These studies suggest that the presence of peptides may be of great significance for the absorption of AA, nutritional supply and protein metabolism of intestinal tissue in animals, especially in newborn animals.

The different effects of peptides and FAA on protein turnover may be related to the different metabolic pathways in vivo. Shibata et al. (1991a, b) found that tryptophan in SP was not easy to enter the degradation pathway of nicotinamide synthesis, but more likely to participate in the pathway of protein synthesis.

When rats were fed FAA mixture, SP or complete protein diet, the conversion efficiency of tryptophan and nicotinamide in vivo was the lowest in SP diet; Snell (1989) also observed this phenomenon in the metabolism of microorganisms.

In addition, it was also found that diglyceptides were more likely to enter protein synthesis than glycine (krizysik et al., 1979);

The uptake of glycyl and leucine in skeletal muscle was higher than that in perfused free form (lochs et al., 1986, 1988); however, Cruz et al. (1988) cultured in vitro with free leucine, the uptake of leucine in tissue increased with the increase of leucine concentration in the culture medium, while the uptake of leucine in tissue cultured in l-tyrosyl-leucine did not The concentration of acyl leucine changed.

Like FAA, the basic function of peptide is to provide nitrogen source for protein metabolism. Adibi et al. (1977) showed that peptides entering the circulation could be hydrolyzed to FAA for tissue utilization;

Backwell (1994) found that the injected peptide marker can directly enter into the milk protein, indicating that the tissue itself has the ability to directly use the AA in the peptide to synthesize the milk protein.

In addition, some peptides may play a direct role in promoting protein synthesis.

Noguchi et al. (19811988) extracted acid soluble peptide rich in glutamic acid or glutamine and glycine from rat muscle, and Nam et al. (l990) found that the excretion of leucine and valine from acid peptide was highly related to protein synthesis and insulin-like growth factor content in urine, which confirmed that some acid peptides in blood circulation might have a strong effect on protein synthesis It plays a catalytic role. Wang et al;

Le Guowei (1996) showed that there was a certain correlation between the total content of peptides and the content of some peptides in the blood circulation of chickens and the protein synthesis rate of tissues;

Shi Yonghui et al. (1996) added a small amount of macromolecular casein hydrolysate to the diet, which changed the SP content and the type and quantity of macromolecular peptide in the plasma of laying hens. The results showed that the egg production rate, egg production and feed conversion efficiency of laying hens were significantly improved.

These studies indicate that the types and contents of peptides in the blood circulation of animals are affected by the type, quality and quantity of protein in the diet on the one hand, and may affect the synthesis of protein in the tissues of animals on the other hand.

Other biological functions of dietary peptide

Some peptides produced by protein hydrolysis can not only be used as the donors of AA, but also have special physiological functions. They can act on the target glands in some form and promote the secretion of hormones or enzymes. For example, a variety of active peptides isolated from the hydrolysates of milk protein in vivo and in vitro can participate in the regulation of nervous and immune functions of the body and promote the proliferation and growth of cells;
Peptides extracted from egg proteins can promote DNA synthesis and cell growth (Azuma et al., 1989)

Many common proteins can also form active peptides under the hydrolysis of intestinal enzymes.

For example, zioudrou et al. (1979) and brantl et al. (1979) first isolated tyrphine from β - casein hydrolysate; petrelli et al. (1984) and brantl (1985) found that β - casein mostly produced tyrphine composed of 7-10 aa residues during gastrointestinal digestion, and its aa sequence was similar to that of N-terminal of endogenous opioid peptide (Morley, 1981; 1982);

The further purified seven peptide (Tyr Pro Phe Pro Gly Pro ILE) and the tetrapeptide (Tyr Pro Phe Pro) generated from the seven peptide also have opioid peptide activity in vitro. At present, it has been known that there are also opioid peptides in the pepsin hydrolysate of wheat gluten.

These opioid peptide active peptides can enter the circulation through complete absorption and play a physiological role as neurotransmitters.

The bioactive peptides released from dietary proteins play an important role in cellular and humoral immunity. Peptides produced by β - casein can promote phagocytosis of macrophages (jolle et al., 19811982; Kishino et al., 1984; Moriguchi et al., 1985; gattegno et al., 1988; Yamauchi et al., 1993), and also can promote the proliferation of mature lymphocytes and immature spleen cells in rats (Coste et al., 1992);

Otani (1992) found that the macromolecular glycopeptide (106-169 peptide) produced by digestion of bovine κ - casein and chylosin can inhibit the proliferation of mouse spleen B cells by lipopolysaccharide (LPS).
Otani et al. (1993) reported that peptides derived from the hydrolysates of bovine α 31 casein, β casein and κ - casein from streptomycin can inhibit the proliferation of mouse spleen cells, while some peptides produced by the hydrolysis of intact κ - casein and its trypsin can weaken the immune response of new living things (Otani et al., 1995).

In addition to casein, other proteins, such as lactoferrin and peptide of soybean protein, also have immune activity. For example, lactoferrin and its hydrolytic released peptide with high concentration and immune activity in human and rat milk have immune protection effect on the intestine of newborn animals (Paul Eugene, 1993).

Adding a small amount of β endorphin or other peptide products to the diet can significantly improve the production performance of animals (morodent et al., 1980; carnie et al., 1989; pansini et al., 1989).

The egg production rate, egg production and plasma iron and zinc content of laying hens were significantly increased by adding a small amount of hydrolyzed casein products (Shi Yonghui et al., 1996). This may be related to the promotion of digestion and absorption of dietary nutrients by peptides, such as some peptides of casein hydrolysate can promote the secretion of CCK in rat intestinal cells (cube, 19891990)).

Bioactive peptides play an important role in the regulation of animal immune status and production performance, and constitute an important aspect of protein nutritional value, which puts forward a new topic for traditional protein nutrition research.

9 summary

SP absorption plays an important role in the absorption of protein digested products, which makes the absorption of most aa residues in peptides faster than that of FAA.

The release of feed protein peptide may not only affect the absorption of AA and protein metabolism, but also participate in the regulation of physiological function in the form of bioactive peptide. The nutritional value evaluation of proteins should not only consider the composition of AA, but also their activity.

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