Mediated transport

Carrier-mediated transport involves cotransport of the absorbable species with a proton. The required proton gradient is hypothesized to be maintained by a Na+-H+ exchanger. The lumen of the intestine is acidic relative to the epithelial cell cytosol. The low cytosolic sodium concentration, required to produce the transporter driving force, is maintained by the Na K ATPase in the basolateral membrane. The sodium/proton exchanger working in concert with the sodium/potassium ATPase, therefore, results in a transport mechanism for the uptake of di- and tripeptides into the intestinal wall (Ganapthy and Leibach, 1985). 

The carriers for free amino acids are not the same as those for di- and tripeptides, and the number of different carriers and the specificity of those pathways are not known (Matthews et al, 1968). The large number of natural di- and tripeptides, furthermore, limits the ability to completely describe all existing mechanisms of carrier transport (Matthews, 1975, 1983). Through extensive study of these mechanisms (Matthews, 1991), however, a number of general guidelines have been developed. 

The carrier transport pathway is stereospecific (Matthews, 1975, 1983 Boyd and Ward, 1982 Asatoor et al, 1973 Cheeseman and Smyth, 1972) peptides of the D-configuration are handled by the transporter (Boyd and Ward, 1982 Asatoor et al, 1973 Cheeseman and Smyth, 1972) but are poorly taken up and slowly hydrolyzed (Matthews, 1975, 1983). An a-pepti bond is preferred (Matthews, 1975, 1983), though not required (Bai et al, 1991) for carrier transport, whereas methylation, acetylation, or other modification of the N-terminal a-amino group (Addison et al, 1974 Das and Radhakrishnan, 1975 Rubino et al, 1971 Addison et al, 1975), as well 

With the carrier transport pathway restricted almost exclusively to uptake of amino acids, dipeptides, and tripeptides, and passive diffusional uptake also limited mostly to smaller, and generally more lipophilic, species, the principal pathway for uptake of macromolec es in the intestine is by endocytosis. Similarly to passive absorption, endocytotic uptake is truly appreciable only in the immature intestine, decreasing significantly after gut closure (Pusztai, 1989). The capacity for uptake of macromolecules in the mature intestine, however, is still significant enough to allow antigen sampling for the development of mucosal immunity or permit appreciable absorption of various toxins. 

In the process of mediated transport, carrier proteins embedded within the plasma membrane assist in the transport of larger polar molecules into or out of the cell. When a given substance attaches to a specific binding site on the carrier protein, the protein undergoes a conformational change such that this site with the bound substance moves from one side of the plasma membrane to the other. The substance is then released. Mediated transport displays three important characteristics influencing its function  

With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+ ATPase, an intrinsic membrane protein. For each ATP molecule hydrolyzed by Na+-K+ ATPase, this pump moves three Na+ ions out of the cell and two K+ ions into it. As will be discussed further in the next chapter, the activity of this pump contributes to the difference in composition of the extracellular and intracellular fluids necessary for nerve and muscle cells to function. 

AHFS Drug Information 2000, American Society of Flealth-System Pharmacists, Bethesda, MD, 2000. 

Core Concepts in Physiology, Lippincott-Raven Publishers, Philadelphia, 1998. 

Guyton, A.C. and Hall, J.E., Textbook of Medical Physiology, 10th ed., W.B. Saunders, Philadelphia, 2000. 

A substance to be carried forms a complex with a component of the membrane on one side the complex is then carried through the membrane, the drug or substance is released, and the carrier returns to the original surface and state to repeat the process. The carrier shows specificity for instance, L-dopa but not D-dopa is transported. 

---

Clathrin-coated vesicles mediate transport within the late secretory and the endocytic pathways. Their major coat constituents are clathrin and various adaptor complexes. 

Yates CR, Chang C, Kearbey JD, Yasuda K, Schuetz EG, Miller DD, et al. Structural determinants of P-glycoprotein-mediated transport of glucocorticoids. Pharm Res 2003 20 1794-803. 

