Protein Metabolism

Protein Metabolism, a concept that encompasses the intake of protein substances in the body, their transformations in the body (see Intermediate Metabolism) and the excretion of combustion products of protein in the form of urea, carbon dioxide, water, and other chemical compounds. Protein metabolism attracts special attention from physiologists because proteins, due to the presence of nitrogen in their molecules, occupy a very unique position among other nutritional substances. Proteins enter as an essential component into the nuclei and protoplasm of all cells of the body; their deficiency in food can lead to the destruction of tissue elements of the body. Therefore, the so-called protein minimum was established long ago, the value of which was determined differently by various authors (Voit, Hindhede) (see Metabolism). Protein metabolism is determined by measuring the amount of nitrogen in food and human excretions (urine and feces). Since the average percentage of nitrogen in protein substances is 16, to calculate the amount

of protein, the figures found for nitrogen are multiplied by 6.25. Not all proteins are nutritionally equivalent; for example, zein (corn protein) does not contain certain amino acids (lysine and tryptophan). Such proteins, in contrast to proteins containing all amino acids, are called incomplete; when feeding exclusively on them, nitrogen balance is not established. For more details, see Metabolism and Intermediate Metabolism, where there is literature on the subject.