Pyridine nucleotide metabolism in Escherichia coli. 3. Biosynthesis from alternative precursors in vivo.

In Escherichia coli, the nicotinamide moiety of pyridine nucleotides may be derived from de novo synthesis of the pyridine ring or through the uptake of either nicotinic acid or nicotinamide. De novo synthesis is the least preferred pathway; it is suppressed if an exogenous source is available. Nicotinamide is kinetically preferred over nicotinic acid; the data are consistent with the hypothesis that nicotinamide can pass through the bacterial membrane much faster than can the charged nicotinic acid, and once inside the cell, the amide is efficiently converted to the acid. Thus even at low concentrations nicotinamide is instantly taken up by cells, but at concentrations of less than 8 X 1O-6 M nicotinic acid in the medium, the rate of entry of the nicotinic acid into the cell appears to be the rate-limiting step in the synthesis of pyridine nucleotide from exogenous niacin. Under conditions where sufficient exogenous niacin or nicotinamide is present so that entry into the cell is not limiting, the rate of pyridine nucleotide biosynthesis is apparently regulated by the rate of conversion of nicotinic acid --) nicotinic acid mononucleotide, as suggested by IMSANDE and PARDEE ((1962) J. BioZ. Chem. 237, 1305). Since the rate of conversion of nicotinamide --f nicotinic acid is not regulated, excess nicotinic acid that is formed from nicotinamide is continually excreted into the medium.