Pyrophosphate Fructose-6-P 1-Phosphotransferase from Tomato Fruit

Abstract

Three forms of pyrophosphate fructose-6-phosphate 1-phosphotransferase (PFP) were purified from both green and red tomato (Lycopersicon esculentum) fruit: (a) a classical form (designated Q(2)) containing alpha- (66 kilodalton) and beta- (60 kilodalton) subunits; (b) a form (Q(1)) containing a beta-doublet subunit; and (c) a form (Q(0)) that appeared to contain a beta-singlet subunit. Several lines of evidence suggested that the different forms occur under physiological conditions. Q(2) was purified to apparent electrophoretic homogeneity; Q(1) and Q(0) were highly purified, but not to homogeneity. The distribution of the PFP forms from red (versus green) tomato was: Q(2), 29% (90%); Q(1), 47% (6%); and Q(0), 24% (4%). The major difference distinguishing the red from the green tomato enzymes was the fructose-2,6-bisphosphate (Fru-2,6-P(2))-induced change in K(m) for fructose-6-phosphate (Fru-6-P), the ;green forms' showing markedly enhanced affinity on activation (K(m) decrease of 7-9-fold) and the ;red forms' showing either little change (Q(0), Q(1)) or a relatively small (2.5-fold) affinity increase (Q(2)). The results extend our earlier findings with carrot root to another tissue and indicate that forms of PFP showing low or no affinity increase for Fru 6-P on activation by Fru-2,6-P(2) (here Q(1) and Q(0)) are associated with sugar storage, whereas the classical form (Q(2)), which shows a pronounced affinity increase, is more important for starch storage.