Protein Digestion and Absorption

Except for the absorption of intact immunoglobulins by the small intestine of mammalian neonates, dietary proteins have no nutritional values until they are hydrolyzed to short-chain peptides and free amino acids in the digestive tract. In nonruminants, the digestion starts in the stomach (pH=approximately 2 3), where protein is denatured by hydrochloric acid, followed by digestion with proteases (pepsins A, B, and C, and renin). The resulting large peptides enter the small intestine to be further hydrolyzed by proteases (including trypsin, chymotrypsin, elastase, carboxyl peptidases, and amino-peptidases) in an alkaline medium (owing to bile salts, pancreatic juice, and duodenal secretions). These enzymes release small peptides and considerable amounts of free amino acids. Oligopeptides composed of more than three amino acid residues are further hydrolyzed extracellularly by peptidases (located mainly on the brush border of enterocytes, and to a lesser extent, in the intestinal lumen) to form tripeptides, dipeptides, and free amino acids. Major mechanisms for the intestinal absorption of amino acids include both Na+-dependent and Na+-independent systems. Dipeptides and tripeptides are absorbed intact into enterocytes of the small intestine

Table l Roles of proteins in animals

Roles

Examples of proteins

Muscle contraction

Actin, myosin, tubulin

Enzyme catalyzed

Dehydrogenase,

reactions

kinase, synthase

Gene expression

DNA binding proteins,

histones, repressor proteins

Hormone mediated

Insulin, somatotropin,

effects

placental lactogen

Protection

Blood clotting factors,

immunoglobulins, interferon

Regulation

Calmodulin, leptin, osteopontin

Storage of

Ferritin, metallothionein, myoglobin

nutrients and O2

Cell structure

Collagen, elastin, proteoglycans

Transport of

Albumin, hemoglobin,

nutrients and O2

plasma lipoproteins

through H+-gradient-driven peptide transporters. Once inside enterocytes, peptides are hydrolyzed by peptidases to form free amino acids. The small intestine transports short-chain peptides (2 3 amino acid residues) at a faster rate than free amino acids.

In ruminants, dietary protein is hydrolyzed by ruminal microbial proteases to form small peptides and free amino acids.[2] Amino acids are further degraded to form ammonia, short-chain fatty acids, and CO2. Small peptides, amino acids, and ammonia are utilized by microorganisms in the presence of adequate energy supply (carbohydrates) to synthesize new amino acids, protein, nucleic acids, and other nitrogenous substances. The most important initial reaction for microbial ammonia assimilation is catalyzed by glutamate dehy-drogenase to produce glutamate, which is then utilized to synthesize glutamine, alanine, aspartate, and asparagine by glutamine synthetase, glutamate-pyruvate transaminase, glutamate-oxaloacetate transaminase, and aspara-gine synthetase, respectively. These amino acids serve as substrates for the synthesis of all other amino acids by microorganisms in the presence of sulfur and adenosine 5'-triphosphate (ATP). Ruminal protozoa cannot utilize ammonia, but derive their nitrogen by engulfing bacteria and digesting them with powerful intracellular proteases. Ammonia that cannot be fixed by ruminal microorganisms is absorbed into blood for conversion into urea via the hepatic urea cycle and may be utilized by ruminal epithelial cells for biosynthetic processes. Microbial cells (bacteria and protozoa) containing proteins and amino acids, as well as undigested dietary proteins, leave the reticulorumen and omasum, and enter the abomasum and small intestine, where digestion of protein is similar to that in nonruminants.

Protein Metabolism

In both nonruminants and ruminants, there is extensive first-pass intestinal catabolism and/or utilization of the amino acids absorbed from the lumen of the small intestine, which substantially reduces their availability to extraintestinal tissues and selectively alters the patterns of amino acids in the portal vein.[3] Amino acids that enter systemic circulation may be oxidized to provide ATP and/ or utilized to synthesize glucose, ketone bodies, protein, urea, uric acid, and other nitrogenous substances.

