Ornithine Decarboxylase, Polyamines, and Pyrrolizidine Alkaloids in Senecio and Crotalaria

Abstract

When tested for ornithine and arginine decarboxylases, pyrrolizidine alkaloid-bearing Senecio riddellii, S. longilobus (Compositae), and Cro- talaria retusa (Leguminosae) plants exhibited only ornithine decarbox- ylase activity. This contrasts with previous studies of four species of pyrrolizidine alkaloid-bearing Heliotropium (Boraginaceae) in which ar- ginine decarboxylase activity was very high relative to that of ornithine decarboxylase. Unlike Heliotropium angiospermum and Heliotropium indicum, in which endogenous arginine was the only detectable precursor of putrescine channeled into pyrrolizidines, in the species studied here using difluoromethylornithine and difluoromethylarginine as the enzyme inhibitors-endogenous ornithine was the main if not the only precursor of putrescine converted into the alkaloid aminoalcohol moiety. In S. riddeliji and C. retusa at flowering, ornithine decarboxylase activity was present mainly in leaves, especially the young ones. However, other very young organs such as inflorescence and growing roots exhibited much lower or very low activities; the enzyme activity in stems was negligible. There was no correlation between the enzyme activity and polyamine or alkaloid content in either species. In both species only free polyamines were detected except for C. retusa roots and inflorescence-with rela- tively very high levels of these compounds-in which conjugated putres- cine, spermidine, and spermine were also found; agmatine was not iden- tified by HPLC in any plant organ except for C. retusa roots with rhizobial nodules. Organ- or age-dependent differences in the polyamine levels were small or insignificant. The highest alkaloid contents were found in young leaves and inflorescence.