Pyridine nucleotide metabolism in Escherichia coli. IV. Turnover

There are two pyridine nucleotide turnover pathways in Escherichia coli, the Gholson turnover cycle, with nicotinamide as an intermediate, and a second new pathway which does not involve nicotinamide. In exponentially growing cultures with a generation time of 60 min, the rates of these cycles are 39 and 102 molecules per s per cell, respectively. We suggest that this second, heretofore undescribed turnover cycle proceeds with nicotinamide mononucleotide as the first intermediate, possibly formed by the DNA ligasecatalyzed cleavage of DPN.