Brush border plasma membrane

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). 

Characterization of cobalamin receptor sites in brush-border plasma membranes of the tapeworm Spirometra mansonoides. Journal of Biological Chemistry, 258 4261-5. 

Knowles, W. J. Oaks, J. A. (1979). Isolation and partial biochemical characterization of the brush border plasma membrane from the cestode Hymenolepis diminuta. Journal of Parasitology, 65 715-31. 

Hydrolysis of sucrose occurs rapidly within the outer portion of the brush-border, plasma membrane, whereupon most of the monosaccharides released are transported across a permeability barrier located in the inner portion of the same membrane. Some hydrolytic products diffuse backward from the mucosal site of formation, and accumulate in the lumen.5-8 The mucosal site of hydrolysis is identified by the use of fluorescent antibodies against sucrase8 and by differential centrifugation of intestinal preparations.2... 

The uptake of lipids across the brush border plasma membrane into the enterocyte is considered to be a transport process that requires no energy [8], The route of transfer of lipids thus has to take place via a process of passive diffusion. A theoretical model for passive lipid solute transfer that takes into account the different factors that affect the rate of transport has been worked out by Dietschy and collaborators [8,68] (cf. Chapter 5). 

Fig. 1. Profile of the concentration gradient (C,-C4) from the luminal bulk phase (L) across the brush border plasma membrane (M) to the intracellular compartment (IC) of the enterocyte. Adjacent to the membrane, on both the luminal and the intracellular side there is an unstirred water layer (UWL). It should be noted that this diagram does not attempt to present the relative dimensions of the two unstirred water layers and the plasma membrane. The concentration gradient will have a different appearance in the case of a lipid towards which the membrane permeability is low (panel A) compared to the case where the resistance of the unstirred water layer against diff.ision of the lipid is high while the lipid readily transverses the plasma membrane (panel B). After Thomson and Dietschy [8).

Walker, J. and Barrett, J. (1993) Evidence for a G protein system in the tegumental brush border plasma membrane of Hymenolepis diminuta. Int. J. Parasitol. 23 281-284. 

Figure 9 Handling of 6-PG in renal proximal tubular cells. Abbreviations BBM, brush border plasma membrane BLM, basolateral plasma membrane 6-MP, 6-mercaptopurine 6-PG, S-(6-purinyl) glutathione 6-PC, S-(6-purinyl)-L-cysteine NAcPC, w-acetyl-S-(6-purinyl)-L-cysteine GGT, y-glutamyltransferase DP, dipeptidase P-lyase, cysteine conjugate P-lyase PLP, pyridoxal phosphate XO, xanthine oxidase 6-ThXan, 6-thioxanthine 6-ThUrate, 6-thiourate AOAA, aminooxyacetic acid and Av /, membrane potential. (Adapted from Ref. 30.)...

Crane (1966) has theorized that the brush border plasma membrane is the site of a mosaic of the enzymes associated with the microvillus. This was based on experiments of Eichholz and Crane (1965) who recovered a fraction of pure microvillous membranes by density gradient centrifugation of brush border homogenate which possessed the total activities of alkaline phosphatase, maltas and sucrase and various peptidases. Evidence in favor of this idea was also collected by Johnson (1967) who demonstrated the presence of knobs 60 A in diameter on the glycocalyx of the luminal side of the plasma membrane which contained the brush border invertase and maltase (see Fig. 6). These knobs could be removed entirely from the microvilli of hamster intestine by papain digestion the remaining membrane, however, still has the alkaline phosphatase incorporated into it (Eichholz, 1969 Oda and Seki, 1966). 

The apical plasma membrane of epithelial cells of small intestinal and renal proximal tubules is characterised by the presence of many microvilli (brush border). These membranes can be isolated relatively easily by centrifugation and free flow electrophoresis techniques. Kinne-Saffran and Kinne [15] found that after free-flow electrophoresis of a rat kidney-cortex membrane preparation, the anion-sensitive ATPase co-migrated with the alkaline phosphatase activity but was separated from the (Na + K )-ATPase activity, which is assumed to be a marker of basolateral plasma membranes. This suggests that the brush-border membrane of the proximal tubule contains an anion-sensitive ATPase. The same conclusion was reached by Liang and Sacktor [17] for a brush-border preparation from rabbit kidney. 

Biochemical studies of plasma membrane Na /H exchangers have been directed at two major goals (1) identification of amino acids that are involved in the transport mechanism and (2) identification and characterization of the transport pro-tein(s). To date, most studies have been performed on the amiloride-resistant form of Na /H" exchanger that is present in apical or brush border membrane vesicles from mammalian kidney, probably because of the relative abundance of transport activity in this starting material. However, some studies have also been performed on the amiloride-sensitive isoform present in non-epithelial cells. 

EPEC causes a degeneration of the microvillus brush border, with cupping and pedestal formation of the plasma membrane at the sites of bacterial attachment and reorganization of cytoskeletal proteins [43, 44], Invasion has been observed in some clinical specimens, but the mechanism of how this bacteria produces diarrhea is not fully understood. Some possibilities include an increase in permeability and loss in microvilli leading to malabsorption. 

It is contended that the renal slice technique measures primarily basolateral uptake of substrates or nephrotoxins, based on histological evidence of collapsed tubular lumens. This results in the inaccessibility of brush-border surfaces for reabsorptive transport (Burg and Orloff, 1969 Cohen and Kamm, 1976). This observation limits the ability of this model to accurately reflect reactions to nephrotoxins that occur as the result of brush-border accumulation of an injurious agent. Ultrastructurally, a number of alterations, particularly in the plasma membrane and mitochondrial compartments, have been shown to occur over a 2-h incubation period (Martel-Pelletier et al., 1977). This deterioration in morphology is very likely a consequence of the insufficient diffusion of oxygen, metabolic substrates, and waste products in the innermost regions of the kidney slice (Cohen and Kamm, 1976). Such factors also limit the use of slices in studying renal metabolism and transport functions. 

The mechanism of iron and heme uptake by the intestine is becoming better understood 70-72), but clearly heme-iron is more efficiently absorbed from the gastrointestinal tract than inorganic iron 73-75), and there is a receptor for heme in the duodenal brush border membrane 76). Duodenal mucosal cells efficiently catabolize heme, and iron-transferrin can be detected in the plasma of blood vessels draining the intestinal segment shortly after Fe—heme—histidine is administered (75). 

The resorption process is facilitated by the large inner surface of the intestine, with its brush-border cells. Lipophilic molecules penetrate the plasma membrane of the mucosal cells by simple diffusion, whereas polar molecules require transporters (facilitated diffusion see p. 218). In many cases, carrier-mediated cotransport with Na"" ions can be observed. In this case, the difference in the concentration of the sodium ions (high in the intestinal lumen and low in the mucosal cells) drives the import of nutrients against a concentration gradient (secondary active transport see p. 220). Failure of carrier systems in the gastrointestinal tract can result in diseases. 

FIGURE 1.6 The glycocalix, G, of the intestinal brush border. MV, microvillous PM, plasma membrane. (From Wheater, P.R. et al., Wheater s Functional Histology, A Text and Colour Atlas, 3rd ed., Churchill Livingstone, Edinburgh, 1993. With permission.)... 

---