Tryptophan, also known as α-amino-β-propionic acid, has three isomers of DL-, D-, and L-tryptophan, and naturally only L-tryptophan. Tryptophan is an essential amino acid for animals to maintain growth. Lack of tryptophan results in decreased feed intake, growth retardation, and rough coat. Tryptophan cannot be synthesized in animals, and tryptophan in plant feeds usually cannot meet the needs of pigs. In recent years, the use of synthetic lysine and methionine in a large amount of compound feed has made the importance of tryptophan in the diet apparent. In order to carry out livestock and poultry production more effectively, it is of great significance to conduct in-depth research on the metabolism of tryptophan.
Metabolic characteristics of tryptophan:
1. The specificity of the transport of tryptophan in animals as a macronutrient, the effect of tryptophan changes in plasma and tissue levels is different from other amino acids.
It is the only amino acid in an animal that binds to albumin by non-covalent bonds (McMenemy et al., 1958). This binding is related to its molecular configuration: L-tryptophan binds to serum albumin at a single site with high stereospecificity, while D-tryptophan has little binding to serum albumin, only L - 1% of tryptophan (McMenemy et al., 1958). In addition, it is also affected by changes in the concentration of free fatty acids in other macromolecular neutral amino acids and plasma. When this state of binding changes, tryptophan metabolism changes in the body (such as affecting the synthesis of serotonin in the brain), and even cause some diseases, such as hepatic coma.
2. Tryptophan can regulate the synthesis of animal liver protein
Sidransky et al. (1980) demonstrated that L-tryptophan can affect the metabolism of liver RNA and protein, which can significantly promote liver polysome accumulation, cytoplasmic poly(A)-RNA synthesis, nuclear-labeled RNA release and nuclear membrane nucleosides. Triphosphatase activity; further studies have shown that L-tryptophan can increase the polymerization of hepatocyte ribosomes and RNA (Garrett et al. (1984), promote the function of liver microsomal complex oxidase system, increase cytochrome p-450 content Increases the activity of NADPH-cytochrome C reductase and NADH-cytochrome b5 reductase per mg of microsomal protein, thereby increasing liver microsomal protein content (Takahashi, 1991). In addition, Cortamira et al. (1991) considered tryptophan It stimulates the synthesis of muscle and liver proteins by stimulating the release of insulin. Thus, tryptophan is a special amino acid that is not only one of the amino acid components of proteins, but also involved in the regulation of protein synthesis. The detailed mechanism has yet to be further studied.
3. The effect of tryptophan on the synthesis and feed intake of serotonin (5-HT) in the nervous medium
Changes in tryptophan concentration will affect the synthesis of serotonin. The serotonin in the brain is synthesized by using tryptophan as a precursor (Fernstrom et al., 1974). The key enzyme is tryptophan hydroxylase, which is affected by tryptophan. Concentration, frequency of neuronal excitation, calcium-dependent phosphorylation of enzymes, and presence or absence of cofactors (Fernstrom, 1983). In vitro or in vivo, the Km value of the enzyme is about 50uM, and the level of tryptophan in the brain is often 10-30uM, that is, the enzyme is in a non-saturated state, so changes in the concentration of tryptophan in the brain will change the brain 5- The rate of synthesis of serotonin (Arimanana et al., 1984). In general, a 1-fold increase in tryptophan levels in the brain (from 15 to 30 uM) increases serotonin levels in the brain by 20%-30% (Arimanana et al., 1984). When passing the blood-brain barrier, tryptophan and long-chain neutral amino acids (LNAA) such as tyrosine, phenylalanine, leucine, isoleucine, alanine and valine, especially Amines and proline compete (Colmenares and Wurtman, 1979). Therefore, the increase or decrease of LNAA in plasma directly affects the amount of tryptophan transferred into the brain, which in turn affects the synthesis of 5-HT.
