Protein digestion and absorption

Table 10.2 Protein digestion and absorption by sheep given 800 g organic matter per day from one of two types of ryegrass...

London began with a study of protein digestion and absorption, and he always reported the amounts of protein and its breakdown products, albuminoses and peptones, in terms of the amount of total nitrogen each contained. He, or more likely some unnamed technicians, determined total nitrogen by the Kjeldahl method. 

Section VI consists of discussions of eleven special topics nutrition, digestion, and absorption vitamins and minerals intracellular traffic and sorting of proteins glycoproteins the extracellular matrix muscle and the cy-toskeleton plasma proteins and immunoglobulins hemostasis and thrombosis red and white blood cells the metabolism of xenobiotics and the Human Genome Project. 

Digestion and absorption in the small intestine. Most digestion and absorption of carbohydrates, proteins, and lipids occurs in the small intestine. A summary of the digestive enzymes involved in these processes is found in Table 18.3. 

The digestion and absorption of fat is considerably more complex than that of carbohydrate or protein because it is insoluble in water, whereas almost aU enzymes catalyse reactions in an aqueous medium. In such media, fat can form small droplets, an emulsion, which is stable in this medium. Formation of an emulsion is aided by the presence of detergents these possess hydrophobic and hydrophilic groups, so that they associate with both the fat and the aqueous phases. Such compounds are known as emulsifying agents and those involved in digestion are mainly the bile salts and phospholipids. 

Abnormalities in protein digestion In individuals with a deficiency in pancreatic secretion (for example, due to chronic pancreatitis, cystic fibrosis, or surgical removal of the pancreas), the digestion and absorption of fat and protein is incomplete. This results in the abnormal appearance of lipids (called steatorrhea, see p. 175) and undigested protein in the feces. 

These agents are administered to aid in the digestion of food. The primary digestant preparations contain pancreatic enzymes or bile salts. Pancreatic enzymes such as amylase, trypsin, and lipase are responsible for digestion of carbohydrates, proteins, and lipids, respectively. These enzymes are normally synthesized in the pancreas and secreted into the duodenum via the pancreatic duct. Bile salts are synthesized in the liver, stored in the gallbladder, and released into the duodenum via the common bile duct. Bile salts serve to emulsify lipids in the intestinal tract and are important in lipid digestion and absorption. 

Effect of a Glucose-Lysine Reaction Mixture on Protein and Carbohydrate Digestion and Absorption... 

Availability of Starting Materials for Maillard Reaction in Vivo Not all starting materials present in the body are available for Maillard reaction. For example, trypotophan in blood, formed by digestion and absorption of tryptophan-rich proteins, is easily bound by plasma albumin, making it unavailable for further transformation. 

FIG. 1 Digestion and absorption of proteins in the small intestine. (1) Brush-border peptidases, (2) brush-border amino acid transport systems, (3) brush-border peptide transport systems, (4) cytoplasmic peptidases, (5) basolateral amino acid transport systems, (6) basolateral peptide transport systems. 

The most abundant amino add in the human organism does not occur in proteins and does not have a carboxyl group. Its addic residue is the sulfonate group, and its name is taurine (N+H3-CH2-CH2-S03 ). It occurs in the free state (exact function often unknown) and in bile salts, in which it plays an important role in fat digestion and absorption (see Chapters 9 and 19). Other amino acids that do not occur in proteins are ornithine and citrulline. They are important intermediates in the urea cycle described in Chapter 20. 

The fourth section deals with various aspects Digestion, Absorption, and Nutritional Biochemistry. The chapter Obesity considers current problems with respect to the ever-increasing incidence of imbalance between energy intake and utilization. Key problems of undemutrition are discussed in the chapters Protein-Energy Malnutrition and Vitamin A Deficiency in Children. The chapters Lactose Intolerance, Pancreatic Insufficiency, and Abetalipoproteinemia focus on the biochemical processes underlying food digestion and absorption. Calcium Deficiency Rickets, Vitamin B12 Deficiency, and Hemochromatosis provide discussions of absorption and utilization of vitamin D, vitamin B12, and iron, respectively. 

Malabsorption is defined as an inadequate assimilation of dietary substances due to defects in digestion, absorption or transport. Malabsorption can affect macronutrients (proteins, carbohydrates, fats), micronutrients (vitamins, minerals) or both, causing excessive faecal excretion and producing nutritional deficiencies and GI symptoms. Digestion and absorption occur in three phases, namely (i) the intra-lumen hydrolysis of fats, proteins and carbohydrates by enzymes, and emulsification by bile salts, (ii) digestion by brush-border enzymes and uptake of end-products and (iii) lymphatic transport of nutrients. Malabsorption can occur when any of these phases is impaired. 

Figure 23.1. Digestion and Absorption of Proteins. Protein digestion is primarily a result of the activity of enzymes secreted by the pancreas. Aminopeptidases associated with the intestinal epithelium further digest proteins. The amino...

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