Characteristics of [D-Trp]-somatostatin-sensitive B50 phosphorylation

Inhibition of the phosphorylation of the synaptic plasma membrane (SPM) protein B50 by D-Trp⁸]-somatostatin in vitro is time-dependent. Increasing the time of incubation of hippocampal synaptic plasma membranes with the peptide from 15 sec to 30 min prior to addition of 7.5 μM γ-³²Ps]ATP results in a complete reduction of B50 phosphorylation. Incubation of synaptic plasma membranes for 30 min in the absence of peptide does not alter basal B50 phosphorylation. Neither ACTH nor β-endorphin produces similar effects—inhibition of B50 phosphorylation by ACTH is maximal at 15 sec and β-endorphin produces only a small inhibition, even after 30 min. D-Trp⁸]-somatostatin is not activating a membrane-bound protease, since maximal inhibition of B50 phosphorylation by the peptide is seen in the presence of leupeptin or bacitracin. Hippocampal synaptic plasma membranes contain protein phosphatase activity. Assays of B50 phosphorylation in synaptic plasma membranes done under conditions that favor either net phosphorylation or dephosphorylation are consistent with inhibition of protein phosphatase activity by D-Trp⁸]-somatostatin. This evidence suggests that D-Trp⁸]-somatostatin interacts with SPM binding sites in the hippocampus, which may alter the activity of an endogenous protein phosphatase to determine the degree of B50 phosphorylation.