Biological Chemistry - Berezov, T. T., Korovkin, B. F. 1998

Metabolism of Simple Proteins
Biological Value of Proteins

The state of Protein METABOLISM in an intact Organism depends not only on the quantity of dietary protein ingested, but also on its qualitative composition. Animal experiments have demonstrated that consuming equal amounts of various dietary Proteins can, in A number of cases, lead to The Development of a negative nitrogen balance. For instance, feeding equal amounts of casein and gelatin to rats resulted in a positive nitrogen balance in the former case and a negative one in the latter*. This difference was due to the distinct Amino Acid Composition of the proteins, which served as the basis for the hypothesis regarding the existence of so-called "deficient" (or "incomplete") proteins in nature. It turned out that out of the 20 Amino Acids, gelatin almost lacks (or contains in very small amounts) valine, Tyrosine, Methionine, and Cysteine; furthermore, gelatin is characterized by a percentage composition of individual amino acids that differs from that of casein. This explains why replacing casein with gelatin in the diet of rats leads to a negative nitrogen balance. These findings indicate that various proteins possess unequal nutritional value. Therefore, to meet the body's plastic (structural) requirements, sufficient quantities of various dietary proteins are needed. Apparently, the principle holds true that the closer The amino acid COMPOSITION OF THE ingested dietary protein is to that of the body's own proteins, its biological value is higher. It should be noted, however, that the degree of dietary protein assimilation also depends on the efficiency of its breakdown under The Influence of gastrointestinal Enzymes. A number of protein substances (e.g., proteins of wool, Hair, feathers, etc.), despite having an amino acid composition similar to that of human body proteins, are hardly utilized as dietary protein because they are not hydrolyzed by the proteinases of the human and most animal intestines.

* The issues of nitrogen equilibrium, positive and negative nitrogen balance are discussed in detail in the physiology course.

THE CONCEPT OF the Biological value of proteins is closely linked to the issue of Essential Amino Acids. Living organisms vary significantly in their ability to synthesize amino acids or other nitrogen-containing compounds that can be used for Amino acid Biosynthesis. Higher plants, for example, can synthesize all the amino acids necessary for Protein Synthesis, and they can use ammonia or nitrates as a nitrogen source. Microorganisms exhibit varying capabilities for Amino acid synthesis. Specifically, while E. coli synthesizes all amino acids using nitrites, nitrates, or ammonia, lactic acid Bacteria lack this ability and obtain preformed amino acids from milk. Higher vertebrates do not synthesize all the necessary amino acids. In the bodies of humans and white rats, only 10 out of the 20 required Amino acids are synthesized—the so-called dispensable (non-essential) amino acids. They can be synthesized from carbohydrate and Lipid Metabolism intermediates. The remaining 10 amino acids are not synthesized in the body and are therefore termed vital, essential, or indispensable amino acids (Table 12.1).

Class="center">Table 12.1. Dispensable and indispensable amino acids

¹ Partially indispensable (conditionally essential) amino acids.

The indispensability of amino acids for the GROWTH AND DEVELOPMENT of animal and human organisms is explained by the Cells' lack of ability to synthesize the carbon skeletons of essential amino acids, since the amination of the corresponding keto derivatives proceeds relatively easily via Transamination reactions (see below). Consequently, to ensure normal vital activity in humans and animals, all 10 of these amino acids must be supplied through the diet.

It should be noted that for adult humans, arginine and histidine have proven to be partially dispensable. G. Rose observed individuals receiving an artificial diet in which the protein was entirely replaced by a mixture of 20 amino acids. He established that in order to maintain normal body weight and performance, not only a specific amount of each Amino Acid and The ratio of essential amino acids in such a diet are important, but also the total nitrogen content of the diet (Table 12.2).

The omission of any essential amino acid from the dietary mixture is accompanied by the development of a negative nitrogen balance, wasting (emaciation), growth retardation, Nervous system dysfunctions, etc. In experiments on rats, the following relative amounts of essential amino acids required for optimal growth were established, taking tryptophan as a unit: lysine 5; leucine 4; valine 3.5; phenylalanine 3.5; methionine 3; isoleucine 2.5; threonine 2.5; histidine 2; arginine 1. There is evidence that approximately the same ratio of essential amino acids is required for humans.

Table 12.2. Minimum daily human requirement for essential amino acids (FAO/WHO recommendations)

¹ Cysteine reduces the requirement for methionine by 80%.

² Tyrosine reduces the requirement for phenylalanine by 70%.

The consequences of an inadequate intake of any essential amino acid have been studied in greater detail in animals. The absence or deficiency of, for example, valine and lysine leads to growth arrest and the development of a severe clinical picture resembling avitaminosis in animals.

It should be especially emphasized that a deficiency of a single essential amino acid in the diet leads to the incomplete utilization of Other Amino Acids. At the same time, animal experiments have demonstrated that the requirements for essential phenylalanine can be partially met by the dispensable amino acid tyrosine, and requirements for methionine by homocysteine with The addition of a necessary amount of methyl group Donors. Glutamic acid reduces the requirement for arginine. Species differences must also be taken into account when determining the essentiality of individual amino acids. For chicks, for instance, glycine proved to be an indispensable growth factor.

Knowledge of its amino acid composition is of great importance for assessing the biological value of dietary protein. Thus, feeding rats casein (milk protein) and zein, a protein isolated from corn that lacks lysine and practically no tryptophan, showed that when casein was received in the diet, animal growth was unimpaired. Replacing casein with zein led to a gradual lagging in growth and a decrease in the body weight of the animals. Adding only tryptophan to zein prevented weight loss but did not increase growth; when lysine was also added to the diet, body weight increased progressively.

Thus, feeding zein—a protein isolated from corn grain that lacks two essential amino acids—leads to growth arrest, a decrease in animal body weight, and the development of a negative nitrogen balance.

Humans and animals do not consume artificially isolated proteins, but rather natural proteins that are part of a mixed diet, which typically contains the full range of essential amino acids. For example, whole corn grain contains 2.5% lysine and 0.7% tryptophan, whereas zein contains no lysine at all, and its tryptophan content is a mere 0.1%. This example once again demonstrates that truly deficient proteins hardly exist in nature and that one should obviously only distinguish between biologically more valuable and less valuable (nutritionally) proteins (Table 12.3).

Table 12.3. Essential amino acid content in proteins of various origins

The biological value of dietary protein depends entirely on the degree of its assimilation by the organism, which in turn is determined by the match between the amino acid composition of the consumed protein and that of the body's own proteins. Such dietary protein is utilized more effectively by the body for tissue protein synthesis. For humans, for example, meat, milk, and egg proteins are biologically more valuable because their amino acid composition is closer to that of human Organs and Tissues. However, this does not preclude the intake of plant proteins, which contain the required set of amino acids, albeit in different proportions. Therefore, to ensure The biosynthesis of the necessary amount of endogenous proteins, a human will require significantly more plant proteins than animal proteins.

Thus, for the normal growth and harmonious Development of the human body, the formulation and Selection of food products that contain an optimal amino acid composition and provide nutritionally adequate diets for various population groups are of paramount importance, taking into account not only age and gender, but also varying climatic conditions, The Nature of labor, the season of the year, etc.