Compared to the unplanted controls, however, acetaminophen in the growth media with plant disappeared significantly faster . For example, the average concentration of acetaminophen in the medium with plant decreased from 5.0 mg L−1 at the beginning to 3.3 ± 0.3 mg L−1 at 48 h, and further to 2.1 ± 0.4 mg L−1 at the end of cultivation . The significant differences in acetaminophen concentration between the treatments suggested that acetaminophen was taken up from the nutrient solution into the plant.Time-dependent concentrations of acetaminophen in cucumber roots and leaves are depicted in Fig. 1b. During the short growth period, acetaminophen was detected in both roots and leaves. The accumulation of acetaminophen in leaves suggested its acropetal translocation in the cucumber seedlings. However, acetaminophen concentrations in roots were 1.2–5.0 times higher than those in leaves. For example, after 96 h, 1.7 ± 0.1 μmol g−1 and 0.5 ± 0.1 μmol g−1 acetaminophen were detected in roots and leaves, respectively . The relatively lower levels of acetaminophen in leaves pointed to moderate transport to the above ground and likely different transformation rates in these tissues. The level of acetaminophen reached the maxima at 96 h in roots and 48 h in the leaves, and a significant decrease was observed in both tissues thereafter , indicating active bio-transformations in plant cells or release of parent and conjugated compounds from plant roots to the bathing solution.
In plant tissues without exposure to acetaminophen, neither acetaminophen nor GSH conjugates were detected,hydroponic farming indicating absence of cross contamination.In plants, GSH conjugates have been extensively studied because of their role in detoxification and selectivity of herbicides, such assulfonylureas , triazines and chloroacetanilides . In comparison, only sporadic studies have considered GSH conjugation of other contaminants in plants. Detoxification via GSH conjugation was recently reported for the antibiotic chlortetracycline in plant cells . In the present study, the concentration of GSHacetaminophen conjugate in cucumber roots after 48 h was 1.7 ± 0.8 nmol g−1 , and it remained relatively constant thereafter . In comparison, the level of GSH-acetaminophen in leaves continued to increase during the exposure and reached 1.2 ± 0.1 nmol g−1 at the end of experiment . The relative differences between roots and leaves also suggested that roots were the main location for acetaminophen detoxification. This observation was in agreement with a previous study where extensive transformation of acetaminophen was also found in the root hairy cell culture of Armoracia rusticana .In the follow-up experiments, GSH-acetaminophen conjugate was also found in the roots of common bean, alfalfa, tomato, and wheat plants . The ability to carry out GSH conjugation appeared to vary among plant species, with the overall level of GSH-conjugates following an order of tomato, cucumber, alfalfa N wheat N common bean . The differences in the conjugation of acetaminophen by GSH among plant species may be attributed to differences in GSH availability and the portfolio of GST isoenzymes, differences in Phase I reactions, or the involvement of other Phase II metabolism pathways.
It must be noted that the commercial cultivars used as genotype may make a difference to the present results. Meanwhile, in all acetaminophen treatment crops, elevated lipid peroxidation was observed, with the damage being less pronounced in cucumber, tomato and alfalfa tissues . Combining with the levels of GSH conjugates, this observation implied that the different tolerance to acetaminophen among different plant species may be due to their differences in GSH conjugation ability. When cucumber plants were cultivated in nutrient solution containing acetaminophen at the lower level or in soil amended with acetaminophen-containing bio-solids, GSH-acetaminophen conjugate was also detected, even though the levels were generally lower . This observation suggested that direct conjugation of acetaminophen with GSH in cucumber plants occurred under environmentally relevant conditions. In mammalian systems, acetaminophen is oxidized by cytochrome P450 and peroxidases to a reactive quinone-like compound , which can then undergo GSH conjugation by GSTs. In the current study, it was noted that in cucumber leaves and roots, the total activity of cytochrome P450 , but not GSH content or GST activity , was significantly inhibited by 1-aminobenzotriazole or piperonyl butoxide, two specific cytochrome P450 inhibitors. With the pretreatment of 1-aminobenzotriazole or piperonyl butoxide, levels of GSH-acetaminophen decreased from 0.7 ± 0.1 nmol g−1 to 0.2 ± 0.1 or 0.3 ± 0.1 nmol g−1 , and from 16.5 ± 1.4 nmol g−1 to 1.9 ± 0.9 and 2.4 ± 0.6 nmol g−1 in cucumber leaves and roots, respectively . These results clearly suggested that cytochrome P450 played a role prior to GSH-acetaminophen conjugation. However, there was only a slight difference in P450 activities between cucumber and common bean ; whereas cucumber displayed significantly higher GST activity than common bean .
