Jean-Claude Prévôt

Ethylene-Induced Increase in Glutamine Synthetase Activity and mRNA Levels in Hevea brasiliensis Latex Cells

Ethylene, used as a stimulant of latex production in Hevea brasiliensis, significantly activates the regenerating metabolism within the laticiferous cells. In this context, attention was focused on glutamine synthetase (GS; EC 6.3.1.2), a key enzyme in nitrogen metabolism. A specific and significant activation of the cytosolic glutamine synthetase (GScyt) in the laticiferous cells after ethylene treatment parallels the increase of latex yield. A marked accumulation of the corresponding mRNA was found, but in contrast, a slight and variable increase of the polypeptide level is at the limit of detection by western blotting. The GS response to ethylene might be mediated by ammonia that increases in latex cytosol following ethylene treatment. The physiological significance for such a regulation by ethylene of the GScyt is discussed in terms of the nitrogen requirement for protein synthesis associated with latex regeneration.

The composition of natural latex from Hevea brasiliensis.

Hevea brasiliensis latex collected by tapping can be considered as cytoplasm, in spite of the absence of nucleus and mitochondrial material. It possesses the complexity of cytoplasm both in its subcellular organization and the diversity of the constituent molecules. Its essential characteristic is the high rubber content. Rubber is a very long chaincis-polyisoprene polymer synthesized in the cytosol compartment. Rubber forms 90% of latex dry weight and 40% of fresh weight, which underlines the astonishing dominance of this anabolic pathway in the laticiferous metabolism. However, the preparation of dry rubber or concentrated latex as used in industry removes a fairly large proportion of the mineral and organic solutes from fresh latex. Latex processing conditions in plantation factories can considerably modify the technological properties of rubber after vulcanization through the influence of the nature and proportion of the residual nonrubber substances. Some of these give natural rubber properties that are still better than those of syntheticcis-polyisoprene.In contrast, it appears to have been clearly demonstrated that the proteins extracted from latex or from rubber tree leaves containing latex display allergenic properties (46,47). Turjanmaa et al. (48) observed that allergenic proteins extracted from latex gloves have molecular weights of between 2 and 30 kDa and are thus similar to various allergens, including those of pollen. It has been mentioned that large quantities of proteins with low molecular weights are found in latex.

Purification and Characterization of Phosphoribosylpyrophosphate Synthetase from Rubber Tree Latex.

Phosphoribosylpyrophosphate synthetase (PRS; EC 2.7.6.1) from Hevea brasiliensis Mull. Arg. latex was located in the cytosol. After purification, its apparent molecular weight under nondenaturing conditions was estimated at 200,000 [plus or minus] 10,000; a single band at 57,000 [plus or minus] 3,000 was detected after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme seemed to be a homotetramer. Its affinity constants were estimated at 200 [plus or minus] 30 [mu]M for adenosine triphosphate and 40 [plus or minus] 2 [mu]M for ribose-5-phosphate. The purified enzyme proved to be functional in a paraphysiological medium (cytosol deproteinized by ultrafiltration). Optimum pH was 7.5 in buffer and 6.5 in a paraphysiological medium. No PRS activity was detected in the absence of the Mg2+ ion. Of the numerous compounds tested, only Mn2+, phosphoribosylpyrophosphate, and inorganic phosphate affected the enzymatic reaction. Mn2+ (inhibitor constant = 20 [mu]M) and phosphoribosylpyrophosphate (inhibitor constant = 30 [mu]M) were inhibitors. PRS responded allosterically (Hills coefficient = 2.3) to ribulose-5-phosphate in the presence of a physiological concentration of inorganic phosphate (10 mM). These results are set in the physiological context of laticifers.

Physiological activators of invertase from Hevea brasiliensis latex

Abstract Potassium or sodium nitrates or phosphates, and thiols such as reduced glutathione or cysteine, stimulate the activity of invertase from Hevea brasiliensis latex. These activators raise the V max but do not affect the K m of the enzyme for sucrose. The action of these effectors is additive. Their efficiency is pH dependent, being higher below pH 7.0 and markedly decreasing above it.

Ethylene, stress and enzymatic activities in hevea latex : the diversity of responses

Latex is a fluid cytoplasm[1] which contains high quantity of rubber (30 to 50% of its fresh weight). It is expeled by tapping from articulated laticiferous cells located in Hevea phloem. Ethrel (Ethephon), an ethylene releaser is used on a large scale in Hevea brasiliensis as a latex production stimulant[21. So, 25 mg of this product mixed with 750 mg of palm oil are applied on the tapping cut of rubber tree, 48 hours before tapping. The treatment causes the extension of latex flow time and the activation of latex regeneration in laticiferous cells between two tappings. The activation is able to cause considerable and transitory modifications in the laticifers metabolism[3]. Study of the mechanisms involved in the modifications has shown that although certain biologically important enzymatic reactions are not (or) little influenced, others may be activated or slowed. An exemple of each case is described and replaced in its physiological context.