Radioimmunoassay of methionine enkephalin and interference by brain factor of immunoreactivity and opiate receptor binding activity.

We developed a radioimmunoassay of methionine-enkephalin (met-enk) which was found to be highly specific to the peptide and 1 pmol was the detection limit. Interference by the following peptides was practically negligible; leucine-enkephalin, LAP- or CNBr-treated met-end, alpha- and beta-endorphins. Morphine and its congeners were totally inert. Measurement of met-enk in rat brain P2 fraction revealed besides met-enk, the presence of the factor which cross-reacted with the antiserum and also appeared to bind with the brain opiate receptor. Though the nature of the factor has not yet been characterized, it is suggested that for qualitative assessment of the tissue level of met-enk, a method to selectively inactivate either the peptide or the factor should be developed first.