Active transport, biological membrane

Such coupling is possible in anisotropic media, such as in active transport at membranes in biological systems. 

There are three major points to be stressed. First, the Hquid/ceUular interface may contribute significantly to mass transfer limitations. Second, when mass transfer Hmitations exist the intrinsic biokinetics parameters cannot be determined. In biochemical reactor design, intrinsic parameters are essential to model adequately the system performance. Furthermore, without an understanding of the intrinsic biokinetics, one cannot accurately study transport mechanisms across biological membranes. The determination of passive or active transport across membranes is strongly affected by the extent of the Hquid/ceUular interfacial resistance. 

Care should be exercised when attempting to interpret in vivo pharmacological data in terms of specific chemical—biological interactions for a series of asymmetric compounds, particularly when this interaction is the only parameter considered in the analysis (10). It is important to recognize that the observed difference in activity between optical antipodes is not simply a result of the association of the compound with an enzyme or receptor target. Enantiomers differ in absorption rates across membranes, especially where active transport mechanisms are involved (11). They bind with different affinities to plasma proteins (12) and undergo alternative metaboHc and detoxification processes (13). This ultimately leads to one enantiomer being more available to produce a therapeutic effect. 

A consequence of this theoretical approach which includes kinetic parameters is the establishment and coupling of certain ion fluxes across the phase boundary (equality of the sum of cathodic and anodic partial currents leading to a mixed potential). If a similar approach can be applied to asymmetric biological membranes with different thermodynamic equilibrium situations at both surfaces, the active ion transport could also be understood. 

Kostyuk, P. G. Electrical events during active transport of ions through biological membranes, in Topic in Bioelectrochemistry and Bioenergetics, Vol. 2, (ed.) Milazzo, G., New York, Wiley 1978... 

Active Transporters use the energy of ATP for vectorial transport through a biological membrane against concentration gradient of the transported substrate. 

Eischer, H. Passive diffusion and active transport through biological membranes - binding of drugs to... 

In addition to the passive diffusional processes over lipid membranes or between cells, substances can be transferred through the lipid phase of biological membranes through specialized systems, i.e., active transport and facilitated diffusion. Until recently, the active transport component has been discussed only for nutrients or endogenous substances (e.g., amino acids, sugars, bile acids, small peptides), and seemed not to play any major role in the absorption of pharmaceuticals. However, sufficient evidence has now been gathered to recognize the involvement of transporters in the disposition of pharmaceuticals in the body [50, 127]. 

The liver plays an important role in determining the oral bioavailability of drags. Drag molecules absorbed into the portal vein are taken up by hepatocytes, and then metabolized and/or excreted into the bile. For hydrophilic drugs, transporters located on the sinusoidal membrane are responsible for the hepatic uptake [1, 2]. Biliary excretion of many drags is also mediated by the primary active transporters, referred to as ATP-binding cassette transmembrane (ABC) transporters, located on the bile canalicular membrane [1, 3-5], Recently, many molecular biological... 

---