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Peptides, amino acids

Free amino Amino acids Amino acids acids

Mouth

Stomach

Small intestine

Digestive system

Mouth

Stomach

Small intestine flora. When undigested amino acids are decarboxylated by intestinal bacteria, the important chemicals such as histamine, tyramine, ammonia, and similar substances are formed, many of which are absorbed. These substances are undesirable in excessive quantities. The ammonia formed by deamination plays a critical role in certain pathological conditions, for the absorbed ammonia can cause body deterioration. The absorption of amino acids is also impaired if the intestinal mucosa is damaged, as occurs in sprue, ulcerative colitis, and resection of a moderate amount of the small intestine. With time, however, there is a functional adaptation of the intestinal mucosa.

Metabolism

The liver may release some absorbed amino acids for body metabolism. Body amino acids may be catabolized or used in body repair, building, and maintenance. The importance of protein to the body is illustrated by its numerous functions and the complexity of its metabolism. If a normal person has a regular diet, three major factors determine the direction of protein metabolism: the quality and quantity of protein consumed, the amount of calories ingested, and the physiological and nutritional status of the body.

Protein metabolism revolves around a body pool of amino acids that are continuously released by protein hydrolysis and resynthesized. The amino acids ingested are the same as those released in the body. Body tissues can utilize only amino acids, not small peptides. About 50-100 g of body protein turns over daily, ranging from the slowest rate in the collagen to the fastest rate in the intestinal mucosa. Any amino acids filtered through the kidneys are reabsorbed, although certain congenital defects in the kidney tubules may interfere with this process. During pregnancy, infancy, childhood, and other conditions of growth, protein synthesis exceeds degradation. Individuals in these categories, therefore, require a large pool of amino acids.

Figure 14 illustrates the general catabolism, or degradation, of protein to amino acids and other metabolites. Figure 15 gives the general outline of protein formation in the body, and Figure 16 provides an overview of protein metabolism. The sections that follow discuss certain aspects of protein metabolism: degradation, protein synthesis, nitrogen balance, and metabolism of creatine and creatinine.

Protein Degradation. Protein is degraded to its individual amino acids in the muscle and other tissues, but the major site of actual destruction (catabolism) of each amino acid is the liver. The amino acids released from all other organs, especially the muscles, reach the blood and are diverted to the liver for degradation (Figure 14). Under normal circumstances, the catabolism of protein is balanced by its formation, although during stresses such as starvation and disease destruction can outstrip synthesis.

The procedure for the oxidation of an amino acid begins with deamination, a process in which the ammonia or amino group of the acid is removed so that only the carbon skeleton of the amino acid is left. Two metabolites are formed: the keto acids and ammonia (Fig. 14). The ammonia is changed to urea, which is released to the blood and eventually excreted in the kidney. Normally, we excrete about 20-30 g of urea in the urine each day. Because urea forms exclusively in the liver, advanced liver disorders raise the blood urea nitrogen level.

The keto acids formed from deamination enter the citric acid cycle to be oxidized. The exact process whereby keto acids and ammonia are formed varies with each individual amino acid, although the goal is the same. Figure 17 shows the points at which each of the acids and their keto acids enter the citric acid cycle or are transformed to pyruvate to form glucose. Within the citric acid cycle, the keto acids may be made to form glucose (gluconeogenesis). The keto acids are also tied to fat metabolism by the interconversion of keto acids and fatty acids. In sum, when protein is degraded amino acids are formed. Some amino acids circulate, some are oxidized, some are converted to glucose, and some are directed to other paths.

Protein Synthesis. Protein synthesis is the process of linking different amino acids by their amino groups (pep-

Liver

Muscle (or other tissue)

Keto acids Ammonia

Glucose Glucose i i i *

Pro Amine

:ein acids

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