Carrier-mediated system

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Although the absence of paracellular transport across the BBB impedes the entry of small hydrophilic compounds into the brain, low-molecular-weight lipophilic substances may pass through the endothelial cell membranes and cytosol by passive diffusion [7]. While this physical barrier cannot protect the brain against chemicals, the metabolic barrier formed by the enzymes from the endothelial cell cytosol may transform these chemicals. Compounds transported through the BBB by carrier-mediated systems may also be metabolized. Thus, l-DOPA is transported through the BBB and then decarboxylated to dopamine by the aromatic amino acid decarboxylase [7]. 

A novel electron carrier-mediator system has been devised by employing a PhCN-2,6-lutidine-Et4NBr-(C/Pt) system in the presence of -octyl methyl sulfide as a mediator. The reaction is probably initiated by the discharge of the bromide ion, which provides bromo cation or bromine. Alkoxysulfonium ions produced by the reaction of the sulfide with bromo cation... 

The passage of most foreign compounds from the blood into the liver normally is not restricted because the endothelium of the hepatic blood sinusoids behaves as a porous membrane. Hence, drugs with molecular weights lower than those of most protein molecules readily reach the hepatic extracellular fluid from the plasma. A number of compounds are taken up into the liver by carrier-mediated systems, while more lipophilic... 

If a drag is sufficiently similar to a substance naturally transported by a carrier-mediated system, the drag may also be transported by the same system. For example, the drags levodopa, methyldopa and... 

It has also been suggested that the oral mucosa contains active or carrier-mediated systems for small molecules such as mono-saccharides and amino acids. However, these processes have not been fully characterized in terms of location, transport capacity or specificity. 

As discussed above, certain nutrients are taken up into the brain by carrier-mediated systems. If a dmg possesses a molecular structure similar to that of a nutrient which is a substrate for carrier-mediated transport (Table 13.1), the pseudo-nutrient dmg may be transported across the BBB by the appropriate carrier-mediated system. For example, the dmg L-dopa crosses the BBB via the neutral amino acid carrier system. Other neutral amino acid dmgs that are transported through the BBB on this transport system are a-methyldopa, a-methylparatyrosine, and phenylalanine mustard. 

Carrier-Mediated Intestinal Transport. Various carrier mediated systems (transporters) are present at the intestinal brush border and basolateral membrane for theabsorption of specific ions and nutrients essential for the body. Many drugs are absorbed by these carriers because of the structural similarity to natural substrates. An intestinal transmembrane protein, P-Glycoprotein (F-Gp) appears to reduce apparent intestinal epithelial cell permeability from lumen to... 

Mercury is unusual in its ability to induce delayed neurological effects. This is especially prevalent with exposure to alkyl mercury compounds. In such cases, the onset of adverse effects may be delayed for months after the initial exposure. The delayed effects of methyl- and dimethylmercury reported in human poisonings are thought, in part, to result from binding to red blood cells, and subsequent slow release. Methylmercury also forms a complex in plasma with the amino acid cysteine, which is structurally similar to the essential amino acid methionine (Aschner and Clarkson 1988). Clarkson (1995) proposed that methylmercury can cross the blood-brain barrier "disguised" as an amino acid via a carrier-mediated system (i.e., transport is not solely the result of methylmercury s lipid solubility). 

Carrier-mediated intestinal transport Various carrier mediated systems (transporters) are present at the intestinal brush border and basolateral membrane for... 

The intracellular glucose is transferred to the portal capillary blood by passive diffusion and by a carrier-mediated system. Intracellular glucose can be converted to lactate... 

A distinct difference between carrier- and receptor-mediated systems should be pointed out. Carrier-mediated systems involve trans-... 

A single carrier-mediated system has been found in leaves of Egeria densa 260) and a single uptake system for all a-amino acids has been... 

Said, H.M., and Redha, R., 1987. A carrier-mediated system for transport of... 

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