Potato sucrose synthetase: purification, properties, and changes in activity associated with maturation

Sucrose synthetase activity is high in young potato tubers but decreases markedly during maturation. The activity decreases rapidly after the tubers are harvested and remains low regardless of storage temperature. This enzyme was purified 34-fold from freshly harvested immature potatoes. It catalyzes both cleavage and synthesis of sucrose but the 2 activities differ in a number of ways. The pH optima are 6.6 and 8.8 for sucrose cleavage and synthesis. respectively. Sucrose cleavage is activated 4-fold by mercaptoethanol and is inhibited by Mn²⁺. In contrast, sucrose synthesis is activated only slightly by either mercaptoethanol or Mn²⁺ alone but 2-fold in the presence of both reagents. However, it was not possible to resolve the 2 activities, their stabilities to partial thermal inactivation are identical, and their ratios are constant over a wide range of activities.