In the previous sharing, we analyzed the intestinal physiological characteristics of weaned piglets and proposed the "three external forces and three internal forces" strategy for formulating non-anti-weaning piglets.
The internal forces can be started by promoting the proliferation and differentiation of intestinal epithelial cells (promoting epithelial development), enhancing the tight junction among intestinal epithelial cells (reducing permeability), and enhancing the anti-inflammatory and antioxidant capacity of intestinal cells. Everyone must be concerned about which substances could improved effects on these three forces, and I hope you will find some answers in article of today.
We have a new understanding of the structure and function of many substances with the rapid development of animal nutrition research technology. For example, the nutrition of protein was only regarded as the nutrition of amino acid at first, and later it was confirmed that peptide was indispensable in protein nutrition.
At first, amino acids were only regarded as the raw materials of synthesis of proteins (nutritional effects). Later on, a large number of amino acids (such as arginine, histidine, valine, leucine, etc.) were found to have functional effects. Similarly, the functional role of peptides is well known today and is widely used in humans and animals.
Some friends' told it’s still not clear on peptide-related issues such as:
How is the peptide absorbed by animals?
Can the peptides taken in the diet be absorbed in a complete form (because structure determines function, biologically active peptides basically need to be absorbed in a complete form to perform their functions)?
What are the functions and amino acid composition of biologically active peptides?
Articles of today will answer these questions.
Regarding how the peptide is absorbed by the intestinal tract of the animal, we must start with the digestion of protein and the absorption of digested substances. It can be seen that the absorption of peptides in intestinal of animals has obvious advantages over absorption of amino acids, and the structural characteristics required for peptides to be absorbed in a complete form without been degraded by the digestive enzymes.
1. Digestion of protein
Digestion of gastric juice in the stomach
Protein digestion starts from the stomach. The gastric glands secrete gastric juice (pH about 0.9-1.5) containing pepsinogen and gastric acid. Pepsinogen is activated by the action of stomach acid and itself. After gastric acid denatures the protein, pepsin hydrolyzes the water-soluble protein into medium and long peptide chains. The main part of the action of pepsin is the peptide bond formed by aromatic amino acids or acidic amino acids.
Digestion of pancreatic juice in the intestine
The pancreatic juice secreted by the pancreas contains endopeptidases and exopeptidases.
Endopeptidases, including trypsin and chymotrypsin (ie, chymotrypsin), exist in the form of inactive zymogens and are secreted into the intestinal lumen and activated by enterokinase, active trypsin, and tissue fluid. Trypsin mainly hydrolyzes peptide bonds formed by basic amino acids such as lysine and arginine; chymotrypsin mainly hydrolyzes peptide bonds formed by aromatic amino acids.
Exopeptidases are mainly carboxypeptidase A and carboxypeptidase B, carboxypeptidase A hydrolyzes the peptide bond formed by the neutral amino acid at the carboxy terminals, and carboxypeptidase B hydrolyzes the carboxy terminals to basic amino acids such as lysine and arginine Peptide bonds formed.
The products produced after pancreatic juice are amino acids and oligopeptides (2-3 AA), which can be absorbed by intestinal epithelial cells.
Digestion in intestinal epithelial cells
After being absorbed into the intestinal epithelial cells, the oligopeptide is hydrolyzed into amino acids by the action of aminopeptidase and carboxypeptidase (hydrolyzing peptide bonds at the amino and carboxyl ends of the peptide chain, respectively).
Fermentation of the large intestine microorganisms
Organic nitrogen-containing substances that have not been digested and absorbed enter the large intestine and are fermented by microorganisms to produce ammonia and amines, which will change the pH and osmotic pressure of the intestine, break the balance of intestinal microorganisms, which are not conducive to intestinal health.
2. Absorption of products produced from protein digestion
Amino acid absorption
There are 4 ways of absorption of amino acids: endocytosis absorption, osmotic absorption, absorption through gap among cells, and active transport absorption across cells depending on the carrier (note that the last one is the main way of amino acid absorption in the intestine). Amino acid transporters of intestinal epithelial cells include neutral, acidic, basic, and special amino acid transporters. They have the characteristics of slow absorption, easy saturation, and high energy consumption. Each carrier has a different rate of amino acid transfer:
the neutral transport system transports aromatic or aliphatic amino acids such as sulfur-containing amino acids, histidine, and glutamine, which is the fastest;
the alkaline transport system transports lysine and refined amino acids Amino acid, which is relatively slow, only 10% of the transport rate of neutral amino acid carriers;
the acid transport system transports aspartic acid and glutamic acid at the slowest rate (Broer et al., 2008).
20-97% of the amino acids transported into the intestinal epithelial cells (different amino acids obtains different proportions) are directly decomposed and utilized by the epithelial cells (Wu et al,. 2014).
Peptide absorption
There are also 4 absorption channels of peptides: transcytosis, passive trans cellular diffusion, tight junction, and PepT1 (note that the latter two are the main ways for peptide absorption in the intestine).
