The nitrogenous groups in natural rubber (NR) from Hevea brasiliensis have been regarded as an important factor governing cured rubber properties, although there is no structural evidence. The nitrogenous compounds in highly purified NR were characterized to disclose the origin of nitrogenous groups. The rubber particles in fresh latex were purified by washing, i.e., successive dilution of the cream fraction and concentration by centrifugation in the presence of surfactant. The nitrogen content of natural rubber from the washed rubber particles decreased from 0.281% to 0.015% level after washing 3–5 times, the level of which was similar to that of deproteinized natural rubber with a proteolytic enzyme. The presence of 1–2 nitrogen atoms per rubber chain was presumed based on the nitrogen content and Mn value. The nitrogen content of fractionated rubber fractions from rubber particles washed 5 times increased with increasing molecular weight. The nitrogenous group in the rubber chain is postulated to originate from mostly phospholipids associating with branch-points and from functional groups formed by oxidation and successive reactions in latex.

A structural study of low molecular weight rubbers from Jackfruit (Artocarpus heterophyllus) and Painted spurge (Euphorbia heterophylla) was carried out as model compounds of natural rubber from Hevea brasiliensis. The rubber content of latex from Jackfruit was 0.4-0.7%, which is very low compared with that of 30-35% in the latex from Hevea tree. The rubber from Jackfruit latex was low molecular weight with narrow unimodal molecular weight distribution (MWD), whereas that obtained from E. heterophylla showed very broad MWD. The 1H and 13C NMR analyses showed that Jackfruit rubber consists of a dimethylallyl group and two trans-isoprene units connected to a long sequence of cis-isoprene units. The alpha-terminal group of Jackfruit rubber was presumed to be composed of a phosphate group based on the presence of 1H NMR signal at 4.08 ppm corresponding to the terminal =CH-CH2OP group.

Farnesyl diphosphate is involved in rubber biosynthesis as an initiating substrate for both polyprenol and mushroom rubber. So far, we have isolated the cDNA of a farnesyl diphosphate synthase (FPS) for the first time from a rare rubber-producing mushroom, Lactarius chrysorrheus, by the degenerate RT-PCR technique based on sequence information of FPS genes from fungi and yeasts. The open reading frame was clarified to encode a protein of 381 amino acid residues with a calculated molecular weight of 42.9 kDa. The deduced amino acid sequence of L. chrysorrheus FPS showed about 50% identity with those of other fungi and yeasts as well as plants. We expressed the cDNA of L. chrysorrheus FPS in Escherichia coli as a glutathione-S-transferase (GST)-fusion protein. The purified obtained protein showed FPS activity in which geranyl diphosphate (GPP) served as primary substrate, with a 2.4-fold higher k(cat)/K(m) value for GPP than for dimethylallyl diphosphate (DMAPP).