ANTIBODIES THAT DISTINGUISH BETWEEN THE SERINE-158 PHOSPHO-FORM 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 phospho rylation site (serine-158) of spinach (Spinacia oleracea) leaf sucrose -phosphate synthase (SPS). The anti-peptide antibodies precipitated hi ghly activated SPS preferentially to ATP-inactivated SPS and interacte d only weakly with the sodium dodecyl sulfate-denatured enzyme bound t o a membrane. The antibodies blocked phosphorylation but not dephospho rylation of SPS. Highly activated SPS was not entirely dephosphorylate d and ATP-inactivated SPS was not completely phosphorylated on serine- 158, as indicated by the sensitivities of immunopurified serine-158 ph ospho- and dephospho-SPS to inhibition by inorganic phosphate. The ant i-peptide antibodies can be used to detect changes in the phosphorylat ion state of serine-158, and they are useful to purify and characteriz e distinct kinetic forms of SPS.