Intestinal membrane-bound enzymes

Four major enzyme groups are secreted lipolytic, proteolytic, amylolytic, and nucleic acid splitting enzymes. These pancreatic enzymes, some of which are secreted in multipile forms, possess specificities complementary to die intestinal membrane-bound enzymes (Tabic 1). Fresh, uncontsnkinated pancreatic juice is without proteolytic activity because these enzymes am in the form of inactive zymogens. An important fraction of the calcium in pancreatic juice accompanies the enzymes, especially ct-amylase. Human pancreatic juice is moat dose to that of the pig, with high proportions of lipase and a-amylase in comparison with other mammals [1]. Therefore, pig pancreas extract, pancreatin, has up to now been die oreferred enzvme source for therapeutic tuncreas substitution. 

These zinc-dependent endopeptidases (meprin A [EC 3.4.24.18] and meprin B [EC 3.4.24.63] ) are members of the peptidase family M12A. They catalyze the hydrolysis of peptide bonds in proteins and peptide substrates. Meprin A, a membrane-bound enzyme that has been isolated from mouse and rat kidney and intestinal brush borders as well as salivary ducts, acts preferentially on carboxyl side of hydrophobic amino acyl residues. Meprin A and B are insensitive to inhibition by phosphora-midon and thiorphan. 

Mucoadhesive polymers exhibiting strong complexing properties are capable of inhibiting intestinal brush border membrane-bound proteases through a far distance inhibitory effect [65]. In vivo, the mucoadhesive polymer is separated from the brush border membrane by a mucus layer [30]. Although there is no direct contact between polymer- and membrane-bound enzymes, it could be shown that inhibition takes place. The exploitation of this far distance effect seems to be a very promising alternative to small molecular mass inhibitors, which are currently used as inhibitors of brush border membrane-bound proteases. 

A small fraction of the polypeptides contacting the brush border of the small intestine is hydrolyzed by membrane-bound enzymes attached to the outside of the enteroey te. Although a variety of peptidases are bound to the brush border, the most abundant is amlnopeptidase N, which catalyzes the hydrolysis of amino... 

As shown in Tables 2,4, 2.5, and 2,6, fruits and vegetables are excellent sources of carbohydrates. Sucrose accounts for about half the solid material of a beet or peach Suciase, a membrane-bound enzyme of the small intestine, catalyzes the hydrolysis of sucrose, yielding one molecule each of glucose and fructose. Meat is a poor source of carbohydrate. However, milk, an animal product, has a high... 

P-Glycosidase A bifunctional, membrane-bound enzyme located on the brush-border membrane of the small intestine. This single polypeptide enzyme has two activities, lactase and glycosylceramidase, located in different domains of the protein. It will hydrolyze lactose to glucose and galactose. 

Note that we have invoked lipase action several times but we have referred each time not to lipase, but to a lipase. This is because it is a different lipase each time. They all carry out the same chemical job, but they have to work in different places under different conditions - the intestinal one is a secreted enzyme working free in the intestinal juice, whereas the lipase at the surface of adipose tissue is a membrane-bound enzyme. Similarly, the lipases of adipose tissues are tailored for their separate jobs by being structured to respond to different physiological regulatory signals. Thus, the lipase involved in fatty acid mobilisation, for instance, is known as hormone-sensitive lipase . 

After the reduction of biliverdin, the bilirubin that is formed then undergoes a series of transport and transformation steps which ultimately lead to its excretion in the intestinal tract. From the sites of its production, bilirubin is released into the plasma where it efficiently binds to albumin, which acts as a plasma-transport system. The bilirubin-albumin complex is carried in the plasma to liver cells (hepatocytes), where the bilirubin is released from its albumin carrier protein and transported across the cell bilayer membrane into the hepatocyte. Once inside, the bilirubin is bound in the cytoplasm to anion-binding proteins such as ligandin. The latter carries the bilirubin to membrane-bound enzymes (localised in the endoplasmic reticulum of the liver cell) which catalyse the esterification of bilirubin, the ester groups (mainly p-D-glucuronoside, but also smaller amounts of fi-D-xylo-pyranosides and p-D-glucopyranosides) being transferred from their uridine diphosphate nucleotides. 

Alkaline phosphatase, a dimeric glycoprotein that catalyses the hydrolysis of orthophosphoric monoesters, is a membrane-bound enzyme comprising four isoenzymes encoded by different structural genes tissue non-specific, intestinal, placental and germ-cell (Moss 1992). 

Diabetic patients have reduced antioxidant defences and suffer from an increased risk of free radical-mediated diseases such as coronary heart disease. EC has a pronounced insulin-like effect on erythrocyte membrane-bound acetylcholinesterase in type II diabetic patients (Rizvi and Zaid, 2001). Tea polyphenols were shown to possess anti-diabetic activity and to be effective both in the prevention and treatment of diabetes (Choi et al, 1998 Yang et al, 1999). The main mechanism by which tea polyphenols appear to lower serum glucose levels is via the inhibition of the activity of the starch digesting enzyme, amylase. Tea inhibits both salivary and intestinal amylase, so that starch is broken down more slowly and the rise in serum glucose is thus reduced. In addition, tea may affect the intestinal absorption of glucose. 

Role of PI in membrane protein anchoring Specific proteins can be covalently attached via a carbohydrate bridge to membrane-bound PI (Figure 17.9). [Note Examples of such proteins include alkaline phosphatase (a digestive enzyme found on the surface of the small intestine that attacks organic phosphates), and acetylcholine esterase (an enzyme of the postsynaptic membrane that... 

NU is membrane-bound and can be found in the liver, brain, heart, blood vessels, pancreas and intestine. It is not yet understood why this enzyme does not increase in bone disease, periods of growth or pregnancy. Even in hepatocellular damage (e.g. acute or chronic hepatitis), it is also normal or only slightly elevated. (38)... 

A variety of membrane-bound proteins are of vital interest to the medical and nutritional ientist, because defects or changes in these proteins can cause such problems as lactose intolerance, cardiovascular disease, cystic fibrosis, and diabetes. Sucrase-isomaltase, an enzyme of the small intestine, is a membrane-bound protein, bound to the plasma membrane of the cnterocyte (gut cell). Part of the production of this enzyme is depicted in Figure 1,26. in Step 1, the polypeptide chain is polymerized on the ribosome (shown in black). In Step 2, part of the amino add chain near the N terminus crosses the membrane of the ER into the lumen but some of the amino acids at the N terminus remain outside. Step 3 shows the protein assuming a three-dimensional shape within the lumen both the C and N... 

---