Studying the molecular basis of presynaptic neurotoxicity of ammodytoxin C, a secretory phospholipase A(2) from the venom of Vipera a. ammodytes snake, we demonstrated the existence of two high-molecular. mass ammodytoxin C-binding proteins in porcine tissues, one in cerebral cortex and the other in liver. These proteins differ considerably in stability and Western blotting properties. However, as shown by immunological analysis and tandem mass spectrometry sequencing of several internal peptides derived from the purified receptors, both belong to secretory phospholipase A2 receptors of the M type, which are Ca2+-dependent multilectins homologous to the macrophage mannose receptor. Based on Southern blot analysis of genomic DNA and deglycosylation of the receptors, the difference between the two proteins most likely stems from the different posttranscriptional. and posttranslational modifications of a single gene product. Our findings raise the possibility that the M-type receptors for secretory phospholipases A., may display different physiological properties in different tissues.