Effect Of Casein Antioxidant Peptide Structure On The Bioavailability And Investigation Of The Mechanism Of Transepithelial Transports

Bioactive peptides have been proved to be potential for applications as health-promoting agents against numerous human health disease conditions, and they are becoming the research focus in the field of functional foods. However, to exert their biological activity, these bioactive peptides have to cross the gastrointestinal barrier, and reach the circulation and target sites in an active form. There are some distinctive features of peptides can influence their transepithelial transport and the in vivo bioavailabilities. Although many researchs about peptide transport have been carried out, the correlations between peptide structural characteristics (molecular weight, charge property, hydrophobicity et al.) and absorption are still not completely understood due to the complex characteristics of peptide transport. Therefore, further studies to correlate the structure with transport are essential. These researchs not only can improve the knowledge of peptide absorption, but also can promote the application of peptides in the field of functional foods. Based on the previous studies, casein peptides were used here to further investigate the effect of peptide structure on their transepithelial transport and bioavailability using an in vitro cell absorption model. The main results were summarized as follows:1. In the present study, the human colon carcinoma Caco-2 cells were cultured as an absorption model. To evaluate the model integrity and permeability, the transepithelial electrical resistance (TEER) and the apparent permeability coefficients of fluorescein sodium and propranolol were assayed. The results indicated that the Caco-2 cell model cultured in our lab was valid to conduct the experiment of peptide transport.2. Peptides, with molecular weight between 500-1600 Da, were mainly transported by paracellular route; however, the PepTl mediated transport was responsible for peptides with molecular weight< 500 Da. Peptidase hydrolysis results revealed that brush-border peptidases (BBPs) and some other peptidases were responsible for peptide degradation in the paracellular route; while the BBPs were the main peptidases involved in the PepTl transport. Therefore, PepTl mediated transport is more efficient than the paracellular route. As for the bioavailability, peptides with molecular weight< 1000 Da showed high transport rate and residual ratios of antioxidant activity and exerted excellent bioavailability between 11% and 16%.3. The uncharged fraction showed higher transport rate and residual ratios of antioxidant activity than that of charged fractions, indicating excellent bioavailability. Moreover, the negatively charged peptide fraction showed higher bioavailability than positively charged ones. For intestinal absorption, the negatively charged fractions were mainly transported via the PepTl and paracellular routes; while the positively charged fractions were mainly transported through PepTl and transcytosis.4. In terms of hydrophobicity, the high hydrophobic peptides had higher antioxidant activity than that of weak hydrophobic and hydrophilic peptides. However, the weak hydrophobic peptides showed excellent transport rate and residual ratios of antioxidant activity. Therefore, the excessive and too low hydrophobicity diminished peptide bioavailability. Only peptides with weak hydrophobicity can exert high bioavailability. For intestinal absorption, peptide fraction with the weakest hydrophobicity was mainly transported via the PepTl and paracellular route; while the other hydrophobic peptide fractions were mainly transported through PepTl and transcytosis.5. In the study of selectivity of transcytosis for peptide structure, we found that the positively charged (≥2) and hydrophobic peptides were easily transported by transcytosis. When peptides were negatively charged (≥2) and had both acidic and basic amino acids in their sequence, they were easily transported only by paracellular route. Peptides, with less charge (≤2), were transported by both paracellular route and transcytosis. Peptide hydrophobicity correlated well with the caveolin-mediated transcytosis and clathrin-and caveolin-independent transcytosis. However, a weak correlation was observed between peptide hydrophobicity and clathrin-mediated transcytosis.