Purification and characterization of an opioid antagonist from a peptic digest of bovine .KAPPA.-casein.

Abstract

A chloroform/methanol extract of a peptic digest of bovine K-casein had binding activity to opioid receptors of rat brain. The active peptide was isolated by reverse-phase liquid chromatography on ODS and CN columns. The structure of the peptide was Ser-Arg-Tyr-Pro-Ser-Tyr-OCH₃, which corresponds to the methyl ester of the 33 - 38th residues of κ-casein. The peptide was esterified during the extraction process, and the structure was confirmed by a chemical synthesis. The IC₅₀ value of the peptide in a radioreceptor assay in the presence of 1 HM [³H]naloxone was 15 μM. The peptide did not have opioid agonistic activities in any of the assay systems tested. In the guinea pig ileum and rabbit vas deferens preparations, however, the peptide antagonized with morphiceptin and dynorphin A-(1-13), respectively. The peptide did not antagonize with [Leu]enkephalin in the mouse vas deferens preparations. Therefore, the peptide was an antagonist selective for the μ- and κ-types of opioid receptors. The peptide was named casoxin.