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Metabolism and Enzymology

Enhancement of Terpenoid Biosynthesis from Mevalonate in a Fraction of the Latex from Euphorbia lathyris

U.S. Department of Agriculture, Eastern Regional Research Center, 600 East Mermaid Lane, Philadelphia, Pennsylvania 19118

A latex pellet fraction from Euphorbia lathyris incorporates mevalonate into triterpenols and their fatty acid esters. Conditions for improved incorporation were determined. CaCl₂ or CaCl₂ plus MnCl₂ stimulated biosynthesis, and the metal ion chelator, ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA) enhanced stimulation. Ethylenediaminetetraacetic acid was almost as effective as EGTA, but phthalic acid and citric acid were relatively poor stimulators. The concentration of the Ca²⁺-EGTA complex was directly measured, and the incorporation data are best fitted by a curve that shows that the receptor for the complex is saturable. In the presence of the metal-chelate complex, the addition of fructose, 1,6-bisphosphate plus aldolase (triose-P) or malate provided additional stimulation. Incorporation was maximum at 40 micromolar R-mevalonate, and inhibition occurred at higher concentrations. The apparent K_m for R-mevalonate was 15 micromolar. Under improved reaction conditions, the rate of triterpenoid biosynthesis from mevalonate is 25 times faster than was previously observed (GJ Piazza, EJ Saggese, KM Spletzer [1987] Plant Physiol 83: 177-180).