Brush-Border transporters

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Some Brush-Border Transporters Involved in Intestinal Absorption Processes, Pertinent to Gl Drug Absorption... 

Tsuji Tsuji A, Tamai I, Nakanishi M et al. (1993) Intestinal brush-border transport of the oral cephalosporin antibiotic, cefdinir, mediated by dipeptide and monocarboxylic acid transport systems in rabbits. J Pharm Pharmacol 45(11) 996—998... 

In eukaryotes there is also evidence that Met(O) is actively transported. It has been reported that Met(O) is transported into purified rabbit intestinal and renal brush border membrane vesicles by a Met-dependent mechanism and accumulates inside the vesicles against a concentration gradient102. In both types of vesicles the rate of transport is increased with increasing concentrations of Na+ in the incubation medium. The effect of the Na+ is to increase the affinity of Met(O) for the carrier. Similar to that found in the bacterial system, the presence of Met and other amino acids in the incubation medium decreased the transport of Met(O). These results suggest that Met(O) is not transported by a unique carrier. 

As might be expected, mRNA for the 5-HT transporter is found in high concentrations in the Raphe nuclei but it is also found in other brain regions. Whether this means that non-5-HT neurons can synthesise this protein is unknown but there is some evidence that it is synthesised in astrocytes, at least. One complication is that there are multiple forms of mRNA for the 5-HT transporter, but there is, as yet, no evidence for transporter subtypes in the CNS. However, it must also be remembered that 5-HT transporters are found in the peripheral tissues, notably platelets, mast cells, the placental brush-border and adrenal chromaffin cells and it is possible that these are not all identical. 

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. 

Further evidence that carboxyl groups are important for transport activity was provided by Igarashi and Aronson [22], Friedrich et al. [23], and Kinsella et al. [24] using the carboxyl group-specific reagent, A,A -dicyclohexylcarbodiimide (DCCD). DCCD irreversibly inactivated the brush border Na /H exchanger in rabbit and... 

Na oi and was partially blocked by amiloride but not by cimetidine. Since these investigators also found that amiloride and cimetidine bound competitively with Na" at the external transport site of the placental brush border Na /H exchanger, they concluded that the vicinal dithiol groups are necessary for transport function but are located at a site distinct from the external transport site. 

Igarashi and Aronson [22] found that the renal brush border Na /H exchanger (resistant-type) was inhibited 40% by 1 mM NEM, and inhibition was not blocked by 1 mM amiloride. Haggerty et al. [13] reported that both the apical and basolat-eral Na /H exchangers in LLC-PKi cells were inactivated by 0.5mM NEM, although the apical Na /H exchanger was more sensitive to inhibition (70% inhibition compared to 20% inhibition of the basolateral transport activity). 

Candidates for the renal brush border Na /H exchanger transport protein identified by covalent labeling, affinity chromatography, or other methods... 

A35 affinity matrix, and eluted with various media. A 25-kDa protein bound to the affinity matrix and was completely eluted with 5 mM free amiloride. The abundance of the 25-kDa protein in brush border and basolateral membranes correlated closely with Na /H exchange activity. Importantly, binding of the 25-kDa protein to the affinity matrix was blocked by MIA > amiloride > benzamil, a rank order identical to that for inhibition of Na /H exchange activity, which suggested strongly that the 25-kDa protein was a structural component of the transporter. 

Whereas the above studies have attempted to identify the Na /H exchanger in renal brush border membranes (a resistant-type), at least one study has reported possible identification of a sensitive-type transport protein [49]. The Na /H exchanger in lymphocytes (a sensitive-type) can be activated by either 12-0-tetradeca-noylphorbol 13-acetate (TPA) or osmotic shrinkage. TPA or osmotic shrinkage... 

A putative regulatory cofactor has been identified for the renal brush border Na /H exchanger (resistant-type) [50]. Morell et al. [50] identified a 42-kDa protein that was distinct from the transporter itself and appeared to be involved in regulation by cAMP-dependent protein kinase (PKA). Evidence supporting this conclusion was ... 

Mucosal brush border membrane vesicles and basolateral membrane vesicles can be isolated to study solute uptake across specific enterocyte boundaries. These more isolated vesicle systems allow for investigation of solute transport across a particular membrane barrier and permit separation of membrane trans-... 

N Piyapolrungroj, C Li, RL Pisoni, D Fleisher. Cimetidine transport in brush-border membrane vesicles from rat small intestine. J Pharmacol Exp Ther 289 346-353, 1999. 

RD Gunther, EM Wright. (1983). Na+, Li+, and CD transport by brush border membranes from rabbit jejunum. J Membr Biol 74 85-94. 

V Ganapathy, FH Leibach. (1990). Peptide transport in intestinal and renal brush border membrane vesicles. Life Sci 30 2137-2146. 

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