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Metabolism and Enzymology

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

Evidence for Change during Ripening

Department of Plant Biology, Genetics and Plant Biology Building, University of California, Berkeley, California 94720

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 Q₂) containing - (66 kilodalton) and - (60 kilodalton) subunits; (b) a form (Q₁) containing a -doublet subunit; and (c) a form (Q₀) that appeared to contain a -singlet subunit. Several lines of evidence suggested that the different forms occur under physiological conditions. Q₂ was purified to apparent electrophoretic homogeneity; Q₁ and Q₀ were highly purified, but not to homogeneity. The distribution of the PFP forms from red (versus green) tomato was: Q₂, 29% (90%); Q₁, 47% (6%); and Q₀, 24% (4%). The major difference distinguishing the red from the green tomato enzymes was the fructose-2,6-bisphosphate (Fru-2,6-P₂)-induced change in K_m for fructose-6-phosphate (Fru-6-P), the `green forms' showing markedly enhanced affinity on activation (K<sub>m</sub> decrease of 7-9-fold) and the `red forms' showing either little change (Q₀, Q₁) or a relatively small (2.5-fold) affinity increase (Q₂). 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-P₂ (here Q₁ and Q₀) are associated with sugar storage, whereas the classical form (Q₂), which shows a pronounced affinity increase, is more important for starch storage.

³ Permanent address: Shanghai Institute of Plant Physiology, 300 Fonglin Road, Shanghai, People's Republic of China 200032.

¹ This work was supported by grants from the Cellular Biochemistry, U.S.-Hungary, and U.S.-China Programs of the National Science Foundation.