EFFECT OF TEMPERATURE ON THE KINETIC-PROPERTIES OF PYROPHOSPHATE-FRUCTOSE 6-PHOSPHATE PHOSPHOTRANSFERASE FROM POTATO-TUBER

The aim of this work was to investigate the potential role of pyrophosphate: fructose 6-phosphate phosphotransferase during cold-induced sweetening in potatoes. The enzyme purified to near homogeneity from mature tubers of Solanum tuberosum cv. Record was reversibly inhibited by low temperature. Under optimal assay conditions the temperature coefficient of the enzyme increased from 1.3 at 25-degrees-C to 2.6 at 5-degrees-C. Low temperature had no significant effect on the affinity of the fully-activated enzyme for its substrates fructose 6-phosphate, pyrophosphate, fructose 1,6-bisphosphate and phosphate. However, the concentration of fructose 2,6-bisphosphate required for half-maximal activation increased about 8-fold as the temperature was lowered from 25-degrees-C to 2-degrees-C. In addition, the extent of activation by this metabolite increased from about 5-fold to 50-fold over the same temperature range. These effects may restrict the interconversion of fructose 6-phosphate and fructose 1,6-bisphosphate at low temperature, and thus contribute to the accumulation of hexose phosphates and subsequent sucrose synthesis in the cold.