Intracellular receptor for somatostatin in gastric mucosal cells: decomposition and reconstitution of somatostatin-stimulated phosphoprotein phosphatases.

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Abstract
Using 32P-labeled histone as exogenous substrate, a potent stimulatory effect of somatostatin was shown on cytosolic phosphoprotein phosphatases (PPPases; phosphoprotein phosphohydrolase, EC 3.1.3.16) in rat gastric mucosal cells. Partial purification of cytosolic fraction in DEAE-Sephadex ion-exchange chromatography, and further gel filtration on Sephadex G-75 and Sephadex G-100 separated somatostatin-dependent PPPases into 3 distinct molecular species. One corresponding to Mr [molecular ratio] 130,000 was devoid of any PPPase activity but specifically bound [Tyr1]somatostatin 125I-labeled on the Tyr ([125I-Tyr1]somatostatin) with an apparent equilibrium dissociation constant of 3 .times. 10-10 M. The 2 other molecular species corresponded to Mr 64,000 and 13,000. They produced catalytic dephosphorylation of 32P-labeled histone, but they were not sensitive to somatostatin and did not show any specific binding to radiolabeled hormone. Mixing of the larger with either of the 2 smaller molecular species resulted in concentration-dependent inhibition of PPPase activity. However this inhibition was reversed by increased concentrations of somatostatin, with the concentration for half-maximal reactivation being close to 0.1 nM. Somatostatin stimulation in reconstituted materials developed according to a rapid time course (t1/2 [half-life], < 5 s), consistent with that observed for binding of [125I-Tyr1]somatostatin. The presence of an intracellular somatostatin receptor in gastric mucosal cells is strongly suggested and this receptor is characterized as a PPPase regulatory subunit. Substrate dephosphorylation could be the primary event triggering physiological effects of somatostatin in stomach and perhaps other organs of the digestive tract.