Study on the nutritional metabolism of tryptophan in animals

Introduction: tryptophan, also known as α - amino - β - indolpropionic acid, has three isomers of DL -, D -, and L-tryptophan, and only L-tryptophan naturally exists Tryptophan is an essential amino acid for animal growth The lack of tryptophan results in the decrease of food intake, growth retardation and rough coat Tryptophan can not be synthesized in animals, and tryptophan in plant feed can not meet the needs of pigs and poultry In recent years, synthetic lysine and methionine have been widely used in compound feed, which makes the importance of tryptophan in diet obvious In order to produce livestock and poultry more effectively, it is of great significance to study the nutrient metabolism of tryptophan The characteristics of tryptophan metabolism 1 The specificity of tryptophan transport in animals as a constant nutrient, the effect of tryptophan changes in plasma and tissue is different from other amino acids It is the only amino acid in animals that binds to albumin through noncovalent bonds (mcmenemy et al., 1958) This binding is related to its molecular configuration: L-tryptophan binds to serum albumin at one site with high stereospecificity, while D-TRYPTOPHAN binds to serum albumin only 1% of L-tryptophan (mcmenemy et al., 1958) In addition, it is also affected by the concentration changes of other macromolecular neutral amino acids and free fatty acids in plasma When the binding state changes, the metabolism of tryptophan in the body will change (such as affecting the synthesis of 5-hydroxytryptamine in the brain), and even cause some diseases, such as liver coma 2 Tryptophan can regulate the synthesis of animal liver protein Sidransky et al (1980) proved that L-tryptophan can affect the metabolism of liver RNA and protein It can significantly promote the accumulation of liver polyribosomes, the synthesis of cytoplasmic poly (a) - RNA, the release of nuclear labeled RNA and the activity of nucleoside triphosphatase Further studies showed that L-tryptophan can increase the ribosome and RNA of liver cells Garrett et al (1984) increased the activity of NADPH cytochrome c reductase and NADH cytochrome b5 reductase per milligram of microsomal protein, and thus increased the content of liver microsomal protein (Takahashi, 1991) In addition, cortamira et al (1991) believed that tryptophan increased protein synthesis in muscle and liver by stimulating insulin release Therefore, tryptophan is a kind of special amino acid It is not only one of the amino acid components of protein, but also participates in the regulation of protein synthesis As for the detailed mechanism of this regulation function, it needs further study 3 The effect of tryptophan on the synthesis and intake of 5-hydroxytryptamine The change of tryptophan concentration will affect the synthesis of 5-hydroxytryptamine in the brain Tryptophan is synthesized with tryptophan as the precursor (Fernstrom et al., 1974) The key enzyme is tryptophan hydroxylase, which is affected by tryptophan concentration, neuronal excitation frequency, calcium dependent phosphorylation of the enzyme and the presence or absence of cofactors (Fernstrom, 1983) In vitro or in vivo, the Km value of the enzyme is about 50um, while the level of tryptophan in the brain is often 10-30um, that is to say, the enzyme is in an unsaturated state, so the change of tryptophan concentration in the brain will change the synthesis speed of 5-hydroxytryptamine in the brain (arimanana et al., 1984) Generally speaking, a doubling of tryptophan levels in the brain (from 15 to 30 UM) can increase serotonin in the brain by 20% - 30% (arimanana et al., 1984) Tryptophan competes with long-chain neutral amino acids (LNAA), such as tyrosine, phenylalanine, leucine, isoleucine, alanine and valine, especially isoleucine and valine, when passing through the blood-brain barrier (Colmenares and wurt man, 1979) Therefore, the increase or decrease of LNAA in plasma directly affects the amount of tryptophan transferred into the brain, and then affects the synthesis of 5-HT Tryptophan regulates the intake of 5-hydroxytryptamine through 5-hydroxytryptamine, which can temporarily inhibit brain activity, make animals tend to be quiet and promote sleep This may be beneficial to reduce the maintenance needs of animals, promote growth and improve feed conversion efficiency Many scholars (Weinberger et al., 1978; lacy et al., 1986) believe that the effect of tryptophan on food intake is related to the synthesis of 5-hydroxytryptamine When tryptophan is seriously deficient, 5-hydroxytryptamine is exhausted, which leads to a sharp decline in intake; when tryptophan level is too high, 5-hydroxytryptamine synthesis in brain increases; because 5-hydroxytryptamine can increase the satiety of animals, when it exceeds the physiological level, it can inhibit animal feeding; the results of central injection study also confirm the role of 5-hydroxytryptamine (Goldman et al., 1971; den - 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 demand, the change of the level is not enough to change the synthesis rate of 5-hydroxytryptamine in the brain, so it works through the peripheral; when the tryptophan level is seriously deficient or too high, the change of the plasma tryptophan level can cause the change of the brain tryptophan level, thus affecting the synthesis of 5-hydroxytryptophan, and then regulating the intake 4 The efficiency of tryptophan to nicotinic acid and nicotinamide nucleotide tryptophan to nicotinic acid partial excess tryptophan (which has met the needs of protein synthesis) can be converted into nicotinic acid mononucleotide In theory, 1.7mg tryptophan can be converted into 1mg nicotinic acid under 100% G molecular transformation However, the conversion efficiency of tryptophan to niacin in animals is relatively low, for example, the hatching rate of eggs is 5.6%, that of human, chicken and piglet is about 2%, and that of duck is 0.5% Factors affecting the conversion of tryptophan to nicotinic acid in the process of tryptophan conversion from canine urine to nicotinic acid, the rate limiting enzyme is tryptophan