Enzyme immunoassay for methionine-enkephalin sulfoxide

1. An immunoassay technique employing an enzyme-labelled ligand was developed for the sulfoxide derivative of the opioid peptide methionine-enkephalin (MOE). 2. The sensitivity of the enzyme immunoassay (EIA) was approximately twofold higher than that achieved in the radio-immunoassay (RIA) using tritiated ligand. Cross-reactivities of leucine-enkephalin and beta-endorphin with the EIA were less than 0.1%, while that with Gly-Gly-Phe-Met and oxidized Gly-Gly-Phe-Met were 2.5% and 10.2%, respectively. 3. These results indicate that the EIA is a useful alternative to the RIA for the assay of small peptides.