Brush border membrane-bound enzymes

In Vitro Test Models for Brush Border Membrane-Bound Enzymes. .. 97... 

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. 

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. 

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... 

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. 

This enzyme and its possible functions have been recently reviewed [78], Cholesterol esterase is secreted by the pancreas into the intestinal lumen where it hydrolyzes dietary cholesterol esters. It has been suggested that the intact enzyme is subsequently bound to the brush border membrane, possibly to high-affinity binding sites, and taken up by the mucosal cell. Once inside the cell, the enzyme is supposed to re-esterify absorbed cholesterol with various free fatty acids [78]. 

Riboflavin is delivered in form of free vitamin, or as its coenzymes, i.e. flavin mononucleotide (FMN) and adenine dinucleotide (FAD), which occurs mainly as a prosthetic group of flavoproteins. Release of coenzymes from flavoproteins by acidification in stomach and proteolysis, both gastric and intestinal, must precede the absorption. This hydrolysis also releases several percentages of covalently bound FAD from 8a-(peptidyl)riboflavins (Chia et al. 1978). Free riboflavin is physiologically preferred form of absorbed vitamin B2 (Daniel et al. 1983). The upper small intestine enzymes which catalyse reversible reactions of conversion nucleotides into riboflavin are located in the brush-border membrane of enterocytes (Figure 36.1). 

The complexity of the cestode tegument was discussed in Chapter 2. The brush border-plasma membrane subtending the tegument is a fluid-mosaic , consisting of proteins, glycoproteins and lipoproteins, embedded in a lipid bilayer (Fig. 2.3, p. 10). An array of membrane-bound proteins, many of which exhibit enzyme activity, is associated with the tegumental brush border plasma membrane. This important area has been comprehensively reviewed by Pappas (624). 

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. 

---