The form, quantity and proportion of protein absorbed into circulation after digestion are not clear. General studies have shown that SP, after entering intestinal cells, will be completely or partially hydrolyzed by dipeptidase and tripeptidase in intestinal cells, and then enter the blood circulation in the form of SP or FAA.

Under normal physiological conditions, most of the SP entering the small intestine cells has been rapidly hydrolyzed to FAA before leaving the serosa (Daniel, 1994); some physiological and pharmacological tests have found that some complete SP can enter the blood circulation (inui et al., 1992; Bronk et al., 1993). For example, it can be observed that diglyceptides can be transported completely from small intestine to blood circulation system by isotope tracing technique (Adibi, 1971). 70% of AA in the blood of bovine portal vein is in the form of peptide (Koeln et al., 1983);

scientists from Newcastle University found that most AA in the mesentery of sheep fed with fish meal is in the form of peptide. Gardner (1975) found that 36% of FAA absorbed from the small intestine to the serous membrane of rats came from peptides; while sleisenger et al. (1977) showed that the amount of FAA absorbed from the plasma of guinea pigs was lower than that of the perfused guinea pigs, which might be related to the existence of the form of peptide absorption;

after the intestinal perfusion of casein hydrolysate of chicken, the content of pbaa in the blood of portal vein increased, and the same as that of the perfusate appeared The content of peptide peak in plasma was significantly higher than that in FAA fed chickens (Le Guowei, 1996) when SP was infused in non anesthetized condition; the content of plasma peptide in portal vein of liver was much higher than that in starving and non nitrogen fed chickens (Wang Yantao, et al., 1997);

these two studies showed that OP and SP in protein digestive products of chicken intestine could be absorbed into circulation completely. Seal et al. (19911993) also believed that peptide could be absorbed completely into circulation, but the change of peptide content in circulation was not detected by blood sampling at 30 and 60 minutes after injection in rats perfused with diglycopeptide and polymethionine.

In this regard, Kee et al. (1993) analyzed that the complete absorption of peptides is not only related to the concentration of intestinal peptide, but also related to the clearance rate of peptides in vivo. The half-life of some peptides, such as cysteine AA peptide, diglycotide and glycyl tyrosine, is short.

In addition, the amount of different absorption substrates entering into the circulation is also different. The type of AA in the circulating pbaa may have a special relationship with the composition of the absorbed intact peptide AA. In some experiments, it has also been confirmed that some synthesized peptides can enter into the circulation in a complete form, but the amount of entry varies according to the composition and configuration of the peptide AA and the hydrolysis ability of peptide bond antipeptidase (Webb et al., 19921993).

SP can be completely absorbed into the blood, for example, diglyceride can appear in the intestinal capsule mucosa of small intestine eversion in rats, but the significance of the phenomenon of SP directly absorbed into the blood from the gastrointestinal tract is still unclear. There are two main reasons for this. First of all, there is no effective method to determine and identify SP species and quantity in body fluids; second, it is due to the differences between animals. The main animals used for SP absorption in laboratory are rats, rabbits and humans.

It may be different for some other animals with special anatomical features and protuberances, especially for ruminants with microbial fermentation function (Le Guowei, 1995).

The effect of op on the contents and types of peptides in circulation and its mechanism are not clear. Gel filtration chromatography showed that the peptides of animal portal vein were mostly 3~5 peptides (Schlagheck, 1984).

The molecular weight of blood peptides in calves was mostly between 500 and 1500 (Koeln, 1993). Matthews (1991) reported that the absorption of two peptides by rumen and true gastric epithelium was linearly related to their concentration in mucosa. Webb et al. (1993) suggested that this unsaturated form of absorption may be carried by osmosis rather than by peptide carrier, that is, the complete absorption of OP may not be related to sp carrier channel absorption.

At present, it has been found that there is an independent energy demand OP transport system, which can drive the transport of 4-5 peptides by ATP or related high-energy substances. It has been confirmed that the transport system has two ATP binding proteins, two intact membrane proteins and one substrate binding protein (Kunji et al., 1993);

Tynkkynen et al., 1993); however, no OP carrier has been found in animals. Simons et al. (1987) and Rerat et al. (1992) perfused the hydrolysates of protein under the condition of adding carbohydrates to supply energy. It was found that the absorption rate of OP was faster than that of FAA, which seemed to show the characteristics of the absorption of OP in animals

Kusaka et al. (1986) suggested that there may be multiple peptide transport systems in chicken intestine, but it is not clear whether there is a new OP carrier in chicken intestine.

The absorption of some FAA by animals can also change the content of peptides in the circulation. Le Guowei (1996) found that 45.9% and 52.6% of the total AA were pbaa in the portal vein of high concentration and low concentration FAA perfusion groups respectively;

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