Binding Characteristics of a Monoclonal β‐Endorphin Antibody Recognizing the N‐Terminus of Opioid Peptides

The present paper describes the isolation and characterization of a clone of hybrid myelomas (3-E7) secreting a mouse monoclonal antibody to beta-endorphin. An examination of its specificity against a series of human beta-lipotropin fragments and other opioid peptides revealed that the N-terminus portion of beta-endorphin is the determinant. Complete or almost complete cross-reactivity was obtained to methionine- and leucine-enkephalin, beta-lipotropin 60-65, and BAM 22; partial cross-reactivity was seen to dynorphin1-13 and alpha-neo-endorphin, whereas beta-lipotropin, alpha-N-acetyl-beta-endorphin, Des-Tyr1-beta-endorphin, in addition to a series of synthetic enkephalin derivatives, completely lacked cross-reactivity. The use of the monoclonal antibody in radioimmunoassay (RIA) for beta-endorphin resulted in a lower sensitivity related to respective polyclonal antibodies. An increase of 100% in tracer binding could, however, be obtained by use of beta-endorphin iodinated with its N-terminal tyrosine protected by coupling to an antibody. A solid-phase RIA was developed involving the internally 3H-labeled monoclonal antibody, which resulted in a 10-fold increase in sensitivity as compared with the homogenous RIA. These data indicate that for the binding to this antibody a tyrosine residue in position 61 is essential, and it thus recognizes a site that is of functional significance for many naturally occurring opioid peptides.