![]() ![]() The absorbed Hyp-containing oligopeptides are distributed to various tissues and finally excreted into the urine 6, 23, 24. ![]() Therefore, although orally ingested collagen hydrolysate is sequentially digested by gastrointestinal enzymes, some parts remain in the peptide form containing Hyp (~20–30% of total Hyp) 20 and are transported into the blood mainly via peptide transporters in the intestinal brush-border membrane 21, 22. Hydroxylation of Pro residues confers high resistance to peptidase/protease digestion of peptides 19. Pro residues at the Yaa position are mostly hydroxylated to 4Hyp, accounting for ~100 residues per 1000 amino acid residues in collagen 18. Prolyl hydroxylation occurs uniquely in collagenous Gly-Xaa-Yaa repeat sequences (Xaa and Yaa are any amino acids). The common feature of those collagen-derived oligopeptides detected in the blood is the presence of 4Hyp within the sequence. The collagen-derived bioactive oligopeptides possessing high oral bioavailability are possibly the major contributing factor to the beneficial effects of collagen hydrolysate ingestion on bone 14, joint 15, skin 16, and other targets 17, which have been reported by many groups, particularly in recent years. These peptides have various biological activities, such as stimulating the growth of skin fibroblasts 3, 8, promoting osteoblast differentiation 9, 10, and chemotactic activity on skin fibroblasts, tenocytes, and peripheral blood neutrophils 11, 12, 13. In addition, many types of oligopeptides, including Xaa-4Hyp (Xaa = any amino acids), Xaa-4Hyp-Gly, and Gly-Pro-4Hyp, have also been detected in the blood 4, 6. The most major collagen-derived peptide in the blood is prolyl-4-hydroxyproline (Pro-4Hyp) 2, and the next is generally 4Hyp-Gly 3. ![]() The blood concentrations of collagen-derived oligopeptides reach µM levels, significantly higher than those of other food-derived peptides 7. However, recently, it has been revealed that various di- and tripeptides exist in the blood after oral ingestion of collagen hydrolysate 2, 3, 4, 5, 6. Most of the orally ingested proteins/peptides are hydrolyzed into amino acids in the gastrointestinal tract and blood. The oral bioavailability of bioactive peptides released from food proteins is important to exert their bioactivity in vivo 1. These results highlight the specific nature of the Gly-3Hyp-4Hyp tripeptide and its potential for health promotion and disease treatment. Furthermore, Gly-3Hyp-4Hyp showed chemotactic activity on skin fibroblasts and promoted osteoblast differentiation. In mice, oral administration of collagen hydrolysate prepared from bovine tendon, which contains a higher amount of 3Hyp, further increased blood Gly-3Hyp-4Hyp levels compared to that from bovine skin. Additionally, Gly-3Hyp-4Hyp uniquely maintained the maximum concentration until 4 h after the ingestion due to its exceptionally high resistance to peptidase/protease demonstrated by incubation with mouse plasma. Here, we identified Gly-3Hyp-4Hyp tripeptide in human blood at high concentrations, comparable to other 4Hyp-containing oligopeptides, after ingesting porcine skin collagen hydrolysate. In contrast, no study investigates the fate of another collagen-specific but minor amino acid, 3Hyp. There are increasing reports demonstrating high bioavailability of 4-hydroxyproline (4Hyp)-containing oligopeptides after oral ingestion of collagen hydrolysate and their bioactivity.
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