Purification and characterization of pyrophosphate-dependent phosphofructokinase from phosphate-starved Brassica nigra suspension cells

Abstract

Previously, we reported that inorganic phosphate (Pi) deprivation of Brassica nigra suspension cells or seedlings leads to a progressive increase in the α:β-subunit ratio of the inorganic pyrophosphate (PPi)-dependent phosphofructokinase (PFP) and that this coincides with a marked enhancement in the enzyme's activity and sensitivity to its allosteric activator, fructose-2,6-bisphosphate (Fru-2,6-P2). To further investigate the effect of Pi nutrition on B. nigra PFP, the enzyme was purified and characterized from Pi-starved B. nigra suspension cell cultures. Polyacrylamide gel electrophoresis, immunoblot, and gel-filtration analyses of the final preparation indicated that this enzyme exists as a heterooctamer of approximately 500 kD and is composed of a 1:1 ratio of immunologically distinct α (66 kD) and β (60 kD) subunits. The enzyme's a subunit was susceptible to partial proteolysis during purification, but this was prevented by the presence of chymostatin and leupeptin. In the presence and absence of 5 μM Fru-2,6-P2, the forward activity of PFP displayed pH optima of pH 6.8 and 7.6, respectively. Maximal activation of the forward and reverse reactions by Fru-2,6-P2 occurred at pH 6.8. The potent inhibition of the forward activity by Pi (concentration of inhibitor producing 50% inhibition of enzyme activity [I50] = 1.3 mM) was attributed to a marked Pt-dependent reduction in Fru-2,6-P2 binding. The reverse reaction was substrate-inhibited by Pi (I50 = 13 mM) and product-inhibited by PPi (I50 = 0.9 mM). The kinetic data are consistent with the hypothesis that PFP may function to bypass the ATP-dependent PFP in Pt-starved B. nigra. The importance of the Pi nutritional status to the regulation and predicted physiological function of PFP is emphasized.