In animal nutrition, protein in feed usually refers to crude protein (CP), defined as the nitrogen in feed ingredients multiplied by 6.25, which includes true protein, non-protein nitrogen (NPN) and insoluble nitrogen. True protein (i.e., protein in the biochemical sense) consists of macromolecules that differ in size, shape, function, solubility and amino acid composition, and can be classified according to their three-dimensional structure and solubility, such as albumins, globulins, glutelins, prolamins, histones and fibrin (Orten and Neuhaus, 1975; Rodwell, 1985; Van Soest, 1994). NPN usually refers to the nitrogen-containing fraction remaining in the filtrate after true protein has been extracted with tungstic acid or trichloroacetic acid (Licitra et al., 1996), including low-molecular-weight compounds such as peptides, free amino acids, nucleic acids, amides, amines and ammonia nitrogen. In common feeds, true protein and NPN together account for only 65% of total feed nitrogen on average (Blethen et al., 1990). The remainder is insoluble nitrogen, which includes protein bound to intact starch granules in grains, most cell-wall-bound protein, a portion of chloroplast protein, and heat-denatured protein bound to neutral detergent fiber (NDF) (Van Soest, 1994).
1 Protein Digestion1.1 Degradation of Dietary Protein in the RumenAfter dietary protein enters the ruminant rumen, it mixes with endogenous protein (protein contained in animal saliva, sloughed epithelial cells and rumen microbial residues) and undergoes fermentation; under the action of protein-degrading enzymes of rumen microorganisms (bacteria, protozoa and fungi), it is degraded to release oligopeptides, amino acids and ammonia. Different microorganisms play different roles: (1) Bacteria participate in the process of feed protein degradation and synthesize bacterial protein, which forms the main body of microbial protein. When amino acid degradation and ammonia production exceed the requirements for bacterial protein synthesis, dietary protein is wasted. For many years, deamination was thought to be limited to the large group of bacteria capable of producing ammonia from protein and protein hydrolysis products. However, recent studies have shown that amino acid deamination results from the combined action of a large number of bacteria with low deaminase activity and a small number of bacteria with high deaminase activity (Wallace, 1996). (2) Protozoa are also active and play an important role in rumen protein degradation. Protozoa only engulf particulate matter (bacteria, fungi and small feed particles). Therefore, protozoa degrade insoluble feed protein (such as soybean meal and fish meal) more strongly than soluble protein (Jouany and Ushida, 1999); at the same time, this engulfment increases nitrogen turnover and consumption in the rumen. In addition, protozoa, like bacteria, have strong deamination activity, but protozoa cannot synthesize amino acids from ammonia (Nolan, 1993), so protozoa are net ammonia-producing microorganisms. Thus, although protozoa contribute greatly to rumen microbial biomass, their contribution to the amount of protein entering the duodenum is not proportional to their contribution to rumen microbial biomass. (3) Because fungi are present at very low levels in rumen fluid, their role in rumen protein degradation is generally considered negligible.The extent of dietary protein degradation in the rumen depends on how easily it is fermented and its retention time in the rumen. The retention time of roughage is relatively long, and very little roughage protein passes through the rumen without being degraded (Ørskov, 1992). Animal protein (such as fish meal) has only a small rapidly degraded fraction, so most of the protein passes through the rumen undegraded and reaches the abomasum and small intestine. Compared with the original feed, the amino acid composition of rumen-undegraded protein after rumen fermentation is somewhat altered. Ganev (1979) found that the amino acid composition of the undegraded protein of roughage differed considerably from that of the original material. Hvelplund and Hesselholt (1987) studied the amino acid composition of rumen-undegraded protein of seven concentrate feeds including soybean cake, and found that the contents of most amino acids did not differ significantly from those in the original feed, while the contents of Ile, Leu, Phe, Try, Thr and Val in the residue tended to increase compared with the feed ingredients. With respect to individual amino acids, the degradation of various amino acids in the rumen differs; for example, branched-chain amino acids (Leu, Ile) are resistant to rumen microbial degradation (Erasmus et al.,1994a), and the resistance of methionine to degradation differs among feeds (Tamminga, 1979).1.2 Synthesis of Rumen Microbial ProteinWhile degrading feed protein, rumen microorganisms also use volatile