Mechanism Of Formation Of Pentachlorophenol Glycoside Conjugates In Plants And Their Inverse Transformation

Pentachlorophenol is a typical organic chlorinated phenolic contaminant and one of the most frequently detected pesticide pollutants in agriculture soils.It was introduced in the 1930 s as a wood preservative and is now widely used in both industry and agriculture as an insecticide,fungicide,and herbicide.PCP is persistent in the environment,and is difficult to dissipate naturally through physical and chemical pathways,including adsorption,volatilization,and leaching.Its bioaccumulation has led to the extensive contamination of soil,surface,ground water,organism and other environmental media,even in the human.PCP is a highly toxic,mutagenic,and carcinogenic organic compound.Thus,the fate of PCP in the environment has been focused owing to its extensive presence and high toxicity.This study aimed to illuminate the fate,occurrence and transformation of PCP in the link of“food crop-digestive tract”.PCP was chosen as the target compound,and the cells,whole plants and physiologically-based-extraction-test(PBET)experiments were designed for three levels of this research.The primary objective of this study is to characterize the formation of PCP glycoside conjugates in plant matrices and its hydrolysis to produce PCP under simulated human digestive conditions.(1)PCP could be rapidly uptake and metabolized by carrot in the whole plant and callus cell experiments.Approximately 90% of the parent compound disappeared in both of the whole plant system and cell system,indicating that carrot had the ability to transform PCP.The parent compound mainly accumulated in the plant roots and cells,which may be related to the physical and chemical properties of PCP and the distribution of lipid in carrot.PCP is a lipophilic organic substance with a partition coefficient of high octanol water(log Kow: 3.00 – 5.17),which was more likely to accumulate in tissues with a higher lipid content,and the ability to transport upward was relatively weak.(2)The cell extracts of the samples at the end of incubation were used for HRMSanalysis to detect and identify metabolites of PCP.Ten metabolites were only present in the treats,including two Phase I metabolites and eight Phase II metabolites.The eight Phase II metabolites included five glycoside conjugates and three sulfate conjugates.All the identified metabolites were also detected in the extracts of carrot plant,showing that the metabolization of PCP underwent in cells.Direct conjugation(Phase II metabolism)with the parent compound at the phenolic hydroxyl site was found to be more predominant than the Phase I transformations.(3)The PCP glucopyranoside standard was synthesized for the qualitative and quantitative analysis of PCP glucopyranoside conjugate in the carrot cell and whole plant metabolism experiment.The purity of the synthesized PCP glucopyranoside standard was 90%.Nearly 20% of the parent compound was transformed as PCP glucopyranoside,but about 90% of PCP disappeared in the incubation system,indicating that PCP was also metabolized as other products.(4)The stability of the PCP glucopyranoside was investigated by a single control variable method to demonstrate the possibility of reverse conversion of the glycoside conjugates.The results showed that the pH value and enzyme content had a great influence on the stability of PCP glucopyranoside,while the temperature had no effect on it.PCP glucopyranoside was in a stable state under neutral conditions(pH = 7.0),and its stability deteriorates with the decrease of pH value with occurring decomposition.When only the enzyme content was changed,the rate of the decomposition of PCP glucopyranoside increased as the enzyme content increased.(5)The deconjugation behavior of PCP glucopyranoside was confirmed by undergoing physiologically-based-extraction-test.The gastrointestinal simulation experiment was divided into two stages: gastric simulation and intestinal simulation.It was found that PCP glucopyranoside had reverse transformation in the gastric simulation stage,and nearly 96% of PCP glucopyranoside underwent deconjugation at the end of the digestion experiment.In the “food crop-digestive tract” system,approximately 17% of the parent compound first concealed by forming the PCP glucopyranoside and then released the parent compound in the digestive tract,causing an underestimation of the environment risk.In the intestinal simulation phase,the content of PCP glucopyranoside was almost no longer decreased,but the content of PCP was still increasing,which may be due to the deconjugation of other conjugates in the digestion process.This study revealed that PCP and other similar compounds with hydroxyl groups may be hidden in crops by forming glycoside conjugates via direct conjugation,and then the glycoside conjugates underwent deconjugation in the digestive tract.The results likely established plant PCP glycoside conjugates as an additional source of PCP exposure to humans,and may stimulate further research on this and similar environmental contaminants in improving their risk assessment.