Plant bound residues

El Zorgani GA, Omer IS, Abdullah AM. 1986. Bound residues of endosulfan and carbofuran in soil and plant material. Proceedings of the Final Research Co-ordination Meeting on Isotopic Tracer-aided Studies of Unextractable or Bound Pesticide Residues in Soil, Plants, and Food. Vienna, Austria International Atomic Energy Agency, 51-56. 

Dec J, K Haider, A Benesi, V Rangaswamy, A Schaffer, E Eernandes, J-M Bollag (1997a) Formation of soil-bound residues of cyprodinil and their plant uptake. J Agric Eood Chem 45 514-520. 

Extraction of residues from soil samples is much more difficult than their extraction from plant or water samples. The pesticide residues in the soil exist often in several forms as bound residue , which may affect the extraction efficiency of pesticides from the soil. Then, various extraction methods such as organic solvent extraction, Soxhlet extraction, sonication extraction, microwave dissolution and supercritical fluid extraction (SEE) are used. Some extraction methods are described in the following. 

Kloskowski, R., Ftihr, F., and Mittelstaedt, W. (1986a). Formation of bound residues of [benzene ring-U-14C]- anilazine and [triazine-U-14C]-anilazine inparabraunerde (Alfisol soil) and their availibility to maize. In Quantification, Nature, and Bioavailability of Bound 14C-Pesticide Residues in Soil, Plants, and Food. Int. Atomic Energy Agency, Vienna, 65-70. 

Scheunert I, Topp E, Schmitzer J, et al. 1985. Formation and fate of bound residues on [14C]benzene and [14C]chlorobenzenes in soil and plants. Ecotox Environ Safety 9 159-170. 

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. 

Aromatic and heterocyclic amines are usually metabolized In higher plants by formation of N-glucosldes or bound residues. In several oases, however, aromatic and heterocyclic amines have been partially metabolized to N-malonyl conjugates. It Is not known whether this type of N-malonyl conjugation is restricted to a few selected amines nor Is It known whether this reaction Is utilized by more than a few plant species such as peanut and soybean. 

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... 

A partial list of xenoblotlca that form bound residues In plant tissues Is presented on Table Vlll. Many heterocyclic and aromatic... 

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

It appears that very similar xenoblotlcs can be metabolized to different types of bound residue and that considerable quantitative variation In bound residue can occur as a function of plant species or specific source of tissue. In wheat cell suspension culture,... 

D, which Is structurally similar to MCPA, appeared to be Inco-porated into lignin (255). This was in sharp contrast to MCPA metabolism in wheat. Carboxln (aniline-l4C) metabolism in peanut cell suspension culture and the fruit of whole peanut plants is an example of tissue variation. In peanut cell suspension culture, only 2.7% of the carboxln was incorporated Into bound residue, but In the fruit of whole plants, 21% Incorporation into bound residue was observed (121). The metabolism of metrlbuzln In tomato and soybean Is an excellent example of species variation. In tomato, metrlbuzln was rapidly metabolized to N-glucosldes and only 2% was Incorporated into bound residue, but In soybean, metrlbuzln was metabolized slowly by homoglutathione conjugation and 20-30% of the metrlbuzln was incorporated into bound residue (46.95). 

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. 

Relatively few studies have been carried out In vivo. However, the results available, for example on carbaryl and carbofuran (36), propham (.37), propanll (3 ), and atrazine (39 ), support the general conclusion of the lUPAC Pesticide Chemistry Commission (34) that covalently-bound non-extractable residues In plants are not generally bloavallable to animals. This conclusion, however. Is based upon limited data and It would be helpful to have the results of some more experiments In which sympathetically-treated radiolabelled bound residues have been fed to animals. 

Nearly all current methods of isolation and purification of lectins rely on affinity chromatography. Naturally, the characteristic ligand must be determined in advance. The properties of lectins can be used to precipitate macromolecules and to agglutinate some types of cells, be they plant or animal. The driving force of this reaction is the association with certain bound residues, generally monosaccharides, from the macromolecule or the cellular periphery. When this type of reaction is observed, the problem is to find the sugar that can inhibit activity at the lowest possible molar concentration. As in the case of immunochemical precipitations, this inhibition is due to the occupation of the recognition site by the small soluble molecule. 

The products of the initial biotransformation of RDX in aquatic vascular plants have not yet been identified. However, [14C]RDX uptake studies with root cultures of the model terrestrial vascular plant Catharanthus roseus confirmed that about 25% of the initial RDX was incorporated into the intracellular bound residues. One recent study [49] provided evidence that RDX taken up by leaves of the wetland plant P. arundinacea was abiotically photodegraded to 4-nitro-2,4-diazabutanal. Just and Schnoor [49] further claimed that plant chloroplasts shuttled electrons to facilitate this reaction, a process they termed phytophotolysis. One RDX biotransformation study [47] provided preliminary evidence for the formation of the nitroso product hexahydro-l-nitroso-3,5-dinitro-l,3,5 triazine (MNX) in the land plant Cyperus esculentus (yellow nutsedge), and so aquatic vascular plants may possess this biotransformation step as well. 

ADNTs with sugars, followed by their sequestration into the cell vacuole or incorporation into bound residues with the cell wall are likely endpoints. The explosives RDX and HMX are also taken up by aquatic vascular plants, but their initial biotransformation products and endpoints are not known at this time. Fundamental studies on RDX and HMX metabolism by aquatic vascular plants and nonvascular macroalgae are needed. 

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