Channels in biomembranes

Short hydrogen-bonded chains are characterized by a large proton polarizability, and they play a role of typical proton channels in biomembranes. That is why it is reasonable to assume that coherent proton transitions in the chain can be involved in the mechanism of real proton transport along the chain [327]. 

The mechanisms of the oscillations in biomembranes have been explained based on the gating of membrane protein called an ion channel, and enormous efforts have been made to elucidate the gating process, mainly by reconstitution of channel proteins into bilayer membranes [9-11]. 

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

O. S. Andersen, D. B. Sawyer and R. E. Koeppe, Modulation of channel function by host bilayers, in Biomembrane Structure and Function (eds B. Baber and K. R. K. Easwaran) p. 227. Adenine Press, Schenectady, 1992. 

Examples showing that metal speciation is important to metal toxicity include arsenic, copper, selenium, and chromium. While ionic copper (Cu2+) and CuClj are highly toxic, Q1CO3 and Cu-EDTA have low toxicity (Morrison et al, 1989). Toxicity tests show that As(III) is about 50 times more toxic than As(VI). Trivalent chromium is much less toxic than hexavalent chromium, probably because Cr(VI) is much smaller and the chemical structure of chromate is similar to sulfate. A special channel already exists in biomembranes for sulfate transport. While modeling metal speciation is not always possible, and redox equilibrium is not achieved in all natural waters, geochemical modeling of equilibrium species distribution remains one of the methods of discerning metal speciation. 

Steroidal alkaloids, such as solanine and tomatine which are present in many members of the Solanaceae, can form complexes with the cholesterol and other lipids present in biomembranes. Important for this interaction is the presence of a lipophilic portion of the molecule (given by the steroidal moiety) and a hydrophilic portion (provided by the sugar side chain). Whereas the lipophilic moiety "dives" into the lipophilic interior of the membrane and interacts with the structurally similar cholesterol, the hydrophilic side chain remains outside and binds to external sugar receptors. Since phospholipids are in a continuous motion (spinning around their axis and horizontal movements), a tension easily builds up which leads to membrane disruption i.e. transient "holes" form in the biomembrane rendering the cell leaky. Since particular steroidal alkaloids can specifically interact with receptors, ion channels or transmitter... 

We will now illustrate the above rationale in application to the sodium channels of biomembranes. Since a sodium channel under physiological conditions obeys the independence principle, one has to conclude that yNa(Na) 1. Now, consider the case where all the sodium ions in the solution have been replaced with another type of cations. Without going into details of the treatment undertaken on the basis of the experimental data reported in Reference (96) and Eqs. (123)-(125), we will only cite the distribution coefficients and relative rate constants thus obtained (Table 2). 

FIGURE 11.8 Artificial ion channels. (a,b)Calixarene rings with attached phosphate groups and long hydrocarbon chains (c) such spacer molecules forming a tailored ion channel in a biomembrane. 

The relationship between biomembranes, ion channels, and peripheral proteins needs to be addressed in studies involving the neurotoxicity of pyrethroids. The multiple target systems for pyrethroids that is presented in Table 21 indicate that these lipid soluble insecticides are well distributed in biomembranes and bind to the proteins (i.e., ion channels) associated with them. The extent of this distribution is not well known however, only 1 % of the pyrethroid concentration in nervous tissue... 

The reduction of O2 is usually believed to proceed accompanying the transfer of protons or other ions through a biomembrane [5], and the reaction rate or even the process is considered to vary depending on the kind of transferring ions. An ion channel or ion pump composed of membrane proteins has often been assumed in the explanation of the dependency of the reaction rate [5]. 

Transporters, particularly those carrying nonlipophilic species across biomembranes or model membranes, can be regarded as vectorial catalysts (and are also called carriers, translocators, permeases, pumps, and ports [e.g., symports and antiports]). Many specialized approaches and techniques have been developed to characterize such systems. This is reflected by the fact that there are currently twenty-three volumes in the Methods in Enzymology series (vols. 21,22,52-56,81,88,96-98,125-127,156-157, 171-174, and 191-192) devoted to biomembranes and their constituent proteins. Chapters in each of these volumes will be of interest to those investigating transport kinetics. Other volumes are devoted to ion channels (207), membrane fusion techniques (220 and 221), lipids (14, 35, 71, and 72), plant cell membranes (148), and a volume on the reconstitution of intracellular transport (219). See Ion Pumps... 

A variety of alkaloids bind to or intercalate with DNA or DNA/RNA processing enzymes and affect either transcription or replication (quinine, harmane alkaloids, melinone, berberine), act at the level of DNA and RNA polymerases (vinblastine, coralyne, avicine), inhibit protein synthesis (sparteine, tubulosine, vincrastine, lupanine), attack electron chains (pseudane, capsaicin, solenopsine), disrupt biomembranes and transport processes (berbamine, ellipticine, tetrandrine), and inhibit ion channels and pumps (nitidine, caffeine, saxitoxin). In addition, these natural products attack a variety of other systems that can result in serious biochemical destabilization... 

The result described in this section suggests that the ion transport from Wl to W2 at a special region of a membrane that resembles the transport at a biomembrane with an ion channel can be realized even in the absence of any chaimel proteins. 

In the following, a unique oscillation of membrane current observed with a liquid membrane system by the present authors [32,37], which has characteristics similar to those of the oscillation at a biomembrane with so-called sodium channel , will be introduced as an example, and the mechanisms for the oscillation will be clarified by using VCTIES, taking into consideration ion transfer reactions and adsorptions at two W/M interfaces in the membrane system. In this connection, various oscillations other than this example and the elucidation of their mechanisms were described elsewhere [32,37,38]. 

---