New aspects of rubber biosynthesis. Following a review of the site of rubber biosynthesis in Hevea brasiliensis and Parthenium argentalum, evidence is given for the initiation of polyisoprene molecules from (ranMerpenoid precursors including geranylgeranyl pyrophosphate. All franj‐14C‐geranylgeraniol has been isolated from incubations of H. brasiliensis latex serum with 14C‐isopentenyl pyrophosphate. Gel‐filtration chromatography of the serum yields very small rubber particles of high biosynthetic activity, and two proteinaceous fractions. One of these increases the biosynthesis of rubber and may contain the enzyme, isopentenyldiphosphate δ‐isomerase, whilst the other appears to inhibit rubber formation. The nature and molecular weight of the rubber formed in vitro is discussed and a mechanism for the de novo formation of rubber particles is suggested.

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1. beta-Hydroxy-beta-methyl[3-(14)C]glutaryl-CoA is efficiently incorporated into rubber on incubation with Hevea brasiliensis latex, and the incorporation is diminished in the presence of unlabelled mevalonate. beta-Hydroxy-beta-methylglutaric acid is not utilized for rubber synthesis, but inhibits the formation of rubber from beta-hydroxy-beta-methylglutaryl-CoA. 2. The incorporation of beta-hydroxy-beta-methylglutaryl-CoA into rubber is stimulated equally by NADP(+) and NADPH and less so by NAD(+) and NADH. ATP is slightly stimulatory and CoA is inhibitory. 3. beta-Hydroxy-beta-methylglutaryl-CoA reductase is concentrated in the sediment (bottom fraction) formed by centrifuging latex at low speed and the enzyme is unstable in the absence of cysteine or GSH. The formation of NADPH takes place in the latex serum. 4. There is a marked seasonal variation in the extent of beta-hydroxy-beta-methylglutaryl-CoA incorporation into rubber in latex, but mevalonate incorporation is relatively constant. This observation, together with the finding that beta-hydroxy-beta-methylglutaryl-CoA reduction is the rate-limiting step in the formation of rubber from beta-hydroxy-beta-methylglutaryl-CoA, suggests that the conversion of beta-hydroxy-beta-methylglutaryl-CoA into mevalonate is of importance in the regulation of rubber synthesis. 5. Evidence suggesting that beta-hydroxy-beta-methylglutaryl-CoA lyase is present in H. brasiliensis latex has been obtained.

Ethephon (I) is used commercially to prolong the flow of latex from the rubber tree after tapping (Yield stimulation). The compound is applied to the bark in the region of the tapping cut and the effect on latex flow is due to the ethylene released by chemical decomposition, since gaseous ethylene itself is also a very effective stimulant. When 14C-I is applied to the bark of a young Hevea seedling, it is absorbed into the plant by processes which appear to be largely non-metabolic. Ethylene formation commences immediately at the site of application, and the gas is quickly translocated throughout the plant. Translocation of I to all parts of the plant also occurs and the accumulation of 14C in the bark above the zone of application is greater than that below. Chromatographic analysis has shown that compounds other than I remain in the plant tissue. Experiments using 14C-I have shown that detached leaves are able to convert a considerable proportion of the compound to at least twelve non-volatile acid products. One of these is a conjugate of I with an unidentified material. A major component of the products is 2-hydroxyethylphosphonic acid (II), which is itself converted to a number of compounds in leaves. The application of I to bark from mature Hevea, results in the formation of a single substance which is also a conjugate of I. Neither I nor II is effective in inducing the formation of ethylene from endogenous precursors in vegetative Hevea tissue. Ethylene is poorly metabolized by Hevea leaves and the evidence available indicates that it is unlikely that any of the compounds produced from I are metabolites of ethylene.

It is now well established that, after removal of C-8, purines are actively incorporated into rings B and c of riboflavin (see e.g. Goodwin, 1959; Plaut, 1961a, b). Further, the addition of purines to cultures of the flavinogenic fungi Eremothecium ashbyii and Candidaflareri stimulates the synthesis