-S, which were validated by their detections in commercial mixtures of CPs. The estimated total concentrations of CPs-S in the human serum samples were higher than the concentrations of medium- and long-chain CPs. The profiles of CPs-S in human serum were similar to those detected in CP commercial mixtures and rats exposed to the commercial mixtures, but CPs-S were not detected in human liver S9 fractions or rat tissues after exposure to CP standards. These results, together with the knowledge of the processes used to chemically synthesize CPs, demonstrate that CPs-S in humans originates from environmental bioaccumulation.

Concentrations and homologue patterns of SCCPs and MCCPs in the serum of the general population of adults in Hangzhou, China

Due to their ubiquitous presence in the environment and humans, chlorinated paraffins (CPs) are a major environmental and public health concern. CPs are known to persist, bioaccumulate and potentially threaten human health, but reports on their internal exposure in the adult general population are still scarce. In this study, serum samples collected from adults living in Hangzhou, China, were quantified for SCCPs and MCCPs using GC-NCI-MS methods. A total of 150 samples were collected and subjected to analysis. ∑SCCPs were detected in 98% of the samples with a median concentration of 721 ng/g lw. MCCPs were found in all serum samples with a median concentration of 2210 ng/g lw, indicating that MCCPs were the dominant homologous group. For SCCPs and MCCPs, ∑C₁₀ and ∑C₁₄ were found to be the dominant carbon chain length homologues. Our results showed that age, BMI and lifestyle were not found to be significantly associated with internal exposure to CPs for the samples in this study. Based on PCA analysis, an age-specific distribution of CP homologues was observed. This suggests that internal exposure to CPs in the general population is related to exposure scenarios and history. The results of this study may contribute to a better understanding of the internal exposure to CPs in the general population and may provide a direction for the investigation of the source of exposure to CPs in the environment and daily life.

Identification and yield of metabolites of chlorinated paraffins incubated with chicken liver microsomes: Assessment of their potential to convert into metabolites

Chlorinated paraffins (CPs) are emerging environmental pollutants. Although metabolism has been shown to affect the differential accumulation of short-chain (SCCPs), medium-chain (MCCPs) and long-chain (LCCPs) CPs in birds, CP metabolites have rarely been reported and the extent to which they are formed is still unclear. In this study, single and mixed CP standards were incubated with chicken liver microsomes in vitro to study the generation of CP metabolites. Putative aldehyde/ketone and carboxylic acid metabolites identified by mass spectroscopy data were shown to be false positive results. Phase I metabolism of CPs first formed monohydroxylated ([M-Cl+OH]) and then dihydroxylated ([M-2Cl+2OH]) products. The yields of monohydroxylated metabolites of CPs decreased with increasing carbon chain length and chlorine content at the initial stage of reaction. Notably, the yield of monohydroxylated metabolites of SCCPs with 51.5% Cl content reached 21%, and that of 1,2,5,6,9,10-hexachlorodecane (C₁₀H₁₆Cl₆) was as high as 71%. Thus, monohydroxy metabolites of CPs in birds should not be ignored, especially those of SCCPs. This study provides important data that could support improvements to the ecological/health risk assessment of CPs.

The potential health risks of short-chain chlorinated paraffin: A mini-review from a toxicological perspective

Short-chain chlorinated paraffins (SCCPs) are ubiquitously distributed in various environmental matrics due to their wide production and consumption globally in the past and ongoing production and use in some developing countries. SCCPs have been detected in various human samples including serum, milk, placenta, nail, and hair, and internal SCCP levels were found to be positively correlated with biomarkers of some diseases. While the environmental occurrence has been reported in a lot of studies, the toxicity and underlying molecular mechanisms of SCCPs remain largely unknown. The current tolerable daily intakes (TDIs) recommended by the world health organization/international programme on chemical safety (WHO/IPCS, 100 μg/kg bw/d) and the UK Committee on Toxicity (COT, 30 μg/kg bw/d) were obtained based on a no observed adverse effect level (NOAEL) of SCCP from the repeated-dose study (90 d exposure) in rodents performed nearly 40 years ago. Importantly, the health risks assessment of SCCPs in a variety of studies has shown that the estimated daily intakes (EDIs) may approach and even over the established TDI by UK COT. Furthermore, recent studies revealed that lower doses of SCCPs could also result in damage to multiple organs including the liver, kidney, and thyroid. Long-term effects of SCCPs at environmental-related doses are warranted.

Bioaccumulation and Biotransformation of Chlorinated Paraffins

Chlorinated paraffins (CPs), a class of persistent, toxic, and bioaccumulated compounds, have received increasing attention for their environmental occurrence and ecological and human health risks worldwide in the past decades. Understanding the environmental behavior and fate of CPs faces a huge challenge owing to the extremely complex CP congeners. Consequently, the aims of the present study are to summarize and integrate the bioaccumulation and biotransformation of CPs, including the occurrence of CPs in biota, tissue distribution, biomagnification, and trophic transfer, and biotransformation of CPs in plants, invertebrates, and vertebrates in detail. Biota samples collected in China showed higher CP concentrations than other regions, which is consistent with their huge production and usage. The lipid content is the major factor that determines the physical burden of CPs in tissues or organs. Regarding the bioaccumulation of CPs and their influence factors, inconsistent results were obtained. Biotransformation is an important reason for this variable. Some CP congeners are readily biodegradable in plants, animals, and microorganisms. Hydroxylation, dechlorination, chlorine rearrangement, and carbon chain decomposition are potential biotransformation pathways for the CP congeners. Knowledge of the influence of chain length, chlorination degree, constitution, and stereochemistry on the tissue distribution, bioaccumulation, and biotransformation is still scarce.