Shin-ichi Fukudome

Opioid peptides derived from wheat gluten: Their isolation and characterization

Four opioid peptides were isolated from the enzymatic digest of wheat gluten. Their structures were Gly‐Tyr‐Tyr‐Pro‐Thr, Gly‐Tyr‐Tyr‐Pro,Tyr‐Gly‐Gly‐Trp‐Leu and Tyr‐Gly‐Gly‐Trp, which were named gluten exorphins A5, A4, B5 and B4, respectively. The gluten exorphin A5 sequence was found at 15 sites in the primary structure of the high molecular weight glutenin and was highly specific for δ‐receptors. The structure—activity relationships of gluten exorphins A were unique in that the presence of Gly at their N‐termini increased their activities. Gluten exorphin B5, which corresponds to [Trp4,Leu5]enkephalin, showed the most potent activity among these peptides. Its IC50 values were 0.05 μM and 0.017 μM, respectively, on the GPI and the MVD assays.

Gluten exorphin C. A novel opioid peptide derived from wheat gluten.

A novel opioid peptide, Tyr‐Pro‐Ile‐Ser‐Leu, was isolated from the pepsin‐trypsin‐chymotrypsin digest of wheat gluten. Its IC50 values were 40 μM and 13.5 μM in the GPI and MVD assays, respectively. This peptide was named gluten exorphin C. Gluten exorphin C had a structure quite different from any of the endogenous and exogenous opioid peptides ever reported in that the N‐terminal Tyr was the only aromatic amino acid. The analogs containing Tyr‐Pro‐X‐Ser‐Leu were synthesized to study its structure‐activity relationship. Peptides in which X was an aromatic amino acid or an aliphatic hydrophobic amino acid had opioid activity.

Release of opioid peptides, gluten exorphins by the action of pancreatic elastase

The release of opioid peptides, gluten exorphins A, which have been isolated from the pepsin–thermolysin digest of wheat gluten, with gastrointestinal proteases was examined. High levels of gluten exorphin A5 (Gly–Tyr–Tyr–Pro–Thr) immunoreactive materials were detected in the pepsin–pancreatic elastase digest by a competitive ELISA. From this digest, gluten exorphin A5, B5 and B4 were isolated. This means that these peptides are released in the gastrointestinal tracts after ingestion of wheat gluten. The yield of gluten exorphin A5 in the pepsin–elastase digest was larger than that in the pepsin–thermolysin digest. The gluten exorphin A5 sequence is found 15 times in the primary structure of the high molecular weight glutenin. The region from which gluten exorphin A5 was released by the action of pancreatic elastase was identified using synthetic fragment peptides.

Effect of gluten exorphins a5 and b5 on the postprandial plasma insulin level in conscious rats

The effect of exogenous opioid peptides, gluten exorphins A5 and B5, which were isolated from the enzymatic digest of wheat gluten, on the postprandial insulin level were examined in rats. The oral administration of gluten exorphin A5 at a dose of 30 mg/kg w. potentiated the postprandial plasma insulin level and the effect was reversed by naloxone. The administration of gluten exorphin B5 showed a similar effect at a higher dose (300 mg/kg w). Furthermore, intravenous administration of gluten exorphin A5 at a dose of 30 mg/kg w. also stimulated the postprandial insulin release. The fact that orally and intravenously administered gluten exorphin A5 stimulates insulin release suggests that it modulates pancreatic endocrine function by the action after the absorption rather than within the the gastrointestinal tract.