Alkaloids transport mechanism

It is interesting to note that the intracellular transport of some alkaloids in plants, such as berberine, also appears to be catalysed hy plant ABC transporters (69,70,359). It was shown that many alkaloids are transported by alkaloid/H antiporters (71). At that time, ABC transporters were unknown. Since these antiporters were ATP-dependent, it might be worthwhile to revisit alkaloid transport mechanisms in plants (359). 

The contractile proteins of the spindle apparatus must draw apart the replicated chromosomes before the cell can divide. This process is prevented by the so-called spindle poisons (see also colchicine, p. 316) that arrest mitosis at metaphase by disrupting the assembly of microtubules into spindle threads. The vinca alkaloids, vincristine and vinblastine (from the periwinkle plant. Vinca rosea) exert such a cell-cycle-specific effect. Damage to the nervous system is a predicted adverse effect arising from injury to microtubule-operated axonal transport mechanisms. 

Nicotine biosynthesis is localized in the roots of Nicotiana plants, and the alkaloids are transported to the shoots in the xylem stream,70 mainly to young leaves and stems and the reproductive parts of the plant.72 At first glance, the costly transport mechanisms seem to be a disadvantage, as there is a time lag of 10 hr from time of induction until the increase of nicotine production.73 The roots, however, as the site of synthesis are well protected against herbivory and continue the production, even when up to 88% of the total leave area is removed.74 Optimization of the cost-value ratio seems to be the reason for the inducible defense acting as a cost-saving... 

The vinca alkaloids, vincristine and vinblastine (from the periwinkle plant, Vinca rosea), inhibit the polymerization of tubulin subunits into microtubuli. Damage to the nervous system is a predicted adverse effect arising from injury to microtubule-operated axonal transport mechanisms. 

In the plant, the biosynthesis of the Vinca alkaloids involves more than 20 enzymatic steps including several cytochrome P450 monooxygenases and one class III plant peroxidase. Removal of the toxic alkaloids from the cytoplasm to the vacuole of plant cells is made by an uncharacterised transport mechanism that we suggest may be an ABC transporter, as already shown for several other plant secondary metabolites. 

In plants the site of alkaloid biosynthesis is often separated from the site of storage for example, tropane alkaloids are produced in the root and stored in the leaves. This means that the alkaloids need to be excreted from the biosynthetically active cells and then transported to and taken up by the storage cells. In undifferentiated tissue like cells in suspension culture this transport mechanism is likely to be seriously affected this might result in low productivity. 

Uptake of alkaloids by isolated vacuoles and transport mechanisms over membranes have been studied in detail. Carrier-mediated active transport... 

At present, only a few membrane compartments, transporters and channels are known to have a defined role in alkaloid transport and compartmentation. One major challenge for research is the high individuality in the cellular organization that rests on the species-specific localization of biosynthetic enzymes and the opportunistic use of transporters, metabolite pools, storage sites and accumulation mechanisms even between taxonomically related genera. 

There is little doubt that the few examples of alkaloid transporters known today will be greatly expanded in the next future as the perfection of molecular and cellular techniques should allow to characterize suspected transport mechanism and proteins and that hitherto escaped the experimental access. Candidates will be found not only among the steps of intracellular accumulation and transport as postulated above, but also within the long known inter-organ transfers, e.g. of nicotine from the root to leave vacuoles in Nicotiana (Hashimoto and Yamada, 1994) or of senecionine-N-oxide from the roots to vacuoles of inflorescences in Senecio (Hartmann, 1999). 

Drug resistance in vitro and probably in vivo results both from inhibition of influx of the vinca alkaloids and, perhaps more frequently, from promotion of their efflux out of cells (34,35). Until relatively recently, the former mechanism was thought to predominate, and, indeed, certain acquired drug-resistant states are clearly associated with the loss of membrane proteins which can be shown to bind and transport agents into cells (34). However, other resistant states have been shown to be associated with the acquisition of membrane transport proteins which remove toxins (and, therefore, chemotherapeutic agents) both from normal and malignant cells. 

Three classes of plant-derived drugs, the vinca alkaloids (vincristine, vinblastine, and vinorelbine), the epipodo-phyllotoxins (etoposide and teniposide and the tax-anes (paclitaxel and taxotere), are used in cancer chemotherapy. These classes differ in their structures and mechanisms of action but share the multidrug resistance mechanism, since they are all substrates for the multidrug transporter P-glycoprotein. 

Fig. 14. Overview of regulatory mechanisms acting at the level of transport and channeling of the alkaloid precursor phenylalanine in Penicillium cyclopium (60). (1) Under the influence of P-factor, the biosynthesis of vacuolar phenylalanine carriers is stimulated. (2) Above a threshold concentration, cellular methionine and cysteine inactivate vacuolar phenylalanine carriers. (3) Distinct concentrations of cellular ATP inhibit the efflux from the vacuole high levels of cytosolic amino acids stimulate efflux in the presence of sufficient ATP. (4) The vacuolar phenylalanine pool is most probably involved in triggering the expression of alkaloid metabolism. (5) In the idiophase, cyclopenin stimulates enzymes involved in the biosynthesis of phenylalanine.

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