fatty acids (VFA), energy (ATP) and some oligopeptides, amino acids and ammonia (including endogenous ammonia) produced during feed fermentation to synthesize microbial protein. Finally, this microbial protein, together with feed rumen-undegraded protein (including intact rumen-undegraded protein and some small peptides), flows with the digesta into the abomasum and small intestine. This system of microbial protein synthesis and breakdown in the rumen not only enables ruminants to make extensive use of NPN, but also plays an important homeostatic control role in ruminant protein metabolism.Many factors affect rumen microbial protein synthesis: (1) Microbial protein synthesis in the rumen is affected by the energy-nitrogen balance and synchronous release. The synchrony of protein and carbohydrate digestion has a considerable effect on microbial protein synthesis. In vivo and in vitro experiments show that the rate of carbohydrate digestion is the main factor controlling the energy available for microbial growth (Hoover and Stokes, 1991). (2) Rumen microbial protein synthesis is also affected by the outflow rate of rumen digesta. Feed intake level, roughage proportion (Owens and Goersch, 1986), increased water intake caused by salt intake (Harrison et al., 1975; Hadjipanayiotou et al., 1982) and saliva secretion (Froetschel et al., 1989) can indirectly affect the amount of microbial protein entering the duodenum by influencing the outflow rate of rumen digesta. (3) For microbial protein synthesis, different nitrogen sources (peptides, amino acids and ammonia) have different effects. Russell et al. (1992) pointed out that fermentation of structural carbohydrates requires only ammonia as a nitrogen source, whereas microorganisms degrading non-structural carbohydrates grow faster when amino acids are supplied. Many reports suggest that rumen microorganisms preferentially use peptides, followed by amino acids (Cooper and Ling, 1985; Prins et al., 1979). But in fact, different bacterial species differ in their preference for peptides and amino acids (Ling and Armstead, 1995). (4) Branched-chain amino acids and ectopic organic acids also regulate microbial growth.Regarding the amino acid composition of rumen microbial protein, most researchers since the 1960s have consistently considered it to be constant. However, extensive experiments by Rulquin and Verite (1990) in France showed that the amino acid composition of rumen microbial protein varies, and subsequently scholars represented by Clark in the United States proposed in 1992 that the amino acid composition of rumen microorganisms is variable. Erasmus et al. (1994b) measured certain differences in the amino acid composition of rumen bacteria with different experimental diets, which may be caused by different microbial species entering the small intestine under different dietary conditions. Domestic scholars have reached similar conclusions (Wang Hongrong, 1998).1.3 Digestion of Protein in the Small IntestineThe digestion of protein in the ruminant small intestine is similar to that in monogastric livestock, but the sources of protein differ. The protein entering the ruminant small intestine includes microbial protein synthesized in the rumen, feed rumen-undegraded protein and endogenous protein of the digestive tract, which have different nutritional significance and roles: (1) Microbial protein synthesized in the rumen accounts for most of the protein flowing into the small intestine (Titgemeyer and Merchen, 1990) and is the main source of small intestinal protein; increasing microbial protein production is the main measure for increasing small intestinal protein. (2) Rumen-undegraded protein is an important component of small intestinal protein and is of great significance in production; adjusting feed rumen-undegraded protein is an important means of regulating the composition of ruminant small intestinal protein. For example, when most or all of the forage is high-quality grass or legume forage, providing forage with a high content of digestible rumen-undegraded protein is extremely important for high-producing dairy cows. (3) Endogenous protein of the digestive tract mainly consists of digestive enzymes, mucin, sloughed epithelial cells of the digestive tract, blood proteins, etc. Under normal circumstances, about 90% of this endogenous protein is reabsorbed by the body before being excreted in feces (Lu Dexun, 1986). This turnover of endogenous protein (release into the intestine and reabsorption by the intestine) also plays an important homeostatic control role in ruminant protein nutrition and metabolism.2 Absorption of Protein Degradation ProductsFor many years, it was thought that protein must be hydrolyzed into free amino acids (FAA) before it can be absorbed, but many studies now show that after degradation, protein can be absorbed in the gastrointestinal tract in two forms, namely free amino