Hevea latex

The Heeva brasiliensis may be tapped for latex by gouging the bark with a tapping knife. The composition of the Hevea latex varies quite widely but the following may be considered to be a typical composition ... 

Wagner S, Breiteneder H, Simon-Nobbe B, Susani M, Krebitz M, Niggemann B, Brehler R, Scheiner O, Hoffmann-Sommergruber K Hev b 9, an enolase and a new cross-reactive allergen from Hevea latex and molds. Purification, characterization, cloning and expression. Eur J Biochem... 

NR occurs as particles dispersed in an aqueous serum, i.e., latex, which are contained in bundles of tubular vessels in the trunk of the tree [7]. Latex is harvested from the tree by a process called tapping. The latex of the Hevea braziliensis tree has been described as a cytoplasmic system containing rubber and nonrubber particles dispersed in an aqueous serum phase. Freshly tapped Hevea latex has a pH of 6.5-7 and a density of 0.98 gcm . The total solids of fresh field latex vary typically from 30 to 40%, depending on weather and tapping frequency and other factors. The dry rubber content is approximately 3 wt% less than the total solids [8]. 

The polymers which will be studied in this book are synthetic polymers and the industrial importance of such materials is well-known. However, we must recall that there also exist numerous natural polymers, and some of these have a great biological importance. Among the most common ones, we may cite natural rubber extracted from hevea latex and cellulose, with its derivatives, extracted from wood. 

The history of the hevea plantation is full of events. This tree was exclusively a product of the Brazilian forest until 1876. At that time, an Englishman, Wickman by name, smuggled hevea seeds from Amazonia, which were sown in Ceylon. Subsequently, rubber planting developed in Malaysia, Indonesia, and Indo-China, Regulating the level aiid the cost of production of the hevea latex has always been difficult in spite of international agreements such as the Stevenson plan (1922). 

Figure 9.5.7 Two examples of potentially high value-added by-products from Hevea latex.

Hevea latex is a suspension in which rubber particles are stabilized by an adsorbed film of phospholipids and proteins triglycerides together with p-sitosterol and related compounds are the principal constituents of the neutral lipids of the rubber phase. 

The composition of Hevea latex varies with age of tree, condition of soil, climatic conditions, season, etc. An average composition is shown in Table 9.4. 

The chemistry and biochemistry of rubber have been investigated extensively, but there are still many unresolved questions. Research on the in vitro biosynthesis of rubber by Hevea latex was especially active in the 1960s [reviewed by Mullins (1%5), Lynen (1%7,1%9), and Archer and Audley (1967, 1973)]. Hepper and Audley (1969) found that up to 50% of the physiological isomer of [3- C]HMG-CoA and 97% of administered IPP were incorporated into rubber by isolated latex. Labeled acetate and MVA were also shown to serve as rubber precursors (Archer and Audley, 1973). Most of the tracer studies have involved the extension of rubber molecules via IPP, taking place on the surface of rubber particles (Archer and Audley, 1973). However, there is evidence that DMAPP stimulates rubber biosynthesis in the presence of washed rubber particles (Lynen, 1%9 Archer and Audley, 1973). This is consistent with the anticipated role of DMAPP as a chain initiator, but the questions of chain initiation and chain termination, and the nature of the rubber particles as organelles, do not appear to have been resolved. It is assumed (Archer and Audley, 1973) that the starter end of the rubber molecule bears an isopropylidene group, and the other end a pyrophosphate group. 

De Corres L, Moneo I, Munoz D, Bernaola G, Fernandez E, Audicana M, Urrutia I (1993) Sensitization from chestnuts and bananas in patients with urticaria and anaphylaxis from contact with latex. Ann Allergy 70 35-39 Dennis M, Henzel W, Bell J, Kohr W, Light D (1989) Amino acid sequence of rubber elongation factor protein associated rubber particles in Hevea latex. J Biol Chem 264 18618-18626 Forstrom L (1980) Contact urticaria from latex surgical gloves. Contact Dermatitis 6 33-34... 

Natural rubber is a constituent of the latex of Hevea brasiliensis a tree grown in the tropics. The latex is present in laticiferous vessels and can be collected after incision of the bark. It contains about 30 % of rubber in form of small droplets and coagulates after the addition of acids. The coagulate is processed to bandlike sheets of raw" rubber (smoked sheets, crepes) which in the industry are heated with sulfur (vulcanization) to harden it. At present time about half of the rubber used in the industry is prepared from Hevea latex, the other half is of synthetic origin. 

The guayule latex value is a mean of 20 latex samples prepared throughout a year from bimonthly harvests of different lines. Each value in the mean was itself a mean of foim protein determinations. All Hevea latex determinations are the mean of 4 assays s.e. Hevea latex samples were obtained from Centro Trade, Virginia Beach, Va, although the enzymatically deproteinized material is Allotex... 

Pig. 9. Particle size distribution of rubber particles purified from guayule stem bark and Hevea latex. Measurements were made on rubber particles in aqueous suspension using a Horiba LA-900 Laser Light Scattering Particle Size Distribution Analyzer. —O—, Hevea — —, Guasnile. Data from Ref 12. 

The NR molecule is widely accepted to be synthesized by the successive addition of IPP to dimethylallyl diphosphate (DMAPP) to build up isoprene units in the cis configuration. Initial biochemical studies presumed that IPP, a precursor of rubber molecules from higher plants, is synthesized via the mevalonate (MVA) pathway,which is operated in the cytoplasm from acetyl-Co A. Later, a non-mevalonate pathway called the methylerythritol phosphate (MEP) pathway has also been considered as responsible for IPP synthesis in the plastids from the condensation of pyruvate and glyceraldehyde-3-phos-phate. Mevalonate and l-deoxy-o-xylulose-5-phosphate (DXP) are intermediates in MVA and MEP pathways, respectively. DMAPP is produced by the isomerization of IPP, catalysed by IPP isomerase, from both pathways. The gene expression of DXP synthase in Hevea latex suggested that the MEP pathway for IPP synthesis is involved in rubber biosynthesis.However, an experiment that involved feeding 600 RRIM seedlings with [l- C] 1-deoxy-o-xylulose triacetate, an intermediate derivative of the MEP pathway, indicated that the MEP pathway is involved only in carotenoid biosynthesis but not in the case of rubber biosynthesis. ... 

Chen, Z., Posch, A., Lohaus, C., Raulf-Heimsoth, M., Meyer, H.E. and Baur, X., Isolation and identification of hevein as a major IgE-binding polypeptide in Hevea latex. J. Allergy Clin. Immunol, 99, 402-409 (1997). 

On gouging the bark of Hevea brasiliensis, hevea latex is collected, which has close to a 33% dry rubber content. Natural mbber, a long-chain polyisoprene, given by... 

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