FIG. 1.

Schematic diagram of NAD⁺ biosynthesis and salvage in yeast cells. The de novo synthesis pathway starts with tryptophan (Trp) and is converted to NaMN by the BNA1 through BNA6 gene products. NaMN is then converted to deamido-NAD (NaAD) by the nicotinic acid/NAM mononucleotide adenylyltransferases NMA1 and NMA2. NaAD is then converted to NAD⁺ by NAD⁺ synthetase (QNS1). If NAD⁺ is hydrolyzed into NAM and o-acetyl-ADP ribose (not shown) by Sir2 or an NAD glycohydrolase, then the NAD⁺ salvage pathway converts NAM back into NaMN for reentry to the biosynthesis pathway. The PNC1 gene product is a nicotinamidase that converts NAM to nicotinic acid (Na), which is then converted to NaMN by the nicotinic acid phosphoribosyltransferase, Npt1. Nicotinic acid can also be imported to the cell by the Tna1 nicotinic acid permease. It is at present unclear how NAM is imported into the cell.