Functions and absorption of bioactive peptides in small intestine

It is well known that a healthy diet plays an important role in disease prevention or modulation. For this reason, food scientists have researched physiological activities of food compounds, in particular, bioactive peptides from food proteins, which can exert positive physiological responses in the body upon their basic nutritional compositions in the provision of nitrogen and essential amino acids. It has been demonstrated that bioactive peptides are essential in the prevention of lifestyle-related diseases such as hypertension, antioxidation, and inflammation. Thus far, many peptides with various bioactive functions have been discovered and identified. It was known that peptides generally consisting of 2 to 9 amino acids may elicit bioactivities. Among them, small peptides showing antihypertensive activity by angiotensin-converting enzyme (ACE) inhibition, renin inhibition, and calcium channel blocking effects are in common.

The source of food-derived bioactive peptides is mainly from dietary proteins (milk, meat, egg, and soybean). So far reported, Sipola et al. demonstrated that a long-term administration (12 weeks) of peptides (Ile-Pro-Pro and Val-Pro-Pro) or sour milk containing both tripeptides to 12- and 20-wk spontaneously hypertensive rats (SHR) resulted in a significant decrease in systolic blood pressure (SBP) of 12 or 17 mmHg, respectively. A dipeptide, Val-Tyr, from sardine muscle hydrolysate, showed a significant clinical antihypertensive effect in mild hypertensive subjects. Trp-His and His-Arg-Trp were reported to block L-type Ca2+ channels. Vallabha et al. identified peptides including Leu-Ile, Leu-Ile-Val, Leu-Ile-Val-Thr, and Leu-Ile-Val-Thr-Gln from soybean hydrolysate with ACE inhibitory activity. A series of oligopeptides Phe-Asp-Ser-Gly-Pro-Ala-Gly-Val-Leu and Asn–GlyPro-Leu-Gln-Ala-Gly-Gln-Pro-Gly-Glu-Arg from squid; Asp-Ser-Gly Val-Thr, Ile-Glu-Ala-Glu-Gly-Glu, Asp-Ala-Gln-Glu-Lys-Leu-Glu, Glu-GluLeu-Asp-Asn-Ala-Leu-Asn, and Val-Pro-Ser-Ile-Asp-Asp-Gln-Glu-Glu-Leu-Met in hydrolysates produced from porcine myofibrillar proteins were found to have antioxidant activity. Other reported peptides were also demonstrated to have physiological activities in preventing lifestyle-related diseases, as summarized in Table 1-1.

Table 1-1. Reported physiological functions of peptides from food proteins

It was believed that dietary proteins were completely hydrolyzed into their constituent amino acids, and then absorbed into the blood via specific amino acid transport systems until the report by Newey and Smyth, who provided the first convincing evidence that dipeptides could be absorbed in intact form. After that, some researchers have reported that a proton-coupled peptide transporter 1 (PepT1) was found to be expressed in the brush border membrane of the small intestine, which plays a role in the intestinal absorption of di-/tripeptides. PepT1 is composed of 708 amino acids. At intestinal epithelial cells, some small peptides (di-/tripeptides) can be transported across the membrane in the intact form with the help of PepT1 transporter, others are hydrolyzed to free amino acids by peptidases in the gut intestinal tract and/or plasma, and released into the portal circulation via the amino acid transporter located in the intestinal basolateral membrane.  The early work by Boullin et al. pointed out the absorption of six dipeptides (Gly-Gly, Gly-D-Phe, Gly-Phe, Gly-Pro, Pro-Gly, and carnosine (β-Ala-His)) in their intact forms into the rat blood stream. There are some evidence on the bioavailability of bioactive peptides such as Val-Tyr and Pro-Gly in humans, and Trp-His in rats. The detection of lactotripeptides, Ile-Pro-Pro and Val-Pro-Pro, in human after oral administration suggests the resistance of the tripeptides to protease digestion. However, there were few reports on the relationship between di-/tripeptide absorption and PepT1 expression, the exceptional report by Jappar et al., who demonstrated that fasting caused a significant upregulation of PepT1 in the small intestine, leading to a significant increase in vivo pharmacokinetics of a model dipeptide glycyl-sarcosine (Gly-Sar) in wild-type and Pept1 knockout mice. Additionally, the intestinal PepT1 was reported to alter the expression during the developmental stages in rats and chicks. In vitro studies reported that oligopeptides could be transported across the brush border membrane (Figure 1). Recently, some reports demonstrated that an ACE inhibitory pentapeptide as Gln-Ile-Gly-LeuPhe and an octapeptide as Gly-Ala-Hyp-Gly-Leu-Hyp-Gly-Pro derived from egg white and chicken collagen were passively transported across Caco-2 cell monolayers through tight-junction (TJ)-mediated passive route. A series of oligopeptides such as Arg-Val-Pro-Ser-Leu, LysVal-Leu-Pro-Val-Pro, and Gly-Gly-Tyr-Arg were demonstrated to be possibly transported via TJ route, along with the reduction in blood pressure in hypertensive rats after orally administered. The aforementioned results strongly implied, bioactive peptides must be absorbed intact without protease hydrolysis in order to reach their target organs intact and exert their biological effects.  Recently, one study by Hanh et al. demonstrated for the first time that tripeptide Gly-Sar-Sar, tetrapeptide GlySar-Sar-Sar, and pentapeptide Gly-Sar-Sar-Sar-Sar could be absorbed in vivo, and in a peptide length-dependent manner in the descending order of Gly-SarSar > Gly-Sar-Sar-Sar > Gly-Sar-Sar-Sar-Sar. Furthermore, no metabolites of Gly-Sar-Sar, Gly-Sar-Sar-Sar, and Gly-Sar-Sar-Sar-Sar were observed in the blood samples, indicating that these peptides remained stable during the in vivo absorption into the blood at > 0.4 nmol/mL-plasma levels (Figure 1).

Reference:

1. Vu Thi Hanh, Weilin Shen, Mitsuru Tanaka, Aino Siltari Korpela, Toshiro Matsui (2017). Effect of Aging on the asorption of snall peptides in spontaneously hypertensive rats. Journal of Agricultural and Foos Chemistry, 65 (29), 5935-5943.

2. Weilin Shen and  Toshiro Matsui (2017). Current knowledge of intestinal absorption of bioactive peptides. Food and Function, 8, 4306-4314.

3. Lourdes Amigo and Blanca Hernández-Ledesma (2020). Current Evidence on the Bioavailability of Food Bioactive Peptides. Molecules, 25, 1-36