Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Proteins
Protein Metabolism in the Body

The sequence of the main Stages of Protein transformation in The Human Body is schematically shown in Fig. 94.

A prerequisite for the vital activity of the Organism is the continuous Biosynthesis of protein. To ensure this process, dietary Proteins must be ingested into the body. In the gastrointestinal tract, dietary proteins undergo enzymatic Cleavage (Hydrolysis) down to structural monomers—Amino Acids—which are capable of penetrating the intestinal wall and being absorbed into the Blood. Via the bloodstream, they are delivered to Tissues, where they are used primarily in The biosynthesis of the body's individual proteins and A number of other substances, or undergo various intratissue transformations (Transamination, deamination, decarboxylation, etc.), resulting in The formation of organic acids, acetyl-CoA, amines, and free ammonia (NH₃), which exhibits toxic effects.

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Fig. 94 Scheme of the MAIN STAGES OF Protein METABOLISM in the human body

Acetyl-CoA can then be oxidized in The Tricarboxylic Acid Cycle to the final degradation products CO₂ and H₂O with the generation of ATP. Ammonia is detoxified through the urea synthesis process and eliminated from the body.

The state of protein metabolism in the human body can be assessed by nitrogen balance. Nitrogen balance is the difference between The amount of nitrogen ingested with food and that excreted from the body in the form of its metabolic end products, expressed in grams of nitrogen per day. Nitrogen equilibrium, positive nitrogen balance, and negative nitrogen balance are distinguished.

Nitrogen equilibrium is observed when the amounts of nitrogen entering and leaving the body are equal. Such a state is the physiological norm for a healthy adult organism. The total nitrogen content in human urine normally ranges from 10 to 18 g ⋅ day^-1.

A positive nitrogen balance is observed during the accumulation of nitrogen in the body. This condition indicates that synthesis processes predominate over protein degradation processes in tissues. This is observed in the growing organism, as well as in athletes during long-term adaptation to physical loads, which is associated with the activation of nucleic acid and Protein Synthesis. A positive nitrogen balance can be achieved by administering anabolic substances that stimulate protein synthesis.

A negative nitrogen balance occurs with enhanced excretion of nitrogen from the body. Such a balance is observed with insufficient Dietary Protein Intake (complete or partial protein starvation), performance of heavy physical workloads leading to The breakdown of Muscle Proteins, and also in certain diseases. A negative nitrogen balance is often noted in elderly individuals. This is due to the predominance of catabolic processes over the Rate of protein synthesis, even against the Background of adequate dietary protein intake. This condition is accompanied by a decrease in human body weight.

Physical activity of various kinds causes a shift in the nitrogen balance, which must be taken into account during the training process to prevent exhaustion of the body.