The characteristics of PepT1 carrier channel: transport oligopeptides composed of 2-3 AA, non-selective to the peptide structure, with the characteristics of fast absorption speed, not easy to saturate, low energy consumption and so on. Due to the abundant expression and high transport efficiency of PepT1 in intestinal epithelial cells, its transport capacity is much higher than that of amino acid transporters. When the 2-3 peptide is absorbed into the intestinal epithelial cells, it will be further hydrolyzed into amino acids under the action of intracellular peptidase. Most of the amino acids are directly decomposed and utilized by the intestinal epithelial cells, and a small amount is transported into the blood circulation.
The characteristics of tight junction channel: mainly transport 3-6 AA peptides that are soluble in water (especially high transport efficiency for negatively charged peptides), and do not consume energy (depending on concentration gradient). When these peptides pass through the cell gap, they enter the blood circulation in a complete form (Xu et al. 2019).
3. Can the peptide added in the diet resist to the degradation of digestive enzymes in the gastrointestinal tract?
Whether the peptide added in the diet can resist the degradation of gastrointestinal protease depends on the molecular weight and amino acid composition of the peptide. Study found that the peptides that can resist the degradation of gastrointestinal proteases have the following characteristics: they do not contain the above-mentioned sites of trypsin, chymotrypsin, carboxypeptidase A and carboxypeptidase B.
It’s clear that 2-3 peptides are mainly absorbed through the transcellular pathway; 3-6 peptides are mainly absorbed through the gap between cells; while longer-chain peptides are difficult to be absorbed in a complete form. Therefore, bioactive peptides composed of more than 6 amino acids are more difficult to function through dietary addition. Even if the requirements of 2-6 amino acids are met, the efficiency of peptide absorption will be affected by the following factors:
The amino acid composition of the peptide
The transport efficiency of peptides is mainly related to the polarity (hydrophilicity) and charging properties of the C-terminal or N-terminal amino acids. Studies have shown that the transport efficiency of polar amino acids at the C-terminals (that is, hydrophilic amino acids) is better than that of non-polar amino acids (hydrophobic amino acids); if both C-terminals are polar amino acids, the transport efficiency of uncharged amino acids is better than that of charged of amino acids.
When the N-terminal is a non-polar amino acid (that is, a hydrophobic amino acid such as cystine, leucine, methionine, proline, valine, or isoleucine), the transport efficiency is higher than that of a polar amino acid (that is, hydrophilic amino acids).
Charge ability and hydrophilicity of peptides
PepT1 vectors preferentially transport neutral 2-3 peptides, followed by charged 2-3 peptides. Studies have also shown that peptides with different charging properties or different hydrophilicities can be absorbed in different ways.
Negatively charged and hydrophilic peptides are absorbed through the gaps among cells, and longer positively charged and hydrophobic peptides are absorbed by endocytosis.
Stability of peptides (ability to resist degradation of digestive tract enzymes)
The lower the molecular weight, the lower the probability of degradation and the higher the absorption efficiency. For example, a peptide composed of 2-3 AA, or the peptide chain does not contain the site of intestinal digestive enzymes (trypsin, chymotrypsin, carboxypeptidase A and carboxypeptidase B), the probability of being degraded is reduced, and the absorption efficiency improve.
What are the functions and amino acid composition of biologically active peptides?
Study found that biologically active peptides have biological functions such as promoting cell proliferation and differentiation, antioxidant, anti-inflammatory, anti-microbial, blood pressure lowering, anti-cancer, and chelating trace elements. For farmed animals, promoting cell proliferation and differentiation, anti-inflammatory, anti-oxidation, etc. has an important role in the health of the body, it is worthy of using in feed. Peptide acquisition methods include:
synthesis (very expensive, only used in human medicine or animal experiments);
natural protein degradation (moderate cost, widely used in human health products or animal production)
Animal or plant protein raw materials are rich in biologically active peptide fragments, which can be released by in vitro enzymatic hydrolysis.
Here, I compiled the structure and function of the bioactive peptides obtained by enzymatic hydrolysis of conventional plant protein raw materials.
Bioactive peptides that promote cell proliferation and differentiation
Antioxidant bioactive peptides
Anti-inflammatory bioactive peptides
Bibliography
1. Broer. Amino Acid Transport Across Mammalian Intestinal and Renal Epithelia. The American Physiological Society, 2008: 249-286.
2. Wu GY, Bazer FW, Dai ZL, Li DF, et. al. Amino Acid Nutrition in Animals: Protein Synthesis and Beyond. The Annual Review of Animal Biosciences, 2014: 387-411.
3. Xu QB, Hong H, Wu JP, Yan XH. Bioavailability of bioactive peptides derived from food proteins across the intestinal epithelial membrane: A review Trends in Food Science & Technology, 2019(86): 399-411.
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