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Metabolism and Enzymology

Evidence for Arginine as the Endogenous Precursor of Necines in Heliotropium¹

Department of Biological Sciences, Union College, Schenectady, New York 12308

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 -DL-difluoromethylornithine, the specific ornithine decarboxylase inhibitor, at 1 to 2 millimolar had no effect on ¹⁴C incorporation into the necines. In contrast, -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.