There are numerous abnormalities of cysteine metabolism. Cystine, lysine, arginine, and ornithine are excreted in cystine-lysinuria (cystinuria), a defect in renal reabsorption. Apart from cystine calculi, cystinuria is benign. The mixed disulfide of L-cysteine and L-homocysteine (Figure 30-9) excreted by cystinuric patients is more soluble than cystine and reduces formation of cystine calculi. Several metabolic defects result in vitamin Bg-responsive or -unresponsive ho-mocystinurias. Defective carrier-mediated transport of cystine results in cystinosis (cystine storage disease) with deposition of cystine crystals in tissues and early mortality from acute renal failure. Despite... 

Figure 46-6. Flow of membrane proteins from the endoplasmic reticulum (ER) to the cell surface. Horizontal arrows denote steps that have been proposed to be signal independent and thus represent bulkflow. The open vertical arrows in the boxes denote retention of proteins that are resident in the membranes of the organelle indicated. The open vertical arrows outside the boxes indicate signal-mediated transport to lysosomes and secretory storage granules. (Reproduced, with permission, from Pfeffer SR, Rothman JE Biosynthetic protein transport and sorting by the endoplasmic reticulum and Golgi. Annu Rev Biochem 1987 56 829.)...

De Jong EK, Dijkstra IM, Hensens M, et al. Vesicle-mediated transport and release of CCL21 in endangered neurons a possible explanation for microglia activation remote from a primary lesion. J Neurosci 2005 25 7548-7557. 

Fig. 9 Schematic representation depicting the movement of molecules from the absorbing (mucosal or apical) surface of the GIT to the basolateral membrane and from there to blood. (A) transcellular movement through the epithelial cell. (B) Paracellular transport via movement between epithelial cells. (Q Specialized carrier-mediated transport into the epithelial cell. (D) Carrier-mediated efflux transport of drug out of the epithelial cell. (Copyright 2000 Saguaro Technical Press, Inc., used with permission.)...

Hilgendorf, C. Spahn-Langguth,H. Regardh,C. G. Lipka, E. Amidon,G. L. Langguth, P., Caco-2 vs Caco-2/HT29-MTX co-cultured cell lines Permeabilities via diffusion, inside- and outside-directed carrier-mediated transport, J. Pharm. Sci. 89, 63-75 (2000). 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

Carrier-mediated transport is linear with mucosal solute concentration until this concentration exceeds the number of available carriers. At this point the maximal solute flux (7max) is independent of further increases in mucosal solute concentration. In the linear range of solute flux versus mucosal concentration (C), the proportionality constant is the ratio of / to the solute-carrier affinity constant (Km). This description of Michaelis-Menten kinetics is directly analogous to time changes in mass per unit volume (velocity of concentration change) found in enzyme kinetics, while here the appropriate description is the time change in solute mass per unit surface area of membrane supporting the carrier. 

Figure 13 Mediated transport kinetic scheme. C = carrier, S = solute 1 and 2 represent sides of the membrane g are rate constants for changes in conformation of solute-loaded carrier k are rate constants for conformational changes of unloaded carrier f and bt are rate constants for formation and separation of carrier-solute complex. (From Ref. 73.)...

Carrier-mediated transport of nutrients in small intestinal epithelia is often promoted by the maintenance of transmucosal ion gradients. A mathematical descrip-... 

Figure 8 A peptide carrier mediated transport to improve oral absorption.

Enalaprilat and SQ27,519 are angiotensin-converting enzyme (ACE) inhibitors with poor oral absorption. Enalapril and fosinopril are dipeptide and amino acid derivatives of enalaprilat and SQ27,519, respectively [51] (Fig. 10). Both prodrugs are converted via deesterification to the active drug by hepatic biotransformation. In situ rat perfusion of enalapril indicated a nonpassive absorption mechanism via the small peptide carrier-mediated transport system. In contrast to the active parent, enalapril renders enalaprilat more peptide-like, with higher apparent affinity for the peptide carrier. The absorption of fosinopril was predominantly passive. Carrier-mediated transport was not demonstrated, but neither was its existence ruled out. 

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

W Wang, VHL Lee. (1991). Carrier-mediated transport of peptides in the rabbit conjunctiva. Pharm Res 8(Suppl) S-129. 

Y Horibe, KJ Kim, VHL Lee. (1998). Carrier-mediated transport of monocarboxy-late drugs in the pigmented rabbit conjunctiva. Invest Ophthalmol Vis Sci 39 1436-1443. 

Describe the factors that affect mediated transport... 

FhuA-mediated transport 54-8, 56 transport across cytoplasmic membrane... 

---