Plasma Protein(s) Yields Met-Enkephalin-Related Peptides in Near-Micromolar Concentrations when Treated with Pepsin*

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Abstract
Treatment of animal and human plasmas with pepsin yielded large quantities of immunoreactive methionine5-enkephalin (i-met-ENK). The concentrations measured after pepsin treatment were 0.1-0.5 .mu.M, about 1000 times the normal circulating level of i-met-ENK (0.03-0.3 nM). The reaction was shown to be time and pH dependent and to involve the action of pepsin on a protein(s) of about 65,000 mol wt. Pepsin-generated i-met-ENK from rat plasma gave three major peaks during reverse phase HPLC, one of which (.apprx. 25% of the total) coeluted with methionine5-enkephalin sulfoxide and also competed in a radioreceptor assay for opiate-related substances. In addition, this material produced met-ENK-like effects on vascular permeability in rat skin and inhibited electrically induced contractions of the isolated guinea-pig ileum in a naloxone-sensitive manner. The plasma substrate(s) that yielded i-met-ENK was distinguished from adrenal proenkephalins, since partially purified plasma substrate(s) did not liberate i-met-ENK upon digestion with trypsin and carboxypeptidase B. Although it is possible that these peptides differ from met-ENK in amino acid sequence, the results presented here suggest that met-ENK-related substances might be formed physiologically by the action of a pepsin-related processing enzyme(s) on plasma substrate(s). Such a mechanism would be analogous to that used in the renin-angiotensin system.