Detection of methionine-enkephalin at the 10(-16) mole level.

A high performance radioimmunoassay for Met5-enkephalin has been developed, using two ways to circumvent the difficulties of obtaining such an assay for the opiate peptides. First, hemocyanin was used as the carrier for the antigen and secondly, a new iodination method for Met5-enkephalin was established, resulting in a [125I]probe with an intact methionyl residue. The Met5-enkephalin antibodies exhibited a very high affinity (Kd = 5 X 10(-12) M) and Met5-enkephalin could be detected at the 10(-16) mole level. The Met5-enkephalin content of individual cervical ganglia and individual cochlea was measured. We also measured the Met5-enkephalin content of brain from different areas of two rat strains: no significant differences were found.