1. β-Hydroxy-β-methyl[3−14C]glutaryl-CoA is efficiently incorporated into rubber on incubation with Hevea brasiliensis latex, and the incorporation is diminished in the presence of unlabelled mevalonate. β-Hydroxy-β-methylglutaric acid is not utilized for rubber synthesis, but inhibits the formation of rubber from β-hydroxy-β-methylglutaryl-CoA. 2. The incorporation of β-hydroxy-β-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. β-Hydroxy-β-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 β-hydroxy-β-methylglutaryl-CoA incorporation into rubber in latex, but mevalonate incorporation is relatively constant. This observation, together with the finding that β-hydroxy-β-methylglutaryl-CoA reduction is the rate-limiting step in the formation of rubber from β-hydroxy-β-methylglutaryl-CoA, suggests that the conversion of β-hydroxy-β-methylglutaryl-CoA into mevalonate is of importance in the regulation of rubber synthesis. 5. Evidence suggesting that β-hydroxy-β-methylglutaryl-CoA lyase is present in H. brasiliensis latex has been obtained.

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