Mass spectrometric characterization of arachidonate-containing plasmalogens in human pancreatic islets and in rat islet .beta.-cells and subcellular membranes

Abstract

Pancreatic islets, when stimulated with D-glucose, secrete insulin by processes requiring glycolytic metabolism and generation of ATP. Hydrolysis of membrane phospholipids also occurs in glucose-stimulated islets, resulting in accumulation of nonesterified arachidonate, which facilitates Ca2+ entry and the rise in beta-cell [Ca2+] that triggers insulin secretion. Glucose-induced hydrolysis of arachidonate from islet phospholipids is mediated in part by an ATP-stimulated, Ca(2+)-independent (ASCI) phospholipase A2 (PLA2) which prefers plasmenyl over diacyl phospholipid substrates. Here we characterize the endogenous plasmalogen content of islet cells and subcellular membranes. Fast atom bombardment mass spectrometric analyses demonstrated that three of the most abundant molecular species of ethanolamine phospholipids in rat pancreatic islets were plasmalogens with sn-2 arachidonate residues and palmitic, oleic, or stearic aldehyde residues, respectively, in the sn-1 position. Purified populations of beta-cells prepared by fluorescence-activated cell sorting were also found to contain these plasmenylethanolamine molecular species in abundance similar to that in intact islets and greater than that in islet alpha-cells. Both islet plasma membranes (PM) and endoplasmic reticulum (ER) also contained these plasmenylethanolamine species, which accounted for 42% (PM) to 64% (ER) of the ethanolamine phospholipid arachidonate content of these membranes, as measured by stable isotope dilution mass spectrometry. Plasmenylethanolamine species were also abundant constituents of human pancreatic islets (accounting for 58% of their ethanolamine phospholipid arachidonate content) and were hydrolyzed more rapidly than diacyl ethanolamine phospholipid by human islet cytosolic ASCI-PLA2. Both secretagogue-induced eicosanoid release and insulin secretion from human islets were attenuated by an ASCI-PLA2 suicide substrate which sterically resembles plasmalogens. These observations are consistent with the hypotheses that islet beta-cell ASCI-PLA2-catalyzed hydrolysis of arachidonate from endogenous plasmenylethanolamine substrates may occur in membrane compartments which participate in regulation of the beta-cell cytosolic [Ca2+] and that this may be an intermediary biochemical event in the induction of insulin secretion.