It is therefore likely that different activities of GST also contributed to the different levels of acetaminophen-GSH conjugates. The results clearly showed that GSH-mediated detoxification of acetaminophen in plants differed from the known detoxification reactions of chlorinated herbicides,such as chlorimuron ethyl , alachlor , and metolachlor . For these chlorinated herbicides, conjugation occurs via a nucleophilic substitution of chlorine by the thiol residue of GSH, forming a polar GSH-conjugate. For acetaminophen, however, the potential site of GSH conjugation may involve alkyl groups and result in the ring opening of acetaminophen. A recent study on ibuprofen metabolism in Phragmites australis also suggested that ibuprofen was not directly amenable to Phase II conjugation; rather, it needed to be first functionalized via Phase I activation . These findings together indicated that conjugation with GSH is a common and important detoxification pathway for xenobiotics of different structures in higher plants. For PPCPs such as acetaminophen, Phase I enzymes may be involved to create favorable reactive sites prior to GSH conjugation.Generally, conjugation to GSH occurs in the cytosol,hydroponic equipment but the accumulation of the conjugates in this compartment may also confer toxicological effects . For example, the conjugate may block GST activity , leading to an accumulation of unconjugated electrophiles in plant cells, or they may encounter cytosolic enzymes , which could result in conversion to toxic metabolites. It is generally assumed that the vacuole serves as a site for the deposition of xenobiotic conjugates in plants and fungi .However, little is known about the biological mechanism underpinning GSH conjugates of organic contaminants across tonoplast. Although vacuole was not collected in this study, GSHacetaminophen conjugate, however, was not detected in the xylem sap of cucumber exposed to acetaminophen . Furthermore, GSH-acetaminophen conjugates were also not detected in the above ground tissues of cucumber exposed to GSH-acetaminophen . These results indicated that the parent compound was not metabolized in the xylem or GSH conjugate produced in the roots was not transported to the above ground. However, all GSH related conjugates were detected in the nutrient solution, ranging from 22.7 ± 4.9 nmol L−1 to 44.6 ± 5.2 nmol L−1 after 5 mg L−1 acetaminophen exposure , suggesting release of GSH-related conjugates from plant roots to the bathing solution. A similar phenomenon where plants released conjugated metabolites was previously observed for triclosan , benzotriazole and 2-mercaptobenzothiazole . Additionally, microorganisms in the growth medium could also convert acetaminophen to GSH conjugates, although this possibility was not explored in this study. Glutathione conjugates produced in plant cells may undergo further transformations to yield cysteine conjugates, as previously shown for animals . Most herbicide-GSH conjugates in plants were rapidly metabolized by dipeptidases to remove the glycine residue of GSH to form cysteine conjugates, which could further yield Nacetylcysteine conjugates .