Dietary protein and energy intake regulate intracellular protein synthesis and degradation (protein turnover). At least four and two ATP molecules, respectively, are required to incorporate one amino acid into a peptide and to hydrolyze one peptide bond. Intracellular protein turnover accounts for approximately 15% and 20% of total energy expenditure in adult and growing animals, respectively. Whereas protein synthesis is well-characterized, the pathways for intracellular protein degradation are less understood.[1] Lysosomal proteases and cytosolic calpains (Ca2+-dependent proteases) contribute substantially to the degradation of long-lived, endocytosed, and myofibrillar proteins. Proteasome (a multisubunit protease complex) selectively degrades intracellular proteins via the ubiquitination pathway. Protein half-lives, which range from <30 min for ornithine decarboxylase to >50 200 h for lactate dehydrogenase, are determined by N-terminal residue and physicochemical properties of a given protein.

Protein Requirements

Proportions of dietary amino acids have a profound impact on the food intake, growth, and health of animals. A limiting amino acid (one that is in the shortest supply from the diet relative to its requirement by animals) impairs the utilization of dietary protein. Likewise, an amino acid imbalance (disproportions of dietary amino acids) reduces the feed intake and growth of animals. Amino acid imbalances may occur among amino acids regardless of their structure and can be prevented by addition of one or more of the limiting amino acids to the diet. Also, an amino acid antagonism (growth depression caused by an excessive intake of an amino acid) commonly occurs among structurally related amino acids (e.g., lysine-arginine, leucine-isoleucine-valine, and thre-onine-tryptophan) but can be overcome by addition of a structurally similar amino acid. Thus, determining optimal amino acid patterns in the diet is very beneficial.

Nitrogen balance studies and growth trials have long been used to determine amino acid and protein requirements of animals.[4] Minimal requirements can also be

Table 2 Nutritionally essential and nonessential amino acids in monogastric animals

Monogastric mammals Poultry

Table 2 Nutritionally essential and nonessential amino acids in monogastric animals

Monogastric mammals Poultry

EAA

NEAA

EAA

NEAA

Argininea

Alanine

Arginine

Alanine

Histidine

Asparagine

Glycine

Asparagine

Isoleucine

Aspartate

Histidine

Aspartate

Leucine

Cysteine

Isoleucine

Cysteine

Lysine

Glutamate

Leucine

Glutamate

Methionine

Glutamine

Lysine

Glutamine

Phenylalanine

Glycine

Methionine

Serine

Threonine

Prolineb

Phenylalanine

Tyrosine

Tryptophan

Serine

Proline

Valine

Tryptophan

Valine

aArginine may not be required in the diet to maintain nitrogen balance in most adult mammals but its deficiency in the diet may result in metabolic, neurological, or reproductive disorders. Proline is an essential amino acid for young pigs.

aArginine may not be required in the diet to maintain nitrogen balance in most adult mammals but its deficiency in the diet may result in metabolic, neurological, or reproductive disorders. Proline is an essential amino acid for young pigs.

estimated by factorial analysis; namely, the sum of fecal and urinary nitrogen in response to a protein-free diet (maintenance), nitrogen deposited in the body, and nitrogen excreted as animal products (e.g., milk, egg, wool, fetus growth). Most recently, direct and indirect (indicator) amino acid oxidation techniques involving radioisotopes or stable isotopes have been developed to estimate requirements of protein and essential amino acids by animals.

Amino acids are traditionally classified as nutritionally essential (indispensable) or nonessential (dispensable), on the basis of whether they need to be supplied in the diet to maintain nitrogen balance or support the maximal growth of animals (Table 2). Essential amino acids are defined as either those amino acids whose carbon skeletons cannot be synthesized by animals or those that are inadequately synthesized in animals relative to needs, and which must be provided from the diet to meet requirements for maintenance, growth, and reproduction. Conditionally essential amino acids are those that normally can be synthesized in adequate amounts by animals, but which must be provided from the diet under conditions where rates of utilization are increased relative to rates of synthesis. Nonessential amino acids are the amino acids whose carbon skeletons can be synthesized in adequate amounts by animals to meet requirements.

Collectively, an ideal protein in the diet would consist of an optimal pattern among essential amino acids that corresponds to an animal's needs. Thus, ideal proteins would likely vary with nutritional and physiological needs, including maintenance, protein accretion, egg and wool production, reproduction, and lactation. Because of extensive catabolism of amino acids by the small intestine, the pattern among amino acids in animal tissues or products is not necessarily similar to that in the diet. Thanks to Baker's seminal work,[5] the concept of ideal protein has gained acceptance for formulating swine and poultry diets in the United States and worldwide.

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