Some of the food-derived tripeptides with angiotensin converting enzyme (ACE)-inhibitory activity have been reported to be hypotensive after being orally administered. The mechanism for the intestinal transport of these tripeptides was studied by using monolayer-cultured human intestinal Caco-2 cells which express many enterocyte-like functions including the peptide transporter (PepT1)-mediated transport system. Val-Pro-Pro, an ACE-inhibitory peptide from fermented milk, was used as a model tripeptide. A significant amount of intact Val-Pro-Pro was transported across the Caco-2 cell monolayer. This transport was hardly inhibited by a competitive substrate for PepT1. Since no intact Val-Pro-Pro was detected in the cells, Val-Pro-Pro apically taken by Caco-2 cells via PepT1 was likely to have been quickly hydrolyzed by intracellular peptidases, producing free Val and Pro. These findings suggest that PepT1-mediated transport was not involved in the transepithelial transport of intact Val-Pro-Pro. Paracellular diffusion is suggested to have been the main mechanism for the transport of intact Val-Pro-Pro across the Caco-2 cell monolayer.

Val-Pro-Pro (VPP) and Ile-Pro-Pro (IPP) are antihypertensive tripeptides isolated from milk fermented with Lactobacillus helveticus and inhibit angiotensin-converting enzyme (ACE). We investigated whether these peptides were generated from beta-casein by digestive enzymes and whether they were resistant to enzymatic hydrolysis, using an in vitro model. VPP and IPP were not generated from beta-casein by gastrointestinal enzymes; instead, a number of longer peptides including VPP and IPP sequences were detected. The fermentation step would therefore be necessary to produce these antihypertensive tripeptides. VPP and IPP themselves were hardly digested by digestive enzymes, suggesting that orally administered VPP and IPP remain intact in the intestine, retaining their activity until adsorption. The present study also demonstrated that various functional peptide sequences in beta-casein were resistant to gastrointestinal enzymes. There may be a strong correlation between the resistance of peptides to gastrointestinal digestion and their real physiological effects after oral administration.

Arphamenine A, an Arg-Phe analog without a peptide bond, has been reported to be a possible inhibitor of the peptide transporter [H. Daniel and S.A. Adibi, FASEB J., 8, 753 (1994)]. The present study demonstrated that arphamenine A is not a real inhibitor, but is a substrate for the transporter, being transported transepithelially under a proton gradient. The substrate specificity of the peptide transporter was wider than expected.