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

Metabolism of Simple Proteins
Factors Determining the State of Protein Metabolism

The direction and intensity of Protein METABOLISM are primarily determined by the physiological state of the Organism and, like all other Types of Metabolism, are undoubtedly regulated by neurohormonal factors. Protein metabolism is more intensive during childhood, physical exertion, Pregnancy, and Lactation—periods when the body’s demand for Proteins increases dramatically. A significant influence on protein metabolism is exerted by dietary habits, particularly the quantitative and qualitative composition of dietary proteins. When protein intake is insufficient, the body breaks down its own proteins in certain Tissues (such as the Liver, Blood Plasma, and intestinal mucosa) to release free Amino Acids. These amino acids ensure the synthesis of vital cytoplasmic proteins, Enzymes, Hormones, and other biologically active compounds. Thus, certain structural tissue proteins are «sacrificed» to sustain the viability of the organism as a whole. Conversely, a high-protein diet does not significantly alter protein metabolism, as excess protein is not stored as reserves but is excreted in the urine as End products of Nitrogen metabolism. However, the qualitative composition of dietary protein is of greater importance, since the absence or deficiency of even a single essential amino acid can become a limiting factor for The Biosynthesis of all proteins in the body.

Protein Synthesis obeys the «all-or-none» law and occurs only when a complete set of all 20 amino acids is present in The Cell. Even if all Amino acids are supplied with food, the body may experience Protein deficiency if the intestinal absorption of any single amino acid is impaired or if it is broken down by intestinal microflora to a greater extent than normal. In such cases, protein synthesis will be restricted, or the organism will compensate for The amino acid shortage by breaking down its own endogenous proteins. The utilization rate of dietary PROTEINS AND AMINO acids also depends on the quantitative and qualitative composition of Introduction/36.html">CARBOHYDRATES and Lipids, which drastically reduce the body's energy reliance on proteins. Experimental and clinical evidence indicates that low-fat and low-calorie diets promote increased excretion of Amino Acids and their degradation products in the urine.

There is experimental evidence of a Direct and Indirect link between protein metabolism and the body's vitamin status, particularly regarding Vitamins B₁, B₂, B₆, PP, and others. Furthermore, protein metabolism is regulated by The activity of the Endocrine glands. Hormones determine, to a certain extent, the direction (toward synthesis or degradation) and intensity of protein metabolism. For example, the administration of ACTH and THYROID HORMONES leads to intensive breakdown of tissue proteins. Conversely, Other Hormones—such as STH, androgens, and estrogens—stimulate anabolic reactions and promote protein synthesis. The administration of certain Adrenocortical Hormones induces dysproteinemia and results in a negative nitrogen balance, which some authors attribute to The stimulation of Gluconeogenesis from the carbon skeletons of amino acids (following their deamination—see below).

Thus, the state of protein metabolism is determined by a multitude of factors, both exogenous (environment, diet, etc.) and endogenous (physiological state, including neurohormonal status, enzymatic capacity, etc.). Any deviation from the normal physiological state of the organism affects nitrogen metabolism. Understanding the patterns of alterations in Protein Metabolism during a specific pathological process is a prerequisite for selecting the appropriate therapeutic strategy to correct the disrupted metabolic pathway.