Molecular biology of pyridine nucleotide and nicotine biosynthesis.

Nicotinamide adenine dinucleotide (NAD) is a ubiquitous coenzyme in oxidation-reduction reactions. Recent animal and fungal studies show that it also plays important roles in transcriptional regulation, longevity, and age-associated diseases. NAD is synthesized de novo from aspartic acid in E. coli or from tryptophan in animals, by way of quinolinic acid. Although the number of biochemical studies on NAD is very limited, a bioinformatic search of genome databases suggests that Arabidopsis (dicots) synthesizes NAD from aspartic acid whereas rice (monocots) may utilize both aspartate and tryptophan as starting amino acids. The salvage pathway recycles the breakdown products of NAD metabolism. In tobacco, an intermediate in the de novo NAD synthetic pathway supplies the pyridine ring moiety of nicotine alkaloids. Gene expression studies in tobacco suggest that part of the NAD pathway is coordinately regulated with nicotine biosynthesis.