Fructose - l , 6 - Bisphosphate 1 s an Allosteric Activator of Pyrophosphate : Fructose - 6 - Phosphate 1 - Phosphotransferase ' Tom

The activity of highly purified pyrophosphate:fructose-6-phosphate 1 -phosphotransferase (PFP) from barley (Hordeum vulgare) leaves was studied under conditions where the catalyzed reaction was allowed to approach equilibrium. The activity of PFP was monitored by determining the changes in the levels of fructose-6phosphate, orthophosphate, and fructose-1,6-bisphosphate (Fru1,6-bisP). Under these conditions PFP activity was not dependent on activation by fructose-2,6-bisphosphate (Fru-2,L-bisP). lnclusion of aldolase in the reaction mixture temporarily restored the dependente of PFP on Fru-2,6-bisP. Alternatively, PFP was activated by Fru-1,6-bisP in the presence of aldolase. I t is concluded that Fru1,6-bisP is an allosteric activator of barley PFP, which can substitute for Fru-2,C-bisP as an activator. A significant activation was observed at a concentration of 5 to 25 PM Fru-l,L-bisP, which demonstrates that the allosteric site of barley PFP has a very high affinity for Fru-1,6-bisP. The high affinity for Fru-1,6-bisP at the allosteric site suggests that the observed activation of PFP by Fru1,6-bisP constitutes a previously unrecognized in vivo regulation mechanism.