Nicotiana Alkaloids and Nicotinic Acid

The most important nicotiana alkaloids (tobacco alkaloids) are nicotine, nornicotine, and anabasine (Fig. 122). Nicotine and nornicotine are the chief alkaloids of Nicotiana tabacum, and anabasine of Nicotiana glauca. Both nicotine and nornicotine consist of a pyridine ring to which a pyrrolidine ring is attached. In the case of nicotine this pyrrolidine ring bears a methyl group on its ring-N. Anabasine also consists of a pyridine 

The formation of nicotinic acid—or, more precisely, of quinolinic acid—via two different pathways is an example of biochemical convergence, Now to the biosynthesis of the pyrrolidine ring of nicotine and nornicotine. Ornithine comes into play here. It furnishes the pyrrolidine component by the route already outlined, and this is then combined with nicotinic acid to give nicotine (Fig. 124). It is not known with certainty at what stage the methyl group is introduced. Very probably ornithine or putrescine is methylated and the methylated precursor is then icorporated into nicotine. Nicotine can be converted into nornicotine by demethyla-tion. 

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