4 factors affecting SP absorption

It is speculated that the factors affecting SP absorption may include three aspects.

Animal factors, including animal species, physiological stage, age and intestinal site

As we all know, the absorption efficiency of AA is different in different intestinal parts of animals. For example, the results of the in vitro intestinal loop method of sheep small intestine showed that the absorption order of AA was different in different small intestine parts (William, 1969); Johns et al. (1973) reported that the absorption rate of lysine in sheep ileum was the highest. Phillips (1979) used perfusion method to study the absorption of AA in sheep's small intestine. It was confirmed that sheep's ileum was the main site of AA absorption, while middle intestine of mice and guinea pigs was the most active site of AA absorption (Baker et al., 1971). This may indicate that animal species play a decisive role in the absorption site of AA. In addition, the different affinity of AA to the carrier, the uneven distribution of the carrier in the intestinal tract, the intestinal environment, the diet and the physiological stage may lead to the different absorption of AA in different parts of the intestinal tract. There are few reports on the absorption site of peptide in intestine. However, it can be inferred from the absorption law of AA that the absorption of SP may also change according to the different intestinal parts, which was confirmed by the research of Zhao Xinhong et al. (1998).

SP itself factors, including peptide chain length, peptide structure, composition of AA in peptide, end of AA in peptide and peptide concentration, etc

Peptide structure plays a decisive role in whether peptide can pass through intestinal mucosa. It is pointed out that there are only transport systems carrying dipeptides and tripeptides in the intestine (craft et al., 1968; Adibi et al., 1975);

Addison et al., 1974a, b), so they can transport dipeptides and tripeptides completely; the large-scale transport of macropeptides in the intestine is very few (Adibi et al., 1977), and it is still controversial whether they can be absorbed completely.

In addition, the position of AA in the peptide affects its absorption, that is, the absorption rate of AA is different at the N-terminal or C-terminal. For example, dipeptides composed of lysine and glycine, the absorption rate of lysine at the N-terminal is higher than that at the C-terminal. In contrast, the absorption rate of L-lysine, a dipeptide composed of glutamic acid and lysine, at the C-terminal is higher than that at the N-terminal (Burston et al., 1972).

Other factors

Rubino et al. (1971) found that AA did not affect the transport of dipeptides and their precursors, while energy was the limiting factor for the inverse concentration gradient absorption of peptides (Addison et al., 19721975; Matthews et al., 1974; nutzenadel et al., 1976). Ganaparty et al. (1985) suggested that electrochemical proton might be the driving force of transport peptide; takuwa et al. (1985) found that there was a H + concentration gradient in the transport of peptide at the brush border membrane, and the driving force of the H + concentration was probably from the electrochemical proton energy supply; however, vincerzini (1989) reported that the absorption of glutathione (GSH) was related to the concentration gradient of Na +, K +, Li +, Ca2 + and Mn2 + Off, independent of H + concentration. In addition, studies at the tissue level have found that SP absorption is dependent or partially dependent on Na + (himukai et al., 1978; shoaf et al., 1980), but there are also studies that Na + is not needed for peptide absorption (bertroot et al., 1981; Cheesman et al., 1982), and Na + is only necessary for FAA released after peptide hydrolysis (himukai et al., 1980; bertroot et al., 1981; himukai et al., 1983) 。 At present, the understanding of peptide transport regulation is not enough, and further research is needed.

5. Types and sources of peptides in animal blood circulation

When animals are hungry or not, there are many kinds of peptides in their blood circulation.The content of peptides in portal vein plasma of cattle and sheep is as high as 70% - 80% (Koeln et al., 1993; seal and Parker, 1991; McCormick and Webb, 1982); the content of peptides in rat plasma is 10% - 52% (Asata et al., 1994; seal et al., 1991; galibois et al., 1991); there are also some peptides in pig and chicken plasma (blahovec et al., 1993). The content of low molecular peptide in blood circulation is very different, which may be related to the absorption state of animals, the influence of the type and content of protein digestion products (including OP and SP) and FAA on the content of peptide in blood circulation besides the different kinds of animals. Adibi et al. (1997) believed that the peptides in human and animal blood circulation mainly come from five sources: digestive tract absorption, body protein decomposition, body synthesis (such as peptide hormones and enkephalin and other physiologically active peptides), taking drugs with peptide structure and parenteral nutrition (such as subcutaneous, muscular, intravenous injection of AA solution containing SP), while the first is the most important source of peptides in blood circulation Way.

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