Sites of Enzymatic Synthesis of Quinolizidine Alkaloids and Their Accumulation in Lupinus polyphyllus

Abstract

Summary The distribution of the first two enzymes of the biosynthetic pathway of quinolizidine alkaloid biosynthesis, lysine decarboxylase and 17-oxosparteine synthase, was studied in the various organs of a flowering Lupinus polyphyllus plant. Lysine decarboxylase activity is present in all plant parts, whereas the activiy of the key enzyme 17-oxosparteine synthase is only expressed in leaf extracts. This indicates that alkaloid synthesis is bound to the leaves, where both enzyme had been localized in chloroplasts previously. The alkaloid content, alkaloid concentration and the alkaloid composition was quantitatively evaluated for the various organs of two flowering L. polyphyllus plants. The highest alkaloid content was found in the roots (38-45% of total alkaloids) and leaves (27-31%), but alkaloids are virtually present in all parts of a plant. Relatively high alkaloid concentrations were found in leaves, in younger parts of the shoot axis and in mature seeds. The alkaloid content of leaves is reduced during senescence by 90% as compared with young leaves. The alkaloid pattern evaluated quantitatively by capillary gas chromatography for the organs of two plants and seeds includes the following compounds: lupanine, sparteine, angustifoline, N-methylangustifoline, dehydrolupanine, 17-oxolupanine, 13-hydroxylupanine and its esters (13-tigloyl-, 13-angeloyl-, 13-benzoyl- and 13-cis and 13-transcinnamoyl-oxylupanine). A high percentage of ester alkaloids and a low content of 13-hydroxylupanine was found in leaves. Roots and seeds, the main sites of alkaloid accumulation, displayed the opposite pattern, i.e. high concentration of 13-hydroxylupanine and low concentration of ester alkaloids.