COMPARISON OF PYROPHOSPHATE TURNOVER AND THE MAXIMUM CATALYTIC ACTIVITY OF PYROPHOSPHATE - FRUCTOSE-6-PHOSPHATE PHOSPHOTRANSFERASE IN LEAVES

Experiments were carried out to assess whether pyrophosphate: fructose-6-phosphate phosphotransferase (PFP) could contribute to the turnover of inorganic pyrophosphate during photosynthetic sucrose synthesis. It was found that: (i) the maximum catalytic activity of PFP exceeded the rate of sucrose synthesis in spinach and barley leaves. Comparison of the measured fluxes and enzyme activities with the previously measured concentrations of pyrophosphate, fructose-6-phosphate and fructose-1,6-bisphosphate suggested that PFP could catalyse the removal of the pyrophosphate produced during sucrose synthesis, provided the cytosolic phosphate concentration were to fall below about 4 mM. (ii) The pyrophosphate level decreased slightly during the induction of photosynthesis in saturating light and CO2. (iii) Pyrophosphate remained high after supplying mannose to leaves to deplete the cytosolic phosphate, and increased after supplying phosphate to leaves. It is discussed how these results provide evidence for a tight regulation of the pyrophosphate concentration, and are consistent with PFP playing a role. © 1990.