The NAD Kinase: A Phosphoryltransferase Displaying an Oxido-reductase Activity - An Electrophoretic Study

The search for a valuable technique for rapid detection, after electrophoresis, of the activity of various NAD kinase isoforms possibly present in different plant materials, has revealed interesting peculiarities of this enzyme (EC 2.7.1.23; also called ATP:NAD+ 2′-phosphotransferase). At first and in the unique but obligatory presence of NAD, the NAD kinase acts almost instantaneously as an oxido-reductase (probably coupled with the transformation of NAD to NADH). In the additional presence of ATP, the transformation of NAD+ to NADP+ reinforced such an oxido-reductase activity. Final assays testing for the specificity of the phosphoryl donor revealed that not only ATP but also GTP, G6P, and even NADP could be the substrate; the efficiencies of these phosphoryl donors varied with the different isoforms of NAD kinase, evidenced in the different seeds tested, and compared with NAD kinase from heterotrophically grown Euglena cells, and NAD kinase purified from chicken liver (from Sigma Chemical Co.).