Conjugation, metabolic reaction plants

In this review, conjugation reactions utilized In xenobiotic metabolism In plants will be discussed In reference to functional groups, phase I reactions necessary to produce a functional group suitable for conjugation, relative rates of reactions, competing metabolic pathways, frequency of occurence, plant species, stability of conjugates, and the relationship between metabolism and herbicide selectivity. Pesticides discussed herein are listed In Table I. 

A direct relationship has been observed between the ability of some plant species to form 0-glucosldes of herbicides and resistance of those species to the herbicides however. In most cases a phase I reaction proceeds glucose conjugation and It Is not known whether the phase I reaction or conjugation results In herbicide detoxification. Chlorpropham is metabolized in plants by ring-hydroxylation and subsequent conjugation with glucose (Equation 3). In vitro, the... 

Xenobiotics that contain a free or potential carboxyl group can be metabolized by amino acid conjugation in both plants and animals. This reaction is illustrated by the conjugation of 2,4-D with aspartic acid (Equation 31). In higher plants, amino acid conjugation is... 

Fig. 1. Diagram of the metabolic reactions that determine pool size of the free lAA in plant cells. Inputs include (A) de novo synthesis from non-tryptophan and tryptophan pathways (B) conjugate hydrolysis and (C) transport. Outputs from the lAA pool inelude (D) oxidative catabolism (E) conjugate synthesis (F) transport and (G) possible lAA use during growth and developmental processes.

Phase III reactions occur primarily in plants presumably because excretion of Phase II conjugates is Insignificant in plants. Phase III reactions are mechanisms whereby plants can reduce the effective concentration of xenobiotlc compounds in the cytoplasm. Thus, conjugation reactions provide mechanisms for the elimination of xenobiotlc compounds from sites of continuing metabolic activity in all organisms ( ) ... 

The above classification of detoxication reactions has been developed for the metabolism of synthetic pesticides In plants. However, the same reactions can occur with natural exocons, such as allelopathic compounds, that have the same functional groups as synthetic pesticides. Most allelopathic chemicals contain functional groups that can be conjugated by Phase II reactions. Thus, detoxication of allelopathic compounds can be expected to proceed by conjugation with the omission of Phase I reactions. The remainder of this review will be concerned with the conjugation of allelopathic compounds. 

The cysteine conjugates appeared to be key metabolil es, occupying pivotal positions in the pathway. S-(Pentachloro-phenyl)cysteine was not demonstrated in vitro, but it was a minor metabolite in peanut plants. This anomoly appeared to be due to the kinetics of the various reactions. A cysteine conjugate was clearly shown to be a key intermediary metabolite in the metabolism of the GSH conjugate of atrazine in sorghum (Figure 1). 

Plants. Metabolized by more than four types of phase I reaction (oxidation of the phenyl ring, oxidation of the methyl group, cleavage of the methyl ester and /V -dealkylation) to form eight metabolites at phase II, most of the metabolites are sugar conjugated... 

The mechanisms utilized by plants and mammals in the metabolism of xenoblotlcs are remeirkably similar. Similar classes of compounds or functional groups are frequently metabolized by comparable mechanisms. Oxidation, reduction, hydrolysis, and conjugation reactions occur with similar frequency in both. In most instances, however. 

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