Figure 1.

Biosynthetic pathways for ornithine-derived alkaloids, polyamine, and nicotinamide adenine dinucleotide (NAD). Each defined step of the pathway is represented by an arrow and named enzyme, while undefined parts dependent on single or multiple steps are shown with broken arrows. Steps in black are hypothesized to be involved predominantly in alkaloid biosynthesis, while those predominantly associated with related primary metabolic pathways are shown in gray. Quinolinate synthase (QS) is shown in dark gray, as it contributes to both nicotine and NAD pathways. It has been proposed that putrescine N-methyltransferase (PMT) and N-methylputrescine oxidase (MPO) have evolved from spermidine synthase (SPDS) and diamine oxidase (DAO), respectively. AO; aspartate oxidase, QPT; quinolinate phosphoribosyl transferase, ODC; ornithine decarboxylase.