title: Plasma native and peptidase-derivable Met-enkephalin responses to restraint stress in rats. Adaptation to repeated restraint.
abstract: Met-enkephalin and related proenkephalin A-derived peptides circulate in plasma at picomolar concentration as free, native pentapeptide and at nanomolar concentration in cryptic forms. We have optimized conditions for measurement of immunoreactive Met-enkephalin in plasma and for generation by trypsin and carboxypeptidase B of much greater amounts of total peptidase-derivable Met-enkephalin in plasma of rats, dogs, and humans. Free Met-enkephalin (11 pM) is constituted by native pentapeptide and its sulfoxide. Characterization of plasma total Met-enkephalin derived by peptidic hydrolysis revealed a small amount (38 pM) of Met-enkephalin associated with peptides of molecular mass less than 30,000 D, and probably derived from proenkephalin A, but much larger amounts of Met-enkephalin associated with albumin (1.2 nM) and with a globulin-sized protein (2.8 nM). Thus, plasma protein precursors for peptidase-derivable Met-enkephalin differ structurally and chemically from proenkephalin A. Met-enkephalin generated from plasma by peptidic hydrolysis showed naloxone-reversible bioactivity comparable to synthetic Met-enkephalin. Prolonged exposure of adult, male rats to restraint stress produced biphasic plasma responses, with peaks occurring at 30 s and 30 min in both free native and total peptidase-derivable Met-enkephalin. Repeated daily exposure to this 30-min stress resulted in adaptive loss of responses of both forms to acute restraint. Initial plasma responses of Met-enkephalin paralleled those of epinephrine and norepinephrine, but subsequently showed divergence of response. In conclusion, Met-enkephalin circulates in several forms, some of which may be derived from proteins other than proenkephalin A, and plasma levels of both free native, and peptidase-derivable Met-enkephalin are modulated physiologically.
authors.affiliation: Department of Medicine, University of Kentucky, Lexington.
authors.forename: K
authors.initials: K
authors.lastname: Pierzchala
authors.forename: G R
authors.initials: GR
authors.lastname: Van Loon
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D043424:Carboxypeptidase B; D002268:Carboxypeptidases / Q000494:pharmacology; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D004744:Enkephalin, Methionine / Q000097:blood* / Q000502:physiology; D004837:Epinephrine / Q000097:blood; D006801:Humans; D008297:Male; D008970:Molecular Weight; D011863:Radioimmunoassay; D051381:Rats; D011919:Rats, Inbred Strains; D012149:Restraint, Physical; D014357:Trypsin / Q000494:pharmacology
chemical_list: D004744:Enkephalin, Methionine; D002268:Carboxypeptidases; D043424:Carboxypeptidase B; D014357:Trypsin; D004837:Epinephrine