Whole cells

Figure Bl.19.4. (a) Local conductance STS measurements at specific points within the Si(l 11)-(7 x 7) unit cell (symbols) and averaged over whole cell, (b) Equivalent data obtained by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPS). (Taken from [19], figure 2.)...

Research conducted by Simons using antiglucocorticoids, including compounds which covalentiy bind to the GR (124), eg, dexamethasone 21-mesylate, has better defined the stmcture and function of the GR. Spiro C-17 oxetanes have shown potent antiglucocorticoid activity in whole cell systems (125,126). 

H. G. Davies, R. H. Green, D. R. Kelly, and S. M. Roberts, Bio-Transformations in Preparative Organic Chemisty The Use of Isolated Enymes and Whole-Cell Systems in Synthesis, Academic Press, London, 1989. 

Direct quantitation of receptor concentrations and dmg—receptor interactions is possible by a variety of techniques, including fluorescence, nmr, and radioligand binding. The last is particularly versatile and has been appHed both to sophisticated receptor quantitation and to dmg screening and discovery protocols (50,51). The use of high specific activity, frequendy pH]- or p lj-labeled, dmgs bound to cmde or purified cellular materials, to whole cells, or to tissue shces, permits the determination not only of dmg—receptor saturation curves, but also of the receptor number, dmg affinity, and association and dissociation kinetics either direcdy or by competition. Complete theoretical and experimental details are available (50,51). 

Recovery. The principal purpose of recovery is to remove nonproteinaceous material from the enzyme preparation. Enzyme yields vary, sometimes exceeding 75%. Most industrial enzymes are secreted by a microorganism, and the first recovery step is often the removal of whole cells and other particulate matter (19) by centrifugation (20) or filtration (21). In the case of ceU-bound enzymes, the harvested cells can be used as is or dismpted by physical (eg, bead mills, high pressure homogenizer) and/or chemical (eg, solvent, detergent, lysozyme [9001 -63-2] or other lytic enzyme) techniques (22). Enzymes can be extracted from dismpted microbial cells, and ground animal (trypsin) or plant (papain) material by dilute salt solutions or aqueous two-phase systems (23). 

Biotransformations are carried out by either whole cells (microbial, plant, or animal) or by isolated enzymes. Both methods have advantages and disadvantages. In general, multistep transformations, such as hydroxylations of steroids, or the synthesis of amino acids, riboflavin, vitamins, and alkaloids that require the presence of several enzymes and cofactors are carried out by whole cells. Simple one- or two-step transformations, on the other hand, are usually carried out by isolated enzymes. Compared to fermentations, enzymatic reactions have a number of advantages including simple instmmentation reduced side reactions, easy control, and product isolation. 

Techniques used in bioseparations depend on the nature of the product (i.e., the unique properties and characteristics which provide a handle for the separation), and on its state (i.e., whether soluble or insoluble, intra- or extracellular, etc.). All early isolation and recovery steps remove whole cells, cellular debris, suspended solids, and colloidal particles, concentrate the product, and, in many cases, achieve some degree of purification, all the while maintaining high yield. For intracellular compounds, the initial harvesting of the cells is important... 

LUMINESCENCE WHOLE-CELL BIOSENSOR ANALYZER FOR WATER TOXICITY ASSESSMENT... 

It is well known that arsenic is one of the most dangerous elements in terms of its potential impacts to both to human and ecosystem health. Therefore the problem of As detection at ppb level remains very important from the point of environmental hazard investigation. The goal of the present work is the developing of very simple and inexpensive assay for arsenite and arsenate determination in environmental samples using whole-cell bacterial biosensors. 

The term biotransformation or biocatalysis is used for processes in which a starting material (precursor) is converted into the desired product in just one step. This can be done by use either of whole cells or of (partially) purified enzymes. Product examples range from bulk chemicals (such as acrylamide) to fine chemicals and chiral synthons (chiral amines or alcohols, for example). There are several books and reviews dealing with the use of bio transformations either at laboratory or at industrial scales [1, 10-13]. 

Biocatalysts in nature tend to be optimized to perform best in aqueous environments, at neutral pH, temperatures below 40 °C, and at low osmotic pressure. These conditions are sometimes in conflict with the need of the chemist or process engineer to optimize a reaction with respect to space-time yield or high product concentration in order to facilitate downstream processing. Furthermore, enzymes and whole cells are often inhibited by products or substrates. This might be overcome by the use of continuously operated stirred tank reactors, fed-batch reactors, or reactors with in situ product removal [14, 15]. The addition of organic solvents to increase the solubility of substrates and/or products is a common practice [16]. 

Whole-cell Systems and Enzymes other than Lipases in Ionic Liquids... 

It is possible that the stationary-state situations leading to an active ion transport occur only in localized regions of the membrane, i.e., at ATPase molecule units with diameters of about 50 A and a length of 80 A. The vectorial ion currents at locations with a mixed potential and special equipotential lines would appear phenomenologically like ionic channels. If the membrane area where the passive diffusion occurs is large, it may determine the rest potential of the whole cell. 

Figure C shows an electron photomicrograph of a broken planar SOFC. The thick portion on the left is the porous anode structure. This is an anode-supported cell, meaning that in addition to collecting current and supporting the anode reaction, the anode layer stiffens the whole cell. The layer on the right is the cathode, and the interface between the two is the thin electrolyte. One of the challenges of this design is to ensure that the rates of expansion of the cathode and the anode match. If the anode expands faster than the cathode, the planar cell tends to curl like a potato chip when the temperature changes.

FIGURE 4.18 Affinity of adenosine receptor agonists in whole cells (dark bars) and membranes (cross-hatched bars, high-affinity binding site). Data shown for (1) 2-phenylaminoadenosine, (2) 2-chloro adenosine, (3) 5 -N-ethylcarboxamidoadenosine, (4) N6-cyclohexyladenosine, (5) (-)-(R)-N6-phenylisopropyladenosine, and (6) N6-cyclopentyladenosine. Data redrawn from [15],... 

An alternative approach involves testing of new drug entities on whole-cell systems and measuring effects on integrated cellular pathways. Favorable phenotypic responses are identified with this approach that may better produce alteration of multicomponent disease processes. 

Enzyme preparations versus whole cell processes 13... 

In designing a process we have the choice of using the whole organism or specific enzymes isolated from it. As always both options have pro s and cons. Broadly speaking we could say that biosynthetic processes mostly rely on whole cells, whereas biotransformations can be catalysed by whole cells and by enzyme preparations. 

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