Met-enkephalin [Arg6, Phe7] immunoreactivity in bovine caudate and bovine adrenal medulla

A radioimmunoassay specific for the COOH-terminus of Met-enkephalin [Arg6, Phe7] and a separate assay specific for the COOH-terminus of Met-enkephalin are described. Immunoreactivity by these two assays was compared in bovine caudate and bovine chromaffin granule preparation after Sephadex G75 chromatography in 50% acetic acid. When the assays were applied to the chromatography fractions of the bovine caudate extract, the majority of the immunoreactivity was found in the fractions corresponding to the heptapeptide and the pentapeptide respectively. When the chromaffin granule chromatography fractions were assayed, both of the radioimmunoassays showed that most reactivity was in several peaks in the larger molecular weight fractions. The major peak for the Met-enkephalin [Arg6, Phe7] assay had an apparent molecular weight of 2800, while with the Met-enkephalin assay the dominant peak of immunoreactivity had an apparent molecular weight of 10,000. The presence of authentic Met-enkephalin [Arg6, Phe7] in both caudate and chromaffin granule extracts was confirmed by reverse-phase chromatography of the previously sized fractions. It appears then that the processing of precursors of opioid peptides is directed, in the caudate, to the synthesis and storage of the enkephalins and of Met-enkephalin [Arg6, Phe7]; in the adrenal medulla the major products of precursor processing are a variety of polypeptides of larger sizes.