X. Proteins, Digestion and Absorption

Digestion is the process by which ingested feed is broken down physically and chemically to simple products for absorption from the digestive tract. In the case of proteins, it involves denaturing of proteins to expose the peptide bonds, followed by hydrolysis and release of free amino acids.

Protein-Digesting Enzymes

Protein-digesting enzymes are either endopeptidase or exopeptidase. Endopeptidases break peptide bonds within the primary structure into smaller fragments. Exopeptidases cleave amino acids off the terminal end of the protein molecule. Carboxypeptidases remove an amino acid from the end with a free carboxyl group, and aminopeptidase act on the terminal amino acid with a free amino group.

Protein Digestion

Protein digestion begins in the stomach.
Gastrin, a hormone, initiates the breakdown of proteins in the stomach. The presence of food in the stomach leads to the secretion of pepsinogen by the chief cells of the gastric mucosa. Pepsinogen is activated to form pepsin (active form) through HCl produced by parietal cells of the gastric mucosa. Pepsin is an endopeptidase. In young animals, milk-coagulating rennin is secreted into the stomach for clot formation, which aids in transport into the small intestine.

The next portion of digestion occurs in the small intestine, which plays a major role in protein digestion. The hormone secretin, in the duodenum, stimulates enzymatic secretions from the pancreas, which includes three inactive forms: trypsinogen, chymotrypsinogen, and procarboxypeptidase. Enterokinase, also secreted at the duodenum, converts trypsinogen into trypsin, which then converts chymotrypsinogen and procarboxypeptidase to their active forms—chymotrypsin and carboxypeptidase.

Digestion is finished off by other enzymes including aminopeptidases and dipeptidases from mucosal membranes. The goal of this process is to bring polypeptides down to single free amino acids.

Just like carbohydrates and fats, absorption is facilitated by the villi within the small intestine into the bloodstream. Normal free proteins are transported via active transport, energy requiring, and use sodium as a kind of cotransported molecule. Whole proteins use a direct transport method that does not require energy. Free amino acids are the major form for absorption into the circulatory system. However, some di-, tri-, and oligopeptides are also absorbed. Specific carrier proteins based on the nature of the amino acid (e.g., neutral, basic, acid, large, small) are involved in amino acid transport. The naturally occurring L-forms of amino acids are absorbed preferentially to D-forms. Some amino acids may compete with others for carrier proteins and transport. For example, arginine inhibits lysine transport and high concentrations of leucine increase the need for isoleucine. Some neutral amino acids inhibit basic amino acid transport.

The Fate of Amino Acids: Absorbed amino acids could be used for tissue protein, enzyme, and hormone synthesis and deamination or transamination, and the carbon skeleton can be used for energy. Undigested proteins in the hindgut are subjected to microbial fermentation leading to the production of ammonia and other polyamines.

Protein Digestion: Ruminants

Protein digestion in the ruminant animals can be divided into two phases: (1) digestion (degradation) in the reticulorumen and (2) digestion in the abomasum and small intestine. Therefore, in ruminant animals, dietary proteins are classified as rumen degradable and rumen undegradable proteins.

Like monogastric animals, the main goal for protein supplementation is to provide amino acids to the animal. However, in ruminants, proteins serve as a source of nitrogen for rumen microbes so they can make their own microbial protein from scratch. Microbes do not “care” where the nitrogen sources come from and can use nonprotein nitrogenous substances such as urea for microbial protein synthesis. Urea is 100% degradable in the rumen by microbial urease (can be toxic at higher levels).

Protein entering the rumen may be degraded by both bacteria and protozoa, which produce proteolytic enzymes. The rumen microbes provide proteases and peptidases to cleave peptide bonds in polypeptides to release the free amino acids from proteins. Several factors such as solubility and the physical structure of protein can affect rumen degradation. These rumen-degraded amino acids release NH3 and the C skeleton by a process called deamination. Along with volatile fatty acids (from carbohydrates), rumen microbes synthesize their own microbial protein, which serves as a primary source of protein to the host ruminant animals.

Microbial protein is enough for maintenance and survival but not for high-producing animals. Ammonia absorbed from rumen is converted to urea and secreted into the blood as blood urea nitrogen (BUN). Urea can be filtered and recycled to the rumen via saliva or through the rumen wall. The concentration of BUN in ruminants reflects the efficiency of protein utilization.

Proteins that are not degraded by rumen microbes are called escaped, “bypassed,” or “undegradable” (rumen undegradable protein, RUP), and have a low rumen degradation rates (e.g. proteins in corn).
RUP enters the abomasum and small intestine of the ruminant animal for digestion and absorption. Proteins reaching the small intestine could be RUP or those from microbial sources. The amino acid needs of the host animal are met by RUP and microbial proteins. Both ruminants and monogastrics require the essential amino acids in their diet, and amino acids cannot be stored within the body, so a constant dietary supply is necessary. Some of the similarities and differences in monogastric and ruminant animals in protein digestion or degradation are shown in the table below.

Research on “Bypass” Potential of Protein Supplements: Among the cereal grains, corn has the highest bypass potential. However, it should be noted that corn is deficient in essential amino acids such as lysine and methionine. Animal protein sources such as fish meal and meat meal have high bypass potential. Drying forages and heat treatment increases bypass potential. Feed processing methods, such as pelleting, steam rolling. or flaking, tend to denature the feed protein due to the generation of heat, thereby “protecting” the protein from lysis in the rumen. Rumen protected protein sources (through formaldehyde treatment) that remain intact in the rumen and dissolve in the abomasum are commercially available.