In food protein, amino acids that are relatively insufficient according to the needs of the human body and their proportional relationship are called limiting amino acids. Limiting amino acid refers to the amino acid whose ratio of the amount of essential amino acid contained in a certain feed or ration to the amount of protein essential amino acid required by animals is low. Insufficiency of these amino acids limits the animal's availability of other essential and non-essential amino acids.
In the process of absorption, digestion and utilization, amino acids need a certain ratio to be fully absorbed and utilized. When a certain amino acid in a certain food is far from reaching this ratio, no matter how high the protein content is, it will not be able to exert its advantages. This amino acid is a limiting amino acid. For example, methionine in beans and lysine in cereals are their respective limiting amino acids.
The most low amino acid is also known as the first limiting amino acid. According to the low degree analogy, there are the second restrictive amino acid, the third restrictive amino acid and so on. Some amino acids are generally lacking in feed, and cannot be converted or synthesized by any amino acid, which limits the nutrition of protein. For laying ducks, methionine is the first limiting amino acid, lysine is the second limiting amino acid, and tryptophan is the third limiting amino acid. For example, in a certain diet, other amino acids are abundant, but methionine can only meet 60% of the requirement, and all amino acids can only use 60%. The remaining 40% is deaminated in the liver and synthesized into urea, which is excreted with urine, resulting in a waste of protein.
Lysine, methionine or tryptophan are often referred to as the first, second and third limiting amino acids in feed protein. The utilization of other amino acids and the biological value of proteins are limited. In the formulation of the diet, the requirements of the first, second and third limiting amino acids are first considered.The following is a brief introduction to the metabolism of the three limiting amino acids:
Lysine
Lysine is metabolized in the body to produce glutaryl-CoA (acetoacetyl-CoA). The further metabolism of acetoacetyl-CoA may have two outlets, one is to generate acetyl-CoA, and the other is to generate a small amount of Q-ketoglutarate to participate in metabolism.
Methionine
Methionine (sulfur-containing amino acid) There are three sulfur-containing amino acids in livestock and poultry, namely cysteine, cystine and methionine (methionine), which are finally metabolized into taurine. Sulfur-containing amino acids can generate pyruvate during catabolism, so they are sugar-producing amino acids, and the sulfur in them is oxidized to sulfuric acid, which is why sulfuric acid is produced during protein decomposition.
Tryptophan
After oxidation and decarboxylation, tryptophan is transformed into 5-hydroxytryptamine, which exists in the brain tissue, and there is also a small amount in the gastrointestinal tract and blood. Its physiological function is to constrict microvessels and increase blood pressure, and it is also a neurotransmitter. Dysregulation of tryptophan metabolism can cause nervous system dysfunction. After tryptophan is oxidized, it can also be converted into nicotinic acid. Niacin is a precursor for the synthesis of nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). NAD and NADP are The coenzyme of anaerobic dehydrogenase participates in various redox reactions in the body.
The limiting amino acids widely used in the feed industry are mainly methionine and lysine. In addition, tryptophan, threonine, valine, etc. are also used in the production of compound feed.