Generation of SP

SP in intestinal lumen of animals comes from degradation of dietary protein or endogenous protein. FAA and op (containing 2-6 aa residues) were produced by protease, endopeptidase and exopeptidase; OP was further hydrolyzed by oligopeptidase to produce FAA and sp.

Absorption mechanism of SP in monogastric animals

There may be three kinds of transport systems for SP: 1) active transport systems that depend on the concentration of H + or Ca2 + and need to consume ATP (Matthews, 1987; vincenzini et al., 1989), which will be inhibited under the condition of hypoxia or the addition of metabolic inhibitors; 2) na + / H + exchange transport systems that are H + dependent and non energy consuming. Daniel et al. (1994) believed that the driving force of SP transport came from the electrochemical gradient of protons. The activity of Na + / H + transport channels in the tip cells of the brush border of the small intestine results in the activity of protons. When SP enters the mucosal cells in the way of facilitation and diffusion, it will increase the H + concentration in the mucosal cells, activate the Na + / H + transport channel, release H +, and restore the H + concentration in the cells to the original level. In the absence of H + concentration, SP absorption depends on the extracellular substrate concentration; when the extracellular H + concentration is higher than that in the cell, it reverses the substrate concentration transport through the co transport system of electricity generation; 3) glutathione (GSH) transport system which may have special physiological significance (vincenzini et al., 1989).

Absorption mechanism of SP in ruminant

The mesenteric and non mesenteric pathways are two forms of SP absorption in ruminants (Webb; 1993). Substances absorbed by jejunum, ileum, cecum and colon enter the mesenteric system; substances absorbed by rumen, reticulum, omasum, abomasum and duodenum enter the non mesentery.

It was found that non mesenteric system was the main way for ruminants to absorb SP, while mesenteric system was the main way for ruminants to absorb FAA (dirienzo, 1990). When using rumen and omasum epithelial cells to study the absorption characteristics of SP, it is found that the absorption of SP by both of them is an unsaturated passive diffusion process; the absorption capacity of SP by the latter is stronger than that of the former (Matthews, 1991).

3 characteristics of SP absorption

After comparing the absorption characteristics of SP and FAA, the researchers considered that SP and FAA are independent and do not interfere with each other, and their transport mechanism in the small intestine is totally different (Adibi et al., 1968; Adibi, 1971; Cheng et al., 1971; buston et al., 1972). Compared with FAA, SP absorption is mainly in two aspects.

Fast speed, low energy consumption, difficult to saturate carrier and avoid competition between AA absorption

Compared with FAA's absorption pathway, SP's absorption speed is fast, energy consumption is low and carrier is not easy to saturate (GANAPATHY et al., 1985; Rerat et al., 1988). Le Guowei (1997) perfused chicken duodenum with a mixture of SP casein and FAA. At 10 minutes, the contents of some peptides and total peptides in portal vein blood circulation in casein group were significantly higher than those in FAA group. Compared with FAA, SP absorption is not only rapid, but also high in absorption rate and intensity. In addition to methionine, SP in the portal vein was faster than that in the corresponding FAA mixture, and the absorption peak was higher; when the perfusion amount was increased, the difference of AA absorption rate between the two sources was further increased.

Hara et al. (1984) reported that the AA absorption intensity of egg protease hydrolysate in rats was 70% - 80% higher than that of FAA. Daniel et al. (1994) suggested that the absorption capacity of SP carrier may be higher than that of AA carrier. Zhao Xinhong et al. (1998) perfused the duodenum of weaned piglets with glycyl-l-lysine dipeptide solution and the mixture of the two FAAS. The results showed that the two perfusates changed the absorption of endogenous AA in different degrees: the latter significantly reduced the amount of glutamate, valine and histidine in the hepatic portal vein; while the former significantly reduced the absorption of most AA compared with the blank group and glycine The absorption of the mixture with lysine is high.

SP itself can promote the absorption of AA or aa residues

Bamba (1993) found that SP not only increased the activity of aminopeptidase and dipeptidase, but also increased the number of SP carriers. Brandsch et al. (1994) reported that under physiological conditions, endorphins from casein hydrolysis in jejunum could increase the kinetic constant km and the maximum absorption rate Vmax of L-leucine into intestinal wall cells.

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