Phosphoenolpyruvate in rat pancreatic islets: a possible intracellular trigger of insulin release?

The content of phosphoenolpyruvate (PEP) has been measured in isolated rat islets of Langerhans incubated in vitro. Islet PEP was higher in islets incubated with 16.7 mmol/1 glucose than in islets incubated with zero or 2.8 mmol/1 glucose. Islet PEP content was also increased in islet incubated with 5 mmol/1 D-glyceraldehyde. Mannoheptulose abolished the glucose-induced rise in PEP content but not that elicited by D-glyceraldehyde. These results are consistent with a role for PEP as an intracellular mediator or glucose- and glyceraldehyde-induced insulin release. The kinetics of pyruvate kinase in extracts of rat islets were studied. The maximal extractable activity was considerably higher than known rates of glycolytic flux. The Km values were found to be 0.16 mmol/1 for PEP and 0.5 mmol/1 for ADP. The control of islet PEP content and the possible role of PEP in insulin release are discussed.