acids and small peptides.2.1 Absorption of Protein Degradation Products in the ForestomachBecause people previously thought that protein must be hydrolyzed into free amino acids before it can be absorbed, the forestomach was considered to play little role in the absorption of protein degradation products. However, recent studies have found that although it is not yet settled whether the forestomach can absorb amino acids, it can absorb small peptides (mainly dipeptides and tripeptides). Studies using isotope labeling have found that the rumen and omasum have the ability to absorb small peptides, and the absorptive capacity of the omasum is even stronger than that of the rumen (Matthews, 1991). mRNA for small peptide transport (Webb et al., 1992) and peptide transporter proteins (Walker and Hirst, 1997) are present in the rumen and omasum. For ruminants, the rumen and omasum are the main sites for absorbing amino acids in the form of peptides (Matthews et al., 1991, 1996; Cheng Maoji, 2000), and they hardly absorb free amino acids.2.2 Absorption of Protein Degradation Products in the Small IntestineWith the deepening of research on the nutritional physiology of peptides, it has been recognized that, in addition to free amino acids, the absorption of small peptides by the animal intestine is another important form of intestinal amino acid absorption. Studies show that there are two mutually independent transport mechanisms for the absorption of small peptides (mainly dipeptides and tripeptides) and free amino acids in the animal intestine (Rubino et al., 1971; Hellier et al., 1972). The small peptide transport system has the characteristics of fast transport speed, low energy consumption and resistance to saturation (Ganapathy and Leiback, 1985; Hara et al., 1984; Matthews, 1987), so small peptides are absorbed in the small intestine faster than free amino acids (Webb, 1990); in addition, peptide absorption can avoid competitive absorption among amino acids (Rubino et al., 1971; Wang Gang, 1999). The animal intestine possesses both mechanisms for absorbing small peptides and free amino acids, which is a self-nutritional regulation mechanism by which animals adapt to changes in the environment and nutrient intake status (Webb, 1990). It has been reported that under long-term nutritional restriction (low-protein diet), the absorptive capacity of the small intestine for amino acids decreases, while the absorptive capacity for peptides is relatively enhanced.The N digestibility of sheep and goats is about 68% (Wang Hongrong, 1998; Peng Yulin, 2000), among which more than half of the N in sheep and dairy cows is absorbed in the form of amino acids (Lapierre et al.,2000a), and the proportion of N retained in the form of amino acids in beef cattle is 42-46% (Huntington and Prior, 1985; Lapierre et al., 2000b). This indicates that free amino acids absorbed by the ruminant intestine are still the main source of nitrogen nutrients, and peptides absorbed in the gastric region are a beneficial supplement (Seal et al., 1991). Therefore, it is generally considered that the intestine mainly absorbs free amino acids and absorbs relatively few small peptides, and that the jejunum and ileum are the main sites of free amino acid absorption.3 ConclusionWith the application of molecular biology, isotope tracing and other technologies in animal nutrition, research on protein digestion and absorption in ruminants continues to make new breakthroughs. At present, research on the composition and role of the rumen microbial community and quantitative research on peptide absorption at different digestive sites have become hot topics in this field. The progress made in these studies will certainly provide a solid theoretical foundation for proposing new nutritional evaluation indicators and nutritional regulation technologies in the future.
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Very good material. Learned a lot. Thanks for your hard work, Dr. Zhao!
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Height determines vision----level; angle determines direction---vertical plane; the two planes intersect in a straight line
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Read with respect, thank you.
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Saved and learned from it, thanks for providing it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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Senior fellow apprentice, the author of this post is Dr. Zhao from our school!
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Learn and learn, there is so much to learn, hehe
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Learn and learn
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Very good, learned from it, thanks for sharing
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Read with respect, thanks to the original poster for sharing!