Views: 0 Author: Site Editor Publish Time: 2022-08-01 Origin: Site
With the in-depth study of protein and amino acid nutrition, people have gradually realized the importance of peptide nutrition. Protein degradation in the animal intestines must be to free amino acid or small molecular peptides can be absorbed by the intestines. The absorption of small molecular peptides in total fodder plays an important role in the absorption of protein. Its absorption use not only in the digestion, absorption and metabolism of amino acids play a crucial role, but also in maintaining the animal's intestinal health, the body's immune function is of great significance.
Absorption mechanisms of small molecule peptides
The protein digestion and absorption of traditional theory, the protein within the lumen, generated by trypsin and chymotrypsin role of free amino acids and oligopeptide (2 ~ 6 amino acid residues), oligopeptide under the action of peptide enzyme was hydrolyzed into free amino acid, and enter the blood circulation in the form of free amino acid, protein is amino acid nutrition. But as people to the research of protein digestion, absorption and metabolism of rules, the theory of protein digestion, absorption, gradually formed a new point of view, the protein within the lumen of the hydrolysis product, in addition to the amino acid, there are some small molecular peptide, and small molecular peptides and free amino acid is absorbed by the intestinal mucosa and transfer into the circulation of the blood.
Newey and Smith (1960) first presented a convincing argument that oligopeptides can be fully absorbed, and people gradually accepted the idea that peptides can be directly absorbed and utilized by animals. Since then, the transport mechanism of oligopeptides in animals has been studied extensively. These studies suggest that there may be a variety of oligopeptide transport systems in animals. The transport modes of oligopeptides in different animal species may be different, and the same animal species may have concentrated different transport modes of oligopeptides.
i. Absorption mechanism of small peptide in monogastric animals
The protein in the diet in the animal after A series of enzymes in the digestive tract, degradation into free amino acid (FAA) and oligopeptide (OP), one of the oligopeptides in the small intestine chorionic brush border by aminopeptidase A and aminopeptidase N, eventually to the free amino acid (FAA) and the form of small molecular peptide (SP) is absorbed by animals. The transport mechanism of small peptides may have the following three forms.
A pH dependent H+-Na+ exchange transport system that does not consume ATP.
Active transport processes, which depend on hydrogen or calcium concentrations, require ATP consumption, and this mode of transport is inhibited in the presence of hypoxia or the addition of metabolic inhibitors.
Glutathione (GSH) transport system. It was found that the transmembrane transport of GSH was related to the concentration gradients of Na+, K+, Li+, Ca2+ and Mn2+, but not to the concentration of H+.
ii. Absorption mechanisms of small molecule peptides in ruminants
Unlike monogastric animals, the absorption of small peptides in ruminants can be divided into mesenteric and non-mesenteric systems. Small molecule peptides absorbed by jejunum, colon, ileum and cecum enter the mesenteric system. However, small peptides absorbed by the rumen, reticulum, omomasum, abomasum and duodenum enter the non-mesenteric system. The non-mesenteric system is the main pathway of small molecule peptide absorption. The absorption of small peptides by ruminants occurs either in the form of passive diffusion or in the form of active transport mediated by carriers.
iii. Characterization of small peptide transporters
At present, at least five kinds of small molecule peptide transporters have been found in animals, among which PepT1 and PepT2 are the most widely studied vectors. Both PepT1 and PepT2 are members of the proton-dependent oligopeptide transporter (POT) family. PepT1 is a low affinity, high capacity peptide carrier, mainly expressed in the digestive tract, but also weakly expressed in the kidney. Pe pT2 is a high affinity, low capacity peptide vector, mainly expressed in the kidney. The factors affecting the transport of small molecule peptide transporters are as follows:
i). Nutrient level. Malnutrition will significantly affect the absorption of small peptides by animals.
ii). Insulin. Insulin promotes the transport of dipeptide by PepT1. Insulin mobilizes PepT1 stored in the cytoplasmic pool, thereby increasing the amount of PepT1 and thus the transport of dipeptide.
iii). The intracellular cAMP level can inhibit the transport of small peptides. Small molecule peptides combined with different carboxyl fatty acids can significantly increase the hydrophobicity of the substrate, thus increasing the affinity of PepT1. In addition, circular dipeptide can be transported more efficiently and stably than linear dipeptide.