In this study, cysteine and Nacetylcysteine conjugates of acetaminophen were detected in both roots and leaves of cucumber, and the levels in roots were about 3.3 ± 0.2 and 0.07 ± 0.01 nmol g−1 , respectively, after exposure to 5 mg L−1 acetaminophen for 96 h . Glutathione-acetaminophen derived cysteine and Nacetylcysteine conjugates were also detected in roots of common bean, alfalfa, tomato, and wheat plants , with levels ranging from 0.2 ± 0.1 to 4.5 ± 0.5 nmol g−1 and from 0.02 ± 0.01 to 0.3 ± 0.09 nmol g−1 , respectively. The amount of acetaminophen conjugates tended to decrease after 48 h and 96, in leaves and roots, respectively , indicating that some conjugates were converted further or the conjugated metabolites were released back to the medium. The content of acetaminophen-cysteine conjugate in leaves was consistently much smaller than that in roots throughout the experiment. Previous studies showed that the generated cysteine or Nacetylcysteine metabolites were exported to the apoplast, where they may be bound to lignin and cellulose . Several metabolites of cysteine conjugates have been identified in plant cells , although the precise fate and route of metabolism of cysteine conjugates is still uncertain. Further research is needed to understand the efflux of cysteine conjugates from the vacuole.Conjugation of xenobiotics with GSH is mediated by enzymes that belong to the super family of GSTs, which were previously shown to respond quickly to xenobiotics . An evaluation of changes in GST activity was made to understand the mechanism by which acetaminophen was conjugated by GSH. Dynamic changes in GST activity in cucumber tissues during exposure are shown in Fig. 5a. The GST activity increased in both leaves and roots after exposure to acetaminophen . Compared to the blank control, GST activity was consistently elevated at each sampling point, amounting to 1.07–1.94 and 1.30–1.60 times that in the blank control in leaves and roots, respectively. A similar behavior was reported earlier in tomato leaves exposed to the fungicide chlorothalonil. Although levels of PPCPs in wastewater effluents are usually below the levels used in this study , it may be argued that continuous irrigation, active water evapotranspiration, and adsorption, may render the actual concentrations of PPCPs in the soil pore water considerably higher. Results from the bio-solid amendment experiment further supported the conclusion that GSH detoxification may occur for acetaminophen under environmentally relevant conditions.The GSH availability has been shown to correlate with the adaptation or response to xenobiotics in plants , and is an important factor in determining cell sensitivity. For example, GSH availability has been used in understanding herbicide selectivity . Any perturbation leading to a depletion of GSH can severely impair a plant’s defense against xenobiotics. Here, the concentration of GSH in cucumber roots decreased immediately after exposure to 5 mg L−1 acetaminophen, although it gradually recovered to the normal level at 120 h; whereas GSH content only decreased at 72 h in cucumber leaves. Compared to the control, the level of GSH was reduced by 55.4%in the root after 48 h and by 15.2% in the leaves after 72 h . These results were in agreement with the observation that roots were the tissues where more GSH conjugated metabolites were found. However, levels of GSH gradually recovered to a level similar to the control at the end of experiment , possibly due to rapid recycling via GR or synthesis of GSH by γ-ECS in plant cells . In order to explore the roles of GR and γ-ECS in maintaining GSH homeostasis, their activities during the exposure were further monitored. In leaves, activities of GR and γ-ECS were elevated slightly, at about 1.53- and 1.54-fold after 144 h, respectively . In contrast, treatment of acetaminophen significantly elevated γ-ECS activity in roots and showed a time-dependent response, increasing by 2.32-fold after 24 h . A similar pattern was observed in GR activity in roots after exposure to acetaminophen ; while significant increase in GR activity was only observed at 144 h in cucumber leaves . The finding indicated that activities of GR and γ-ECS were elevated to ensure sufficient GSH turnover in response to acetaminophen induced consumption of GSH through GSH conjugation.Based on the results of this study, we propose a model to illustrate the operation of the GST detoxification pathway, which involves conjugation, accumulation and processing of these metabolites . The present study provided strong evidence that transformation of acetaminophen, one of the most used antipyretic and analgesic drugs worldwide, was likely catalyzed first by cytochrome P450, and followed by GSH conjugation mediated by GSTs.