Antibodies That Distinguish between the Serine-158 Phospho- and Dephospho-Form of Spinach Leaf Sucrose-Phosphate Synthase

Serum antibodies were raised against a synthetic peptide corresponding to the amino acid sequence surrounding the major inactivating phosphorylation site (serine-158) of spinach (Spinacia oleracea) leaf sucrose-phosphate synthase (SPS). The anti-peptide antibodies precipitated highly activated SPS preferentially to ATP-inactivated SPS and interacted only weakly with the sodium dodecyl sulfate-denatured enzyme bound to a membrane. The antibodies blocked phosphorylation but not dephosphorylation of SPS. Highly activated SPS was not entirely dephosphorylated and ATP-inactivated SPS was not completely phosphorylated on serine-158, as indicated by the sensitivities of immunopurified serine-158 phospho- and dephospho-SPS to inhibition by inorganic phosphate. The anti-peptide antibodies can be used to detect changes in the phosphorylation state of serine-158, and they are useful to purify and characterize distinct kinetic forms of SPS.