Tryptophan regulates animal feed intake of serotonin through serotonin to temporarily inhibit brain activity, making animals quiet and promoting sleep. This may help the animal to reduce maintenance needs, promote growth and improve feed conversion efficiency. Many scholars (Weinberger et al., 1978; Lacy et al., 1986) concluded that the effect of tryptophan on feed intake is related to the serotonin synthesized. When severe tryptophan is deficient, serotonin is depleted, resulting in a sharp drop in feed intake; when the level of tryptophan is too high, serotonin synthesis in the brain increases; since serotonin increases the satiety of the animal when it exceeds At physiological levels, animal feeding can be inhibited; the results of studies conducted by central injection have also confirmed this effect of serotonin (Goldman et al., 1971; Dené„„bow et al., 1983). Tryptophan is likely to act directly on amino acid receptors or by increasing peripheral serotonin (Deutsch et al, 1980; Friedman et al, 1984; Lacy et al, 1986). When the dietary tryptophan level is close to the required amount, its level is not changed enough to change the synthesis rate of serotonin in the brain, so it acts through the periphery; when the tryptophan is severely deficient or too high, the plasma is colored. Changes in the level of the acid can cause changes in the level of tryptophan in the brain, which affects the synthesis of serotonin, which in turn regulates feed intake.
4. Tryptophan synthesis of niacin and nicotinamide nucleotides
The partial conversion of tryptophan to niacin with a partial excess of tryptophan (which meets the needs of protein synthesis in the body) can be converted to niacin mononucleotides. Under 100% molar conversion, theoretically 1.7 mg tryptophan can be converted to 1 mg niacin. However, the actual efficiency of conversion of tryptophan to niacin in animals is relatively low, such as 5.6% hatching, 2% for humans, chickens and piglets, and 0.5% for ducks.
Factors affecting the conversion of tryptophan to nicotinic acid During the conversion of tryptophan to niacin by the kynurenine pathway, the rate-limiting enzyme is tryptophan oxygenase, which is only present in the liver and is affected by chromogenic ammonia. Acid levels, glucocorticoids, glucagon, high concentrations of coenzymes (especially NADPH), estrogen, heme (Badawy et al., 1975) and other factors, in addition, some other enzymes and certain nutrients in the diet It also affects the conversion efficiency of tryptophan, and combines the following aspects: the activity of methylpyridinium carboxylate carboxylase (PAC) and 3-hydroxy-2-aminobenzoic acid oxygenase (3-HAAO). Chen et al (1996) showed that the addition of tryptophan or niacin to the diet can improve the growth of broilers and prevent niacin deficiency, while Beijing duck and female muscovy can only prevent niacin deficiency by adding niacin. They found that the ratio of PAC activity and 3-HAAO to PAC activity in chicken liver was significantly affected by the level of tryptophan or niacin in the diet, whereas ducks did not. The PAC activity in duck liver was 4-5 times that of chicken, so The reaction substrate 3-hydroxy-2-aminobenzoic acid is converted to acetyl CoA by a carboxylation pathway, while the precursor quinolinic acid which is converted to nicotinic acid by an enzymatic pathway is less.
Vitamins B2 and B6. The kynurenase in the nicotinic acid synthesis nicotinic acid reaction pathway is very sensitive to VB6 deficiency. When VB6 is deficient, the enzyme activity is decreased (Knox, 1953), and a large number of metabolites of tryptophan, such as uric acid and canine quinine Porphyrin and 3-hydroxyphosphoramidobenzoic acid are excreted from the urine (Sauberlich, 1985). Henderson et al (1995) found that vitamin B2 deficiency attenuates the hydroxylation of kynurenine.
Leucine. Previous studies have found that corn and sorghum contain hyper-leucine, which causes pellagra, which is thought to inhibit certain enzymes in the process of converting tryptophan into niacin. However, results from cell level studies indicate that leucine has no effect on the activity of tryptophan oxygenase (Salter et al., 1985). Lowry et al. (1989) used slope technique to analyze chicken growth data and found no excess leucine to affect the metabolism of tryptophan or niacin. Later studies found that leucine inhibits the absorption of tryptophan in the intestine and thus affects its utilization.
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