Biochemical and functional similarities between human eosinophil-derived neurotoxin and eosinophil cationic protein: homology with ribonuclease

Abstract

Eosinophil-derived neurotoxin (EDN) and eosinophil cationic protein (ECP) were isolated from lysates of human eosinophil granules by gel filtration and ion exchange chromatography on heparin-Sepharose. Radioimmunoassay, using monoclonal antibodies, of fractions from the heparin-Sepharose chromatography showed one peak of EDN activity and two peaks of ECP activity (termed ECP-1 and ECP-2). EDN, ECP-1, and ECP-2 each exhibited heterogeneity in charge and molecular weight when analyzed by two-dimensional nonequilibrium pH gradient electrophoresis and NaDodSO4/PAGE. Digestion of EDN with endoglycosidase F (endo F) decreased its molecular weight and charge heterogeneity. Thus, END likely contains a single complex oligosaccharide. Endo F digestion of ECP-1 and ECP-2 decreased the molecular weight of both polypeptides, indicating that both likely contain at least one complex oligosaccharide. Amino acid sequence analyses showed that ECP-1 and ECP-2 are identical from residue 1 through residue 59 and that the sequences of EDN and ECP are highly homologous (37 of 55 residues identical). Both EDN and ECP NH2-terminal sequences showed significant homology to RNase, especially in regions of the RNase molecule involved in ligand binding. EDN, ECP-1, and ECP-2 had neurotoxic activity, causing the Gordon phenomenon at doses down to 0.15 micrograms when injected into the cisterna magna; the proteins were comparable in their activities. These results indicate that EDN and ECP are related proteins and suggest that they derived from genes associated with the RNase family.