Activation of chloroplast-localized enzymes of quinolizidine alkaloid biosynthesis by reduced thioredoxin

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Abstract
Previous studies from the laboratory of the authors have shown that the tetracyclic quinolizidine alkaloids are synthesized in leaf chloroplasts of Lupinus polyphyllus. Additionally, alkaloid formation reveals a light dependent diurnal rhythm in vivo. The present study shows that the principal biosynthetic enzymes, lysine decarboxylase and 17-oxosparteine synthase, assayed in acetone powder extracts and isolated chloroplasts of L. polyphyllus, were activated by reduced E. coli thioredoxin. Since both enzymes display optimal activity at pH 8 and were rather inactive at pH 7, both thioredoxin and the light mediated shift in the hydrogen ion concentration of the chloroplast stroma from pH 7 to pH 8 may be involved in the light controlled alkaloid formation.