oxygenase, which only exists in the liver and is regulated by tryptophan level, glucocorticoid, glucagon, high concentration of coenzyme (especially NADPH), estrogen, heme (Badawy et al., 1975), and other factors Some enzymes and some nutrients in the diet also affect the conversion efficiency of tryptophan, which can be summarized as follows: the activities of PAC and 3- hydroxy -2- aminobenzoate oxygenase (3-HAAO) Chen et al (1996) showed that adding tryptophan or niacin to the diet could improve the growth of Broilers and prevent niacin deficiency, while only adding niacin to Beijing duck and female Muscovy Duck could prevent niacin deficiency They found that the activity of PAC in chicken liver and the ratio of 3-haao to PAC activity were significantly affected by the level of tryptophan or niacin in diet, but not in duck The activity of PAC in duck liver was 4-5 times of that in chicken, so the reaction substrate 3-hydroxy-2-aminobenzoic acid was converted into acetylcoa by carboxylation, while the precursor quinolinic acid which was converted into niacin by non enzymatic way was less Vitamins B2 and B6 In the reaction pathway of tryptophan synthesis of nicotinic acid, canine urine aminotransferase is very sensitive to the deficiency of VB6 When VB6 is deficient, the activity of this enzyme is reduced (Knox, 1953) A large number of metabolites of tryptophan, such as xanthuric acid, canine urine quinoline acid and 3-hydroxyphosphoaminobenzoic acid, are excreted from urine (sauberlich, 1985) Henderson et al (1995) found that vitamin B2 deficiency can weaken the hydroxylation of canine urinary ammonia Leucine Previous studies have found that maize and sorghum contain too much leucine, which leads to pellagra It is believed that it can inhibit some enzymes in the process of transforming tryptophan into nicotinic acid However, at the cellular level, leucine had no effect on the activity of tryptophan oxygenase (Salter et al., 1985) Lowry et al (1989) did not find that excessive leucine affected the metabolism of tryptophan or nicotinic acid when analyzing the growth data of chickens with slope technique Later, it was found that leucine could inhibit intestinal absorption of tryptophan, thus affecting its utilization Iron and copper levels in the diet Iron deficiency can lead to a reduction in heme, which in turn reduces or alters tryptophan oxygenase activity (Badawy et al., 1975) Oduho (19931994) thought that iron deficiency only affected tryptophan metabolism when tryptophan was the precursor of nicotinic acid Hollister et al (1966) reported that dietary copper levels affected the conversion efficiency of tryptophan to niacin Physiological state and molecular configuration of tryptophan In pregnancy, the conversion efficiency of tryptophan to nicotinic acid is higher than that in non pregnancy Nicotinic acid cannot be synthesized by D-TRYPTOPHAN Tryptophan may be an important precursor of nicotinamide nucleotides Nicotinic acid is generally considered as a normal precursor of NAD and NADP But most evidences, such as Hana's disease (tryptophan oxygenase deficiency), carcinoid syndrome and drug-induced pellagra, indicate that even with niacin, pellagra is still serious Therefore, tryptophan is more likely to be an important precursor of nicotinyl peptide nucleotides under normal conditions, because quinoline acid phosphoribosyl transferase generally plays a catalytic role in an environment much lower than its Km value When quinolinic acid is increased, its binding speed with nucleotides is also increased; however, nicotinamide and nicotinic acid transphosphorylase generally work under their Vmax, so the excess nicotinic acid can not be used and is discharged after methylation (bender et al., 1982) Therefore, although the conversion efficiency of tryptophan and niacin is low, it is of great significance to study the relationship between tryptophan and niacin from the biochemical and physiological levels 5 Tryptophan can synthesize melatonin and affect the physiological function of animals in the pineal gland cells Tryptophan can generate melatonin under the catalysis of 5-hydroxytryptamine-N-acetyltransferase and hydroxyindole-o-methyltransferase It participates in a series of metabolic regulation activities, controls the biological rhythm of animals, and has antagonistic effect on sex hormones Bowman (1989) showed that tryptophan and discontinuous light could improve the immunity of broilers, which may be due to the melatonin, which is believed to have immune promoting effect and regulated by light 6 The utilization of tryptophan with different molecular configuration (D and L) in animals All kinds of animals can make good use of L-tryptophan There are great differences in the utilization of L-tryptophan among different animal species: pigs and rats can make good use of D-TRYPTOPHAN (Ohara et al., 1980), while mice, chickens, dogs and people make little use of D-TRYPTOPHAN (Friedman et al., 1982; czarnechi et al., 1982) Morrison et al (1956) observed that intraperitoneal injection of D-TRYPTOPHAN had a higher utilization rate than oral injection, and found that the intestinal absorption rate of D-TRYPTOPHAN was low, which partly explained the low utilization rate of D-TRYPTOPHAN in poultry In addition, the D-type amino acids with biological effects play a role through two steps: first, they oxidize to form ketone analogues on A-site carbon, and then generate L-type amino acids through amino transfer Therefore, the low utilization rate of D-TRYPTOPHAN may also be due to the low efficiency of the animal variety in transforming D-type to L-type When tryptophan is added to the diet, its molecular configuration should be considered The relationship between tryptophan and other nutrients metabolism tryptophan has complex interaction mechanism with carbohydrate, protein, fat, vitamin and microelement in the process of metabolism 1 Carbohydrates were fed diets with high carbohydrate content to stimulate insulin secretion, increase plasma tryptophan concentration and decrease LNAA concentration, and increase the ratio of tryptophan to LNAA (Fernstrom et al., 1972) Disaccharide and polysaccharide