The formation of inositol 1,2-cyclic phosphate on agonist stimulation of phosphoinositide breakdown in mouse pancreatic minilobules. Evidence for direct phosphodiesteratic cleavage of phosphatidylinositol

It is generally thought that formation of inositol 1,2-cyclic phosphate (IcP) on agonist-stimulated “breakdown” of endogenous phosphatidylinositol in intact cells would provide strong evidence for the direct phosphodiesteratic cleavage of phosphatidylinositol. We report here that on ionophoresis of extracts of pancreatic minilobules incubated with the cholecystokinin/pancreozymin congener, caerulein, the usual inositol phosphates, i.e. inositol 1-phosphate (IP), inositol 4,5-bisphosphate (IP2), and inositol 1,4,5-trisphosphate (IP3) were seen. In addition, an [3H]inositol-labeled unknown was present with the correct electrophoretic mobility of IcP. There was only a trace of “IcP” in the unstimulated pancreatic minilobules. Several lines of evidence indicate that the unknown peak was IcP. 1) It ran on ionophoresis with standard [14C]IcP, and the ratio of 3H to 14C for each point on the peak was a constant within experimental error. 2) The putative IcP peak which had been eluted from the electropherogram also coincided with standard [14C]IcP on paper chromatography. 3) On mild acid hydrolysis in the presence of standard 14C-labeled IP, the putative [3H] IcP peak disappeared and appeared in the exact position of the standard [14C]IP peak, as to be predicted of IcP. The formation of IcP on agonist stimulation supports direct phosphodiesteratic cleavage of phosphatidylinositol on stimulation of phosphoinositide breakdown in pancreatic minilobules.