N-oxidation and degradation of pyrrolizidine alkaloids during germination of Crotalaria scassellatii

Abstract

The pyrrolizidine alkaloids (axillaridine, axillarine, desoxyaxillarine) found in the seeds of Crotalaria scassellatii occur exclusively as tertiary alkaloids. However, they are rapidly converted into the respective N-oxides during the first days of seedling development. N-Oxidation is followed by a progressive degradation of the seed alkaloids; five-week-old seedlings are almost depleted of alkaloids. Dry seeds contain ca 2% alkaloids which account for less then 2% of total seed nitrogen and ca 20% soluble nitrogen. Mature plants accumulate pyrrolizidine alkaloids exclusively in the form of N-oxides in all tissues. It is suggested that the polar pyrrolizidine N-oxides are the preferred storage compounds in vacuolated tissues, whereas the desiccated seeds accumulate the lipophilic tertiary alkaloids.