Dietary protein exists in three forms. The major portion is conjugated with other substances, a small fraction is associated with fats and carbohydrates, and only a very small part exists as free protein, such as that in egg white. To be used by the cells, all proteins must be broken down into their constituent amino acids. In the sections that follow, the paths of protein digestion, absorption, and metabolism will be traced.


Digestion of protein begins in the stomach (Table 1 and Fig. 13) where acid activates the pepsinogen (an enzyme) to release pepsin (another enzyme). The pepsin cleaves peptide linkages in the protein to produce polypeptides, each with two or more amino acids. When the stomach content reaches the duodenum, the pH is raised to about 6.5 by the presence of alkaline pancreatic juice. In the small intestine, chymotrypsin and trypsin hydrolyze most of the protein molecules to form small polypeptides and dipep-tides. These are further digested to form free amino acids by the pancreatic enzyme carboxypeptidase and the intestinal enzymes aminopeptidase and dipeptidase. Those small peptides not split into individual amino acids may gain enter to mucosal cells to be digested later, for body tissues can utilize only amino acids.

Because protein molecules also contain nonpeptide linkages, they must be denatured first (by heat or stomach acid) before the appropriate enzymes can digest them. The denaturation process exposes the protein molecules, providing more surface area for enzymatic action. On the

Figure 9. The respiratory chain or electron transport system (S = substrate).

other hand, excess heating or cooking can also reform some other linkages, making digestion more difficult.


Normally only amino acids (and certain small peptides) are absorbed. In the small intestine all free amino acids, whether ingested or digested products, are absorbed via the mucosal cells into the hepatic portal vein. The D-amino acids are not absorbed as well as the L forms. The L-amino acids, the biologically active ones, are absorbed by active transport (a process requiring energy). Absorption occurs along the entire small intestine, with the slowest rate along the ileum. The stomach and colon may also absorb some amino acids. About 20-30% of ingested proteins are unabsorbed and excreted in the stools.

Although normally only amino acids are absorbed, it is well known that in small infants some undigested proteins are also absorbed. The subsequent antigen-antibody interaction causes the child to develop an allergic reaction when ingesting the same protein foods later. This explains the high allergic incidence among infants to foods such as eggs and cereals. If adults show allergy to ingested protein foods, they are still probably capable of absorbing whole protein molecules. For the majority of the population, this ability disappears with age.

Amino acids seem to be utilized best when they are absorbed in accordance with the body need for growth and function. Any excess absorbed will not be stored but excreted in the urine or metabolized to ammonia. Studies have shown that absorption and utilization of proteins are optimized when adults evenly distribute their protein intake throughout the day.

In some cases, erratic patterns of protein absorption may be due to stomach irregularities rather than irregular protein ingestion, for the stomach regulates the emptying of the nutrient into the small intestine. In patients with partially or completely removed stomachs, the rate of protein emptying is so disturbed that much of this valuable nutrient is lost in the fecal waste or degraded by intestinal


Glucose (6C)


Pyruvic acid (3C)

Acetyl-CoA (2C)



Respiratory chain or electron transport system

Figure 10. The three stages of converting glucose to carbon dioxide, water, and ATP.

Respiratory chain or electron transport system



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