Characterization of ATP-Dependent Fructose 6-Phosphate 1-Phosphotransferase Isozymes from Leaf and Endosperm Tissues of Ricinus communis

Plastid and cytosolic isozymes of ATP:fructose 6-phosphate 1-phosphotransferase ($\text{PFK}_{\text{p}}$ and $\text{PFK}_{\text{c}}$, respectively) have been isolated from leaves and developing endosperm tissues of the castor oil plant (Ricinus communis L). Endosperm $\text{PFK}_{\text{p}}$ has been purified to apparent homogeneity. Polyclonal antibodies raised against one of the four polypeptides associated with potato tuber PFK (molecular mass, 46 kilodaltons) immunoprecipitated developing endosperm and leaf $\text{PFK}_{\text{p}}$, but not $\text{PFK}_{\text{c}}$ isozymes. Western blots, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and analytical gel filtration show that $\text{PFK}_{\text{p}}$ from developing endosperm is a 220 kilodalton homotetramer composed of 57 kilodalton subunits. Kinetic studies of leaf $\text{PFK}_{\text{p}}$ and $\text{PFK}_{\text{c}}$ isozymes reveal both similarities and differences to the characteristics of the respective endosperm isozymes studied previously (WJ Garland, DT Dennis [1980] Arch Biochem Biophys 204: 302-317). The immunological and kinetic data suggest that leaf and developing endosperm $\text{PFK}_{\text{p}}$ are different but structurally related proteins.