Factors affecting the release and absorption of small peptides
i. Factors affecting the release of small peptides
i). The content and quality of dietary protein. The activity of intestinal brush border peptidase increased when animals were fed diets with high protein content. The activity of peptidase and the absorption of peptide were decreased when the diet was fed with low protein or no protein. During digestion, the quantity and proportion of small molecule peptides formed are also related to the quality of dietary protein. Protein with balanced amino acids can produce more small molecule peptides. Poor quality protein produces a large number of free amino acids and a small number of large molecular weight peptide fragments. Not only the proportion of amino acids in the diet affects the release of small peptides, but also the type of amino acids. It was found that the release of protein peptide was positively correlated with the available lysine.
ii). Processing and storage conditions. Processing and storage conditions are important factors affecting the release of small molecular peptides during protein digestion. It was found that the peptide release of soybean meal after heating or long-term storage was only 63% of that of fresh soybean meal with high available Lys content. The reason may be that Maillard reaction occurs, which makes the peptide bond between Lys residue and its adjacent amino acid residue difficult to break, thus affecting the digestibility of protein. In vitro hydrolysis experiments showed that the small molecule peptides released from steamed meat were less, while frozen, dried or fresh meat released more small molecule peptides.
iii). Protease in digestive tract. Protein ingested by animals is hydrolyzed by proteases in the digestive tract to produce peptides and free amino acids. Trypsin and hydroxypeptidase act only on peptide bonds composed of the carboxyl groups of lysine and arginine; Pepsin hydrolyzes several different peptide bonds and degrades most proteins into polypeptides and a few amino acids. In different intestinal segments, lumen and intestinal cells, the amount, relative proportion and activity of these enzymes are different, which affect the release of small-molecule peptides to different degrees.
ii. Factors affecting the absorption of small peptides
i). Protein quality in feed
During digestion, the amount and proportion of peptides formed are related to protein quality. The results show that protein can be easily hydrolyzed into oligopeptides with low molecular weight and large quantity in the diet with high balanced content of essential amino acids. But the essential amino acid deficiency, the imbalanced feed protein produces the quantity small, the molecular mass large peptide fragment. Animal proteolysis produces a higher proportion of small molecule peptides, while plant proteolysis produces more free amino acids, and the release of small molecule peptides is directly proportional to the content of available basic amino acids (such as lysine).
ii). Physical and chemical properties of small peptide
One is the length of the peptide chain. Current studies suggest that dipeptides and tripeptides can be fully absorbed, but whether oligopeptides above tripeptides can be fully absorbed remains controversial. Two is the configuration. In general, L-type is more easily absorbed than D-type, and neutral amino acid residue configuration is more easily absorbed than acidic and basic amino acid residue configuration peptide. The third is the affinity of small molecule peptide and carrier. Peptides composed of hydrophobic amino acids with large side chains are easier to absorb, while hydrophilic peptides with small charged molecules are more difficult to absorb.
iii). Feed processing and storage conditions
Processing and storage conditions are important factors affecting the proportion of small molecular peptides released to free amino acids during protein digestion. The Maillard reaction during protein processing and storage also has a certain effect on the absorption of small molecule peptides.
iv). The physiological state of an animal
The physiological state and metabolic changes of the animal can also affect the absorption of small peptides. Animals have different ability to utilize small molecule peptides at different growth stages. Age, health status and so on will affect the absorption and utilization of small molecule peptides.
v). Metabolic regulatory factor
Recent studies have shown that small peptides are not only substrates for protein metabolism, but also important physiological regulators. Such as growth hormone, stimulant and other regulatory factors can be directly as neurotransmitter, but also indirectly stimulate the secretion of intestinal receptor hormone or enzyme and play a role. Similarly, metabolic changes of growth hormones, stimulants and other regulatory factors will in turn affect the efficiency of protein utilization.
vi). Use of protective agents
Since there is non-mesenteric absorption of peptides in ruminants, and peptides can be used by rumen microorganisms and tissues, the absorption of small molecule peptides can be changed by adding protective agents. The rumen peptide concentration increased by 19.69% after adding sodium acetate in Hu sheep diet. When ionophore was added to goat diet, peptide protection in the rumen was increased by 20%, and the overall nitrogen digestibility was also significantly improved.
In recent years, great progress has been made in the study of oligopeptides. As mentioned above, the absorption mechanism of small peptides plays an important role in the utilization of proteins by animals. This mechanism allows the absorption of amino acids faster than the degradation to free amino acids for reabsorption, thus improving the utilization of protein in animals. In addition, some oligopeptides can be used as physiological active substances, which can be directly absorbed by animals and participate in physiological function and metabolic regulation of animals. Therefore, the absorption of small peptides is of great importance to animals.