Utilization of metabolic energy under NaCl stress: Activity of nucleoside diphosphate kinase

Nucleoside diphosphate kinase (NDPK) is an essential and ubiquitous enzyme in the synthesis of nucleoside triphosphates (such as UTP, GTP and CTP) and deoxynucleoside triphosphates (dATP, dTTP, dGTP and dCTP). It catalyzes the following reaction: (d)NDP + ATP mutually implies (d) NTP + ADP The activity of nucleoside diphosphate kinase (NDPK) was investigated in roots and leaves of barley exposed to NaCl. Exposure to NaCl had no effect on tissue length or fresh weight gain; there was a transitory increase in NDPK activity in roots only. NDPK activity in roots increased after 1-day exposure to NaCl but the activity returned to a similar level as in non-exposed seedlings within 2 days. Km and Vmax toward ATP and dCDP were measured. Vmax towards ATP was unaltered after 1-day exposure to NaCl, while Vmax towards dCDP increased 3-fold. The total amount of enzyme in roots did not change during the 4-day exposure to NaCl, indicating that the increase in Vmax was not due to more enzyme but to higher turnover number. Km towards dCDP was unaltered after 1-day exposure to NaCl, while Km towards ATP decreased after 1-day exposure. These results suggest that NDPK in NaCl exposed roots has a higher affinity towards ATP, making it more efficient under conditions of increased competition for ATP due to NaCl stress.