Lipid membranes transducers

Use of bilayer lipid membranes as a generic electrochemical transducer is an exciting future for food biosensors. A taste sensor with multichanneled lipid membrane electrode was recently developed (93). The electric patterns generated from the sensor are similar to human response. The sensor can distinguish different brands of beer. More details on the taste sensor can be found in Chapter 16 of this book. 

One method to realize the taste sensor may be the utilization of similar materials to biological systems as the transducer. The biological membrane is composed of proteins and lipids. Proteins are main receptors of taste substances. Especially for sour, salty, or bitter substances, the lipid-membrane part is also suggested to be the receptor site [6]. In biological taste reception, taste stimulus changes the receptor potentials of taste cells, which have various characteristics in reception [7,8]. Then the pattern constructed of receptor potentials is translated into the excitation pattern in taste neurons (across-fiber-pattem theory). 

In the present study, therefore, lipid membranes were used as transducers of taste information. Artificial lipid materials, such as dioleyl phosphate (DOPH) or dioctadecyl-dimethyl-ammonium, were used to construct a lipid membrane and responses of electrical potential and resistance of the membranes were measured [9-15]. It was confirmed that the lipid membranes could discriminate five primary taste substances. Moreover, they could detect the interactions between taste substances observed in biological systems. The response properties were different in different types of lipids. If a hydrophobic part of a lipid was different, taste substances which can be detected were different. These facts indicate that the taste sensor can be realized by the use of various kinds of lipid membranes as transducers. 

Although the above lipid membranes had the ability to sense the taste by responding to many taste substances, information was insufficient to recognize quality of the taste. This weakness was overcome by means of a multichannel sensor, where transducers were composed of lipid membranes immobilized with a polymer [16-23]. We investigated responses of the sensors to various taste solutions. The electrode showed five different response patterns to five primary tastes with small experimental deviations. The patterns looked alike when the applied substance elicited the same taste in humans. 

Acetylcholineesterase Bilayer lipid membranes were prepared by adding a solution of egg phosphatidylcholine and dipalmi-toyl phosphatidic acid dropwise into the surface of aqueous 0.1 M KC1/10 mM HEPES, near the Saran Wrap partition of a two compartment plexiglass cell. A portion of AChE solution in 10 mM Tris hydrochloride buffer solution of pH 7.4 was applied. The electrolyte level was momentarily dropped below the orifice and raised to form a membrane. The membranes were used as transducers for the reaction of AChE with ACh. An external voltage (25 mV) was applied across the membrane between two Ag/AgCl reference electrodes. Enzymatically generated hydronium ion causes transient current due to alteration of the electrostatic field by the ionization of dipalmitoyl phosphatidic acid. The response delay time was directly related to the substrate concentration where acetylcholine can be determined from 1 pM upto mM level. [113]... 

Planar supported lipid membranes were first prepared and studied as simplified structural models of cell membranes [4,6, 32], and more recently as biocompatible coatings for sensor transducers and other synthetic materials [33-37], A major advantage of the planar geometry relative to vesicles, and a major contributor to the expansion of this field, is the availability of powerful surface-sensitive analyti-cal/physical techniques. Confining a lipid membrane to the near-surface region of a solid substrate makes it possible to study its structural and functional properties in detail using a variety of techniques such as surface plasmon resonance, AFM, TIRF, attenuated total reflection, and sum frequency vibrational spectroscopy [38 -2]. 

A limited improvement in this context may be possible by the use of more stable proteins, e.g. from thermophilic bacteria. However, many principles demonstrated by nature could be transposed to sensor development. For example, biomimetic channel and carrier molecules could be used in conjunction with stabilized lipid membranes to prepare sensitive and selective electrochemical transducers which embodied the principle of intrinsic amplification by depolarization. The use of artificial receptor sites would probably result in a substantial reduction of the desired selectivity coefficients, but this could easily be compensated by the application of array processing for background correction. 

Thin metal films on optical fiber transducers Lipid membrane coated piezoelectric crystals Polymeric films on ISEs ... 

Mueller and Rudin and Lev and Buzhinsky found, however, that valinomycin induced an increase in the ion permeability of black lipid membranes which obviously were free of all complicated biological transducers. An alternative mechanism was proposed to explain the activity of valinomycin in which it was suggested that valinomycin forms an ion conducting channel across the lipid bilayer This mechanism was rejected in favor of the currently accepted mobile carrier mechanism when it was demonstrated that both valinomycin and nigericin could transport ions across bulk phases too thick to accommodate stacked molecular channels The generic term ionophore, i.e. ion carrier, was chosen to describe these compounds by Pressman et al. in order to emphasize the dynamic aspects of the transport mechanism Although this term is widely accepted, Ovchinnikov... 

PKC is a family of enzymes whose members play central roles in transducing information from external stimuli to cellular responses. Members of this family of serine/ threonine kinases respond to signals that cause lipid hydrolysis. PKC isozymes phosphorylate an abundance of substrates, leading to both short-term cellular responses such as regulation of membrane transport and long-term responses such as memory and learning. 

The fluidity of lipid bilayers permits dynamic interactions among membrane proteins. For example, the interactions of a neurotransmitter or hormone with its receptor can dissociate a transducer protein, which in turn will diffuse to interact with other effector proteins (Ch. 19). A given effector protein, such as adenylyl cyclase, may respond differently to different receptors because of mediation by different transducers. These dynamic interactions require rapid protein diffusion within the plane of the membrane bilayer. Receptor occupation can initiate extensive redistribution of membrane proteins, as exemplified by the clustering of membrane antigens consequent to binding bivalent antibodies [8]. In contrast to these examples of lateral mobility, the surface distribution of integral membrane proteins can be fixed by interactions with other proteins. Membranes may also be partitioned into local spatial domains consisting of networks... 

Mechanosensitive ion channels can be looked at as membrane-embedded mechano-electrical switches. They play a critical role in transducing physical stresses at the cell membrane (e.g. lipid bilayer deformations) into an electrochemical response. Two types of stretch-activated channels have been reported the mechanosensitive channels of large conductance (MscL) and mechanosensitive channels of small conductance (MscS). 

A cell is enclosed by a lipid bilayer known as the plasma membrane. In Vignette 1.2 in Chapter 1 we discussed an example of a membrane, a complex structure with a mosaic of embedded or adsorbed moieties such as proteins. It is these membranes that protect the intracellular contents from the exterior environment of the cells and regulate the transport of materials into and out of the cells. They can also act as signal transducers and control the electrical excitation in the nervous system by altering the (membrane) permeability to particular ions in response to stimuli. Such electrical activities can propagate over long distances and represent one of the most spectacular of the membrane functions. 

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