Brush border enzymes, carbohydrate digestion

Final digestion of dietary carbohydrates and proteins occurs on the lumen face of small intestinal enterocytes by so-caUed brush border enzymes , including maltase, sucrose, lactase and peptidases. 

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. 

The first member of this class, acarbose, was introduced in the early 1990s. a-Glucosidase inhibitors slow the intestinal process of carbohydrate digestion by competitive inhibition of the activity of a-glucosidase enzymes located in the brush border of the enterocytes... 

The mucosa of the small intestine has an enormous surface area because of the presence of villi. Villi are covered by absorptive columnar epithelial cells whose surface is further increased by microvilli (brush border), on which carbohydrate and peptide digestive enzymes and transport processes involved in absorption are situated. Pits between the villi contain undifferentiated cells which move up the villi, mature, function for a few days and are shed into the lumen of the gut. 

Pancreatic amylase hydrolyses starches to maltose and oligosaccharides. Final digestion of carbohydrates takes place in the brush border of the epithelial cells. Oligosaccharides are hydrolysed to monosaccharides and reabsorbed. Proteins are hydrolysed to peptides by pepsin, trypsin and several other proteolytic enzymes from the pancreas. The brush border attaches proteases which hydrolyse the di- and tripeptides to amino acids and absorbs them. 

The intestinal oligo- and disaccharidases are fixed components of the cell membrane of the brush border region of the wall of the small intestine. These enzymes digest dietary carbohydrate to monosaccharides which are absorbed through the intestinal wall. They include sucrase, maltase, isomaltase, lactase, trehalase, and hetero-p-glucosidase. In the late 1970s, it was realized that inhibition of all or some of these activities could regulate the absorption of carbohydrate, and that these inhibitors could... 

Digestion of closed, right-side-out vesicles of porcine, rabbit, and rat brush-border jejunum with papain released aa-trehalase and other carbohydrate hydrolases that had been bound to the membrane without unduly affecting the lipid bilayer that limits the vesicles." The enzymes appear to be surface components attached to the outside of the membrane. 

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