Identiflcation of a Proenkephalin Precursor in Striatal Tissue

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Abstract
Recent studies have supported the suggestion that proenkephalin is the same in both adrenal medulla and brain. However, although previous investigations have characterized enkephalin‐containing adrenal intermediates derived from proenkephalin, as yet no such intermediates have been isolated from the brain. This has led to the belief that the processing of proenkephalin in the brain is extremely rapid and enkephalin‐containing intermediates do not accumulate. In this investigation Sephacryl‐300 gel filtration chromatography of guinea pig striata, extracted in 8 M urea, demonstrated several peaks of both bioactive and immunoreactive enkephalin‐like peptides after enzymatic digest (trypsin followed by carboxypeptidase B). Comparable profiles were obtained using rat and bovine striatal tissue. In guinea pig the major species emerging from gel filtration, eluting with an apparent molecular weight of 29,000, represented approximately 9% of the total (methionine) enkephalin immunoreactivity. It had an apparent pI of 5.0 when subjected to chromatofocusing. This species was further characterized using sodium dodecyl sulphate‐polyacrylamide gel electrophoresis and nitrocellulose blotting techniques as well as highly specific radioimmunoassays to (Met5)‐enkephalin, (Leu5)‐enkephalin, and (Met5)‐enkephalin‐Arg6‐Phe7. This species was found to contain these opioid peptides in an approximately 6:1:1 ratio, respectively, and to have an apparent molecular weight of 31,000. It was also indicated that (Met5)‐enkephalin‐Arg6‐Phe7 constituted the C‐terminal seven residues of this molecule.