High-affinity binding sites for 125I-labelled pancreatic secretory phospholipase A 2 in rat brain

Abstract

Porcine pancreatic secretory phospholipase A 2 (ppsPLA 2) has been shown to modulate agonist and antagonist binding to α-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) receptors and to effect neurotransmission in the central nervous system (CNS). To further elucidate the mechanism of action of ppsPLA 2 in the CNS, the binding profile of 125I-labelled ppsPLA 2 to rat whole-brain membranes was assessed. Two classes of calcium-dependent binding sites were detected using unlabelled ppsPLA 2 as a displacer with IC 50 values of 3 and 217 nM. Similar values were obtained for [ 125I]ppsPLA 2 binding to membranes prepared from isolated cortical and hippocampal rat brain regions. [ 125I]ppsPLA 2 binding displayed bell-shaped concentration-dependence curves to Ca 2+, Zn 2+ and pH. Binding was not inhibited by AMPA, the false substrate, oleoyloxyethyl phosphocholine (OOPC), or by BSA-galactose or wheat germ agglutinin. [ 125I]ppsPLA 2 binding was reduced by treatment of the rat brain membranes with mercaptoethanol and proteinase K treatment or by their pre-incubation at 95°C. These results show a different binding profile to the previously characterised snake venom sPLA 2 N-type receptors and suggest the existence of novel class of sPLA 2 N-type binding sites.