Biosynthesis of the pyrrolidine ring of nicotine in Nicotiana glutinosa

The biosynthesis of the pyrrolidine ring of nicotine has been studied using short-term steady-state exposures of Nicotiana glutinosa seedlings to ¹⁴CO₂. The pyrrolidine ring of the labeled nicotine has been degraded in a systematic manner to ascertain the radioactivity at each carbon, and a new method has been developed for obtaining C-2′ with complete radiochemical integrity. Some of the labeling patterns obtained were symmetrical while others were clearly unsymmetrical. The duality of the labeling patterns found in these ¹⁴CO₂ biosyntheses, together with other data on pyrrolidine ring biosynthesis which are critically examined, is best rationalized by postulating two biosynthetic pathways for formation of the pyrrolidine ring, one involving a symmetrical precursor and the other an unsymmetrical one.