SUCROSE-PHOSPHATE SYNTHASE PHOSPHATASE, A TYPE-2A PROTEIN PHOSPHATASE, CHANGES ITS SENSITIVITY TOWARDS INHIBITION BY INORGANIC-PHOSPHATE INSPINACH LEAVES

The activity of a type 2A protein phosphatase from spinach leaves was monitored using phosphorylated sucrose-phosphate synthase (SPS) as a s ubstrate. After partial purification the overall activities of sucrose -phosphate synthase phosphatase (SPS-P) recovered from leaves harveste d in the dark and in the light did not vary. However, SPS-P preparatio ns from darkened leaves were more strongly inhibited by inorganic phos phate and certain phosphorylated compounds than preparations from illu minated or mannose fed leaves. We conclude, that activation of SPS inv olves an interconversion of multiple forms of SPS-P activity.