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2,5diChl-2OH biphenyl 2,5diChl-3OH biphenyl 2,5diChl-4OH biphenyl

To identify chemical structure, position of chlorines and hydroxyl groups plant cells of four different species were incubated with monochlorobiphenyls. GC-MS analysis revealed that metabolites identified in plant tissue were mono- and dihydroxy-compounds. Some of them could be identified by GC-MS by comparison with the structures of the standards. The results are summarized in Table 3. Altogether, data showed that although different plant species metabolise PCB congeners following a similar pattern for any PCB congener, the different plant species form the same types of hydroxychloroderivatives but number of products and positions of hydroxyl group(s) differ with the plant species. Alfalfa has quite low potential to metabolise PCBs, and its ability is limited only to transformation of 4-chlorobiphenyl, which is metabolised to 4Cl-4'OH biphenyl, the same product found in cells of other plant.

5.1.2 Enzymes responsible for PCB transformation in plants

As was previously documented in case of many environmental xenobiotics, phytotransformation refers a process by which organic contaminants are uptake by plants of organic contaminants from soil and groundwater and subsequently the xenobiotics are metabolised or transformed. Various plant xenobiotics-converting enzymes, cytochrome P450, peroxidases, glutathione-S-transferases, carboxyesterases, O-glucosyl transferases, O-malonyl transferases, N-glucosyl transferases and N-malonyl transferases, were isolated and purified. They were proved to metabolise efficiently various xenobiotics such as PCBs, organic solvents and chlorinated pesticides [70, 71].

Organic pollutants undergo in plants a three -stage metabolic processes similar to mammalian metabolism [72]. First phase which includes oxidation, reduction or hydrolysis of xenobiotics often generates metabolites with increased polarity [67, 71, 72, 73]. These reactions are catalyzed by mixed functional oxidases (cytochrome P450) and/or peroxidases to introduce an hydroxyl group or to substitute common functional groups (nitro, carboxyl, alkyl or halogens) [68]. Plants hydroxylate PCBs to form more soluble hydroxyderivatives [67, 74, 75, 76]. This reaction suggests that polychlorinated biphenyls oxidases, cytochrome P450 and/or peroxidases present in plant cells are involved in PCB conversion. The identity of the enzymes responsible for plant transformation of xenobiotics was already a matter of discussion in the early seventieth, when the first reports about PCB metabolisation were published [77]. In 1992 it was discussed by Lee and Fletcher [74] who studied involvement of cytochrome P450 and peroxidase in PCB transformation in presence of specific inhibitors for cytochrome P450 and peroxidases. Their study provided evidence that cytochrome rather than peroxidases were the major contributors to plant metabolism of PCB. More recently, we got evidence that peroxidases are also likely [62, 63] involved in PCB transformation. It was shown that plant tissue cultures of various species, exhibiting higher peroxidase activities in PCB presence, possess higher capabilities to transform them. Chroma et al. [63] used different specific inhibitors for both enzymic systems and measured not only PCB removal but also peroxidase activities. Presence of all inhibitors resulted in a decrease of PCB degradation efficiency compared to the controls without any inhibitor. Nevertheless, further following analyses showed a decrease of peroxidase activity not only when peroxidase's but also cytochrome P450 inhibitors were added (see Table 4). This phenomenon supported idea that both these enzymic systems are involved in the metabolism of PCBs in plant cells.

Tab. 4. The effect of inhibitors on peroxidase of SNC-9O and its PCB transformation. Modified according [63].

Inhibitor

(Enzyme inhibited)

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