Methionine has become a consensus as the first limiting amino acid in poultry. As an essential amino acid, animals cannot be synthesized or synthesized in small amounts and must be provided by an external diet to maintain animal production. The content of methionine in common corn-soybean feed is limited, and additional methionine is needed in the feed to meet the animal's demand. At present, the price of methionine is quite high in the market, and choline and betaine have been widely used as feed substitutes as part of methionine. The relationship between metabolism and function of methionine, choline and betaine in poultry was reviewed.
1 Physiological effects of methionine, choline and betaine
Methionine is a sulfur-containing amino acid, and its chemical name is 2-amino-4-methylthiobutyric acid, and its molecular formula is C5H11NO2S. The choline is a hydroxide of (β-hydroxyethyl)trimethylammonium having a molecular formula of C5H15NO2S. Betaine, also known as trimethylglycine, has the molecular formula C5H11NO2. There are similarities in structure.
1.1 The physiological role of methionine
Most animals, especially birds, do not synthesize methionine in the body, or the amount of synthesis is minimal. Methionine is directly involved in the synthesis of proteins in most of the animal's body. When the amount of cystine in the diet is insufficient to meet the needs of the animal's synthetic protein, methionine is converted to the cysteine ​​required for the synthesis of the protein.
Another important role of methionine is to provide methyl groups for various methylation reactions in animals. Methyl group is necessary for the synthesis of a number of substances with important physiological effects. For example, a process for avian synthesis of uric acid requires a methyl group such as methionine to provide a methyl group. Methionine participates in a series of methylation reactions in the animal body by providing methyl groups. These methylation reactions synthesize some important metabolites, including choline, carnitine, creatine, phospholipids, adrenaline, RNA and DNA.
1.2 Physiological effects of choline
Choline is mainly present in the form of lecithin, lysolecithin, phosphorylcholine, neurocholine, choline acetate, etc., and the content of free choline is very small. Choline is an important substance in the synthesis of phospholipids and lecithins in animals. It is also involved in the synthesis of fat in the liver and transported to adipose tissue for storage. It can effectively prevent bone short leg disease and fatty liver in poultry. After acetylation in the body, choline participates in neural activity in the form of acetylcholine. Another important role of choline is to provide methylation for the synthesis of methionine. On the other hand, choline also accepts the methyl group provided by methionine for the synthesis of choline. In this process methionine is both a methyl acceptor and a methyl donor.
1.3 Physiological effects of betaine
Betaine, like choline, also promotes fat metabolism and inhibits fatty liver. In aquaculture, betaine has been widely used as an attractant. At the same time, betaine is a direct and effective methyl donor. One of its three methyl groups can directly participate in methyl transfer, while the other two are oxidized and indirectly participate in methylation.
2 Relationship between choline, betaine and methionine as methyl donor
The synthesis of methionine in animals requires choline to provide a methyl group. Choline must first be oxidized to betaine in the mitochondria, and methylation is provided by betaine. Thus choline is the precursor of betaine and this process is irreversible. Betaine transfers methyl groups to homocysteine ​​to synthesize methionine. However, homocysteine ​​can only be metabolized by methionine in the body. Natural proteins contain almost no such amino acids, so betaine cannot replace methionine to synthesize proteins. However, if the supply of choline or betaine is insufficient, the above-mentioned transmethylation cycle is inhibited. On the one hand, it will affect the synthesis of its own methionine. On the other hand, due to the lack of methyl group, the methionine which cannot be regenerated in the diet will be used. The methyl group is provided to participate in the methylation reaction to meet various physiological needs of the animal. This will affect the rate of protein synthesis and affect the growth of animals.
If the supply of methionine is excessive and there is a lack of choline and betaine, a large amount of homocysteine ​​accumulates in the body, resulting in tibial dysplasia and atherosclerosis.
3 choline, betaine and methionine substitution
Betaine acts as a methyl donor and is more efficient than choline as a methyl donor. Studies on radioisotopes have shown that betaine as a methyl donor is 12-15 times more efficient than choline, so as a methyl donor, betaine can completely replace choline. However, the important physiological functions of choline are phospholipids, fat transport, etc., and betaine cannot be converted into choline. Therefore, betaine cannot completely replace the function of choline. Studies have shown that 75.00% of the body's need for choline must be provided by choline itself, and the remaining 25.00% can be replaced by betaine.
The substitution of betaine for methionine is mainly related to the choline content of the diet. Numerous studies have shown that when the choline content in the diet is insufficient, the addition of betaine can partially replace the function of methionine, providing a methyl group to improve growth rate and save methionine. However, this substitution is not complete and the feed must contain about 0.50% methionine. Yan Yuming and other studies have shown that in broiler diets, betaine replaces methionine, the best replacement in the early stage is 1/2, and the best replacement in the later period is 2/3. However, if the amount of choline in the diet meets the growth requirements of the animal, the addition of betaine is not a substitute for methionine-synthesized protein and does not exhibit better production performance.
In actual production, the corn-soybean-type diet has less choline content, and there is a weakness of methyl deficiency. Adding choline can alleviate the negative effect caused by methyl deficiency. Whether choline can completely replace methionine has always been a big controversy, the key is related to the level of methionine in the diet and the age of the test chicken. Dietary methionine deficiency, choline addition effect is obvious. Chicks that provide methylated choline due to phosphatidylethanolamine and methionine in vivo do not meet growth requirements, and a sufficient amount of choline must be ensured in the diet.
In summary, betaine as a methyl donor can partially replace choline; choline and betaine can partially replace methionine, but its substitution effect should be based on dietary composition, animal specific nutritional needs and price in actual production. And other factors are considered together.
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