Biosynthetic incorporation of the aminobutyl group of spermidine into pyrrolizidine alkaloids

Abstract

In short-term tracer experiments [ 3H]putrescine and [ 14C]spermidine were fed simultaneously to root cultures of Senecio vulgaris. The specific incorporation of the two tracers into putrescine, spermidine and pyrrolizidine alkaloids was followed over a period of 30 to 480 min. The results show that the aminobutyl moiety of spermidine is directly incorporated into the necine base of pyrrolizidine alkaloids. The same experiment carried out in the presence of the diamine oxidase inhibitor β-hydroxyethylhydrazine, which blocks the alkaloid biosynthesis after homospermidine, revealed the same specific incorporation into accumulating homospermidine. The results are in accordance with the substrate specificity of plant homospermidine synthase (EC 2.5.1.44) which uses both putrescine and spermidine as donor for the amino butyl group. Under physiological conditions more than half of the aminobutyl moiety of homospermidine comes directly from spermidine. This is the first report for the role of spermidine as aminobutyl donor in alkaloid biosynthesis. The importance of the linkage between spermidine and pyrrolizidine alkaloid biosynthesis is discussed. © 1997 Elsevier Science Ltd. All rights reserved