Fig. 1.9.4. Sequestration of xenobiotics, after conjugation, in the cell wall or vacuole. A Hydroxylation and conjugation of 2,4-dichlorophenoxyacetic acid and pentachlorophenol. b Map of reactions which result in the sequestration of xenobiotics after uptake into the cell. (Sandermann 1994)

Natural Substrates of GS-X Pumps Of course, organisms did not develop GST-GS-X pumps for xenobiotics. Their natural substrates are nucleophilic substances of secondary metabolism: anthocyanins (flavonoids), auxin, salicylic acid, cinnamic acid, hydroxy fatty acids and degradation products of DNA. The Michae-

lis constant of AtMRP 1 and 2 for cyanidin-3-glucoside-GS conjugate (an anthocyanin) is ca. 50 |iM and Vmax is % 5 nmol/mg protein per minute, for example. Similar kinetic data were reported for glucosyl conjugates, e.g. of degradation products of chlorophyll.

4. In phase IV the conjugates are immobilised in the vacuole. Conjugates with glutathione are often further metabolised in the vacuole in order to avoid their diffusion back into the cytoplasm. Thus, for example, a carboxypepti-dase (exopeptidase) may cleave the glycine residue from glutathione. A vacuolar dipepti-dase is able to remove the amino terminal glutamic acid, with the consequent formation of cysteine conjugates. The final fate of these cysteine conjugates is unclear. One possibility would be that they leave the vacuole again via the Golgi system and become irreversibly incorporated into the cell wall as "bound residues" (Fig. 1.9.4).

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