Use of methionine-enkephalin sulfoxide and leucine-enkephalin radio-immunoassays for the measurement of enkephalins in the rat brain.

1. A radio-immunoassay (RIA) for methionine-enkephalin sulfoxide was developed and was used to measure methionine-enkephalin, after oxidation, in the rat brain. 2. The RIA was performed together with assays that use commercially available met- and leu-enkephalin antisera. 3. Evidence is presented here indicating that antisera raised against the native met-enkephalin peptide underestimate met-enkephalin content due to loss of immunoreactivity after oxidation of peptide. 4. These results show that a more accurate measurement of met-enkephalin may be obtained by using the met-enkephalin sulfoxide RIA. The cross-reactivity of met-enkephalin with the leu-enkephalin RIA was diminished following oxidation of peptide.