Summary: The utilization of exogenous nicotinamide adenine dinucleotide (NAD) by was studied in suspensions of whole cells using radiolabelled NAD, nicotinamide mononucleotide (NMN), and nicotinamide ribonucleoside (NR). The utilization of these compounds by has the following characteristics. (1) NAD is not taken up intact, but rather is degraded to NMN or NR prior to internalization. (2) Uptake is carrier-mediated and energy-dependent with saturation kinetics. (3) There is specificity for the β-configuration of the glycopyridine linkage. (4) An intact carboxamide group is required on the pyridine ring. The intracellular metabolism of NAD was studied in crude cell extracts and in whole cells using carbonyl-C-labelled NR, NMN, NAD, nicotinamide, and nicotinic acid as substrates in separate experiments. A synthetic pathway from NR through NMN to NAD that requires Mg and ATP was demonstrated. Nicotinamide was found as an end-product of NAD degradation. Nicotinic acid mononucleotide and nicotinic acid adenine dinucleotide were not found as intermediates. The NAD synthetic pathway in differs from the Preiss-Handler pathway and the pyridine nucleotide cycles described in other bacteria.


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