Biochemical properties of the eosinophil cationic protein and demonstration of its biosynthesis in vitro in marrow cells from patients with an eosinophilia.

The eosinophil cationic protein (ECP), which has been shown to be secreted both in vitro and in vivo, is a cytotoxic unique constituent of eosinophil granules. To increase the understanding of the mechanisms behind the role of the eosinophil as a cytotoxic effector in disease, a detailed biochemical characterization of ECP was performed. A considerable molecular heterogeneity was revealed when purified ECP was eluted isocratically from a high-resolution cation exchange resin; the separation, reproducibly achieved, of five components was probably due to hydrophobic interaction with the resin. These polypeptides, which reacted quantitatively with anti-ECP antiserum, showed molecular weights (mol wt) of 19,500 and 16,700 and showed almost identical amino acid compositions. The amino-terminal sequence for one of the polypeptides was (in the standard one-letter code) (R-P-X-Q-F-T-R-A-Q-W-F-A-I-Q-H-I-S-L-N-P-R-R-C-T-I-A-M-R-A-I-N-N-Y-). The biosynthesis of ECP was demonstrated in marrow cells from patients with eosinophilia using labeling with (14C)-leucine, followed by immunoprecipitation with anti-ECP, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and fluorography for visualization of labeled ECP. Biosynthesis was demonstrated of mol wt 22,000 ECP, which may represent precursor ECP, since with time some of it was processed into ECP with a mol wt of 18,000 to 19,000. Monensin, a proton ionophore, blocked the processing of mol wt 22,000 ECP. This study shows that ECP consists of a family of similar polypeptides. These may, however, have different biological activities.