Formation of NADP ؉ from NAD ؉ is catalyzed by NAD kinase (NadK; EC 2.7.1.23). Evidence is presented that NadK is the only NAD kinase of Salmonella enterica and is essential for growth. NadK is inhibited allosterically by NADPH and NADH. Without effectors, NadK exists as an equilibrium mixture of dimers and tetramers (K D ‫؍‬ 1.0 ؎ 0.8 mM) but is converted entirely to tetramers in the presence of the inhibitor NADPH. Comparison of NadK kinetic parameters with pool sizes of NADH and NADPH suggests that NadK is substantially inhibited during normal growth and, thus, can increase its activity greatly in response to temporary drops in the pools of inhibitory NADH and NADPH. The primary inhibitor is NADPH during aerobic growth and NADH during anaerobic growth. A model is proposed in which variation of NadK activity is central to the adjustment of pyridine nucleotide pools in response to changes in aeration, oxidative stress, and UV irradiation. It is suggested that each of these environmental factors causes a decrease in the level of reduced pyridine nucleotides, activates NadK, and increases production of NADP(H) at the expense of NAD(H). Activation of NadK may constitute a defensive response that resists loss of protective NADPH.NAD metabolism ͉ pyridine nucleotides ͉ metabolic control ͉ NADP synthesis ͉ sedimentation equilibrium

Amino acid analyses of the crystalline enzymes aldolase and D-glyceraldehydephosphate dehydrogenase from rabbit skeletal muscle indicate a considerable excess of free basic over free acidic groups (20). If these groups were to exhibit, dissociation constants within the ranges observed for corresponding groups in other proteins, one would expect the enzymes to lie isoelectric in the vicinity of pH 9 or higher. Actually the enzymes in phosphate buffers of ionic strength 0.1 have been reported to be isoelectric on the acid side of neutrality (5, IS).1 Presented at the Twenty-second National Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society at Cambridge, Massachusetts, June 23-25, 1948.

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