Evidence for arginine as the endogenous precursor of necines in Heliotropium

• Published in: Plant physiology (Bethesda) 1987-05, Vol.84 (1), p.42-46
• Main Authors: Birecka, H, Birecki, M, Frohlich, M.W
• Format: Article
• Language: English
• Subjects: ALCALOIDE; ALCALOIDES; ALCOHOLES; ALCOHOLS; ALCOOL; ALKALOIDS; Amino acids; ARGININA; ARGININE; BIOCHEMICAL PATHWAYS; Biological and medical sciences; Biosynthesis; ENZYME INHIBITORS; FOTOSINTESIS; Fundamental and applied biological sciences. Psychology; HELIOTROPIUM; heliotropium angiospermum; HELIOTROPIUM INDICUM; Hydrolysis; INHIBIDORES DE ENZIMAS; INHIBITEUR D'ENZYME; Leaves; LIASAS; LYASE; LYASES; Metabolism; Metabolism and Enzymology; Metabolism. Physicochemical requirements; ORNITHINE; ORNITINA; PHOTOSYNTHESE; PHOTOSYNTHESIS; Plant biochemistry; Plant physiology and development; Plants; Pyrrolizidine alkaloids; Radioactive decay; Radiocarbon; VIA BIOQUIMICA DEL METABOLISMO; VOIE BIOCHIMIQUE DU METABOLISME
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.84.1.42

In pyrrolizidine alkaloid-bearing Heliotropium angiospermum and H. indicum shoots exposed, in the light, to 14C-labeled CO2 for 44 hours, the incorporation of 14C into 1,2-epoxy-1-hydroxymethylpyrrolizidine and retronecine amounted to 0.23 and 0.15%, respectively, of the total carbon assimilated. Treatment of the shoots with α-DL-difluoromethylornithine, the specific ornithine decarboxylase inhibitor, at 1 to 2 millimolar had no effect on 14C incorporation into the necines. In contrast, α-DL-difluoromethylarginine, the specific arginine decarboxylase inhibitor, prevented the incorporation of 14C into the necines of both species; the inhibitor did not affect the absolute incorporation of 14C from exogenous [1,4-14C] putrescine in either species. Thus, arginine is the only apparent endogenous precursor of the putrescine channeled into pyrrolizidines, at least in these two Heliotropium species that exhibited a relatively much higher in vitro activity of arginine decarboxylase than of ornithine decarboxylase. However, within 28 hours after administration, not only exogenous L-[5-14C]arginine, but also exogenous L-[5-14C]ornithine exhibited significant incorporation of their label into the necines, incorporation that could be partially prevented by both inhibitors. Neither inhibitor affected the rates of 14C-labeled CO2 assimilation, transformation of labeled assimilates into ethanol-insoluble compounds, or the very high degree of conversion of the introduced amino acids into other compounds. Methodology related to alkaloid biosynthetic studies is discussed.