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Evidence for Arginine as the Endogenous Precursor of Necines in Heliotropium

Helena Birecka, Mieczyslaw Birecki and M. W. Frohlich
Plant Physiology
Vol. 84, No. 1 (May, 1987), pp. 42-46
Stable URL: http://www.jstor.org/stable/4270574
Page Count: 5
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Evidence for Arginine as the Endogenous Precursor of Necines in Heliotropium
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Abstract

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.

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