ELECTROSPRAY-IONIZATION MASS-SPECTROMETRIC ANALYSES OF PHOSPHOLIPIDS FROM RAT AND HUMAN PANCREATIC-ISLETS AND SUBCELLULAR MEMBRANES - COMPARISON TO OTHER TISSUES AND IMPLICATIONS FOR MEMBRANE-FUSION IN INSULINEXOCYTOSIS

Glucose-induced insulin secretion from pancreatic islets involves hydr olysis of arachidonic acid from phospholipids as an intermediary event . Accumulation of nonesterified arachidonate in islet membranes may in fluence both ion fluxes that trigger insulin secretion and fusion of s ecretory granule and plasma membranes. Recent findings indicate that p lasmenylethanolamine species may also participate in fusion of such me mbranes, but high-performance liquid chromatographic (HPLC) and gas ch romatographic/ mass spectrometric (GC/MS) analyses of islet secretory granule phospholipids suggested that they contain little plasmenyletha nolamine. Here, electrospray ionization mass spectrometry (ESI/MS) of intact phospholipid molecules is used to demonstrate that the most pro minent components of all major glycerophospholipid headgroup classes i n islets are arachidonate-containing species. Such species contribute the majority of the ESI/MS negative ion current from rat and human isl et glycerophosphoethanolamine (GPE), and the fraction of GPE negative ion current contributed by plasmenylethanolamine species in rat islets is higher than that for rat Liver or heart and similar to that for br ain. The most prominent sn-2 substituent of plasmenylethanolamine spec ies in brain is docosahexaenoate and in islets is arachidonate. Arachi donate-containing plasmenylethanolamine species are also prominent com ponents of GPE from islet secretory granules and plasma membranes. Fus ion of islet secretory granule and plasma membranes is demonstrated to be catalyzed by cytosolic components from insulinoma cells and rat br ain with chromatographic similarities to a rabbit brain factor that sp ecifically catalyzes fusion of plasmenylethanolamine-containing membra nes.