Nicotinate, quinolinate and nicotinamide as precursors in the biosynthesis of nicotinamide–adenine dinucleotide in barley
1. The relative efficiencies of nicotinate, quinolinate and nicotinamide as precursors of NAD+ were measured in the first leaf of barley seedlings. 2. In small amounts, both [14C]nicotinate and [14C]quinolinate were quickly and efficiently incorporated into NAD+ and some evidence is presented suggesting that NAD+ is formed from each via nicotinic acid mononucleotide and deamido-NAD. 3. [14C]Nicotinamide served equally well as a precursor of NAD+ and although significant amounts of [14C]NMN were detected, most of the [14C]NAD+ was derived from nicotinate intermediates formed by deamination of [14C]nicotinamide. 4. Radioactive NMN was also a product of the metabolism of [14C]nicotinate and [14C]quinolinate but most probably it arose from the breakdown of [14C]NAD+. 5. In barley leaves where the concentration of NAD+ is markedly increased by infection with Erysiphe graminis, the pathways of NAD+ biosynthesis did not appear to be altered after infection. A comparison of the rates of [14C]NAD+ formation in infected and non-infected leaves indicated that the increase in NAD+ content was not due to an increased rate of synthesis.