Bound xenobiotic residues

Zitzelsberger, W., W. Ziegler, and P.R. Wallnofer. 1987. Stereochemistry of the degradation of veratrylglycerol-(3-2,4-dichlorophenyl-ether, a model compound for lignin bound xenobiotic residues by Phenerochaete chrysosporium, Corynebacterium equi and photosensitized rioboflavin. Chemosphere 16 1137-1142. 

In conclusion, toxic chlorinated phenol intermediates formed during the chemical, photochemical and/or enzymatic degradation of chlorophenoxyalkanoic compounds would temporarily be detoxified when they are incorporated into the humic acid, since their bioavailability and movement into terrestrial and aquatic ecosystems would be greatly reduced. However, the knowledge of the potential toxicity problems which these bound-residues could give rise to in the environment is still very limited. Xenobiotic chemicals incorporated into humic polymers are not really removed from the ecosystem and they may maintain their identity and toxic properties for unknown time spans, eventually causing time-delayed pollution problems, if and when they will be released from humic substances. 

Several xenobiotics that are metabolized by GSH conjugation In plants, Including atrazine, propachlor, and PCNB, produce significant levels of bound residue (15). It appears that the bound residue may be formed from the GSH pathway with either a cysteine conjugate or a thiol as a precursor 1 5). The chemical nature of these bound residues has not been determined. 

Xenobiotics are frequently metabolized in plants by mechanisms that lead to the incorporation or inclusion of the xenobiotic into biological polymers or tissue residues that are not soluble in commonly used nonreactive solvents. These residues are frequently refered to as bound, insoluble, or nonextractable residues (2 ). Bound residues in plants have most commonly been detected in plant tissues treated with radloactlvely-labeled pesticides. These residues were an important topic of a symposium held in Vail, Colo, in 1975 (17) they have been discussed in mauiy more recent papers (11,154-1577"and they were discussed at a symposium at the l88th ACS National Meeting, 1984 "Non-extractable Pesticide Residues Characteristics, Bioavailability and Toxicological Significance". 

Occasionally, xenobiotics may be extensively metabolized in plants to CO2 or other low MW endogenously occuring products which can produce bound residues by reincorporation into biological polymers. Residues of this type are generally of little concern to toxicologists and residue chemists because these residues do not represent an unusual hazard to the biosphere. A recently proposed... 

Table VIII. Bound Residues of Xenobiotics In Plants...

The fate of plant xenobiotic conjugates I.e. glycosides, malonates, N-acyl-amlno acids, alkyl/aryl glutathiones and derivatives, lipophilic conjugates and polymer conjugates (bound residues). In animals Is reviewed. Some classes are reasonably well-studied but no Information Is available for others. 

The anticipated fate of xenobiotic plant conjugates in mammals is that they will be hydrolyzed at the conjugating bond and processed as phase I metabolites (or possibly as the parent if an alcohol, phenol, carboxylic acid, etc.). Relatively few such studies have been reported to date but the purpose of this chapter is to review the available results and to judge whether or not the expectation is justified and whether or not generalizations may be useful. Macromolecular conjugates (bound residues) are also considered but, in the absence of enough definitive data, only approaches to their study are suggested. 

It Is worth emphasizing that a plant bound residue that has relied upon high chemical reactivity for Its formation (e.g. an electrophile reacting with a nucleophilic centre) will be devoid of that reactivity. Xenobiotics Incorporated via energy-dependent biochemical mechanisms, however, cannot be viewed In this way. 

Although the globally distributed DDT is a very well investigated xenobiotic regarding the environmental occurrence and behaviour, detailed information about the fate of DDT in the bound residues fraction is very limited. Already in 1977 Lichtenstein et al. (1977) reported the formation of bound 14C-labelled DDT on agricultural soil accompanied by a drastically reduced insecticidal activity of the associated proportion. Also recent studies confirmed the decrease of DDT toxicity with time after application to soils as a result of less bioavailibility due the incorporation into the non-extractable particulate matter (Robertson and Alexander 1998). For a better understanding of the processes leading to these observations more information is required about the incorporation of DDT residues into the non-extractable particulate matter not only of soils but also of particulate matter within the aquatic environment. 

Xenobiotic Species Tissue xenobiotic concentration (nmol g- wet wt.) Metabolites Macromolecular bound residues Rate of metabolism (pmol min g wet wt.) Reference... 

Basically, the metabolism of pesticides and xenobiotics in plant cells is considered to be a detoxification process, this also being true for the portion of the applied chemical that is transformed into nonextractable residues. Xenobiotics bound to the plant cell wall are immobile and, thus, are effectively kept away from the sites of primary and secondary plant... 

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