A model mechanism for the enzymatic synthesis of lupin alkaloids.

A crude enzyme preparation obtained from cell suspension cultures of Lupinus polyphyllus catalyzes the pyruvate dependent conversion of cadaverine into the tetracyclic lupin alkaloids. As the first reaction product 17-oxosparteine could be identified by gas-liquid chromatography and mass spectroscopy. In some experiments sparteine was found additionally. A participation of diamine oxidase could be ruled out. The cadaverine-pyruvate transaminating enzyme system (17-oxosparteine synthase) catalyzes the formation of 17-oxosparteine from three cadaverine units without releasing free intermediates. These results are inconsistent with the hypothetical mechanism thus far formulated for the lupin alkaloid biosynthesis. A new enzymatic model mechanism is proposed regarding both the results of the enzymatic experiments and those of the in vivo tracer studies.