Intact microbial cells

The potential use of immobilised cells in fermentation processes for fuel production has been described previously. If intact microbial cells are directly immobilised, the removal of microorganisms from downstream product can be omitted and the loss of intracellular enzyme activity can be kept to a minimum level.11... 

Intact microbial cells have also been used as biocatalyst for this particular type of bioconversion. Incubation of the racemic 2-acetoxy-l-silacyclohexane rac-(SiS,CR/SiR,CS)-81 with growing cells of the yeast Pichia pijperi (ATCC 20127) yielded the optically active l-silacyclohexan-2-ol (Si/t,CS)-70 (Scheme 16)66,67. Under preparative conditions, this biotransformation product was isolated as an almost enantiomerically pure compound (enantiomeric purity >96% ee) in ca 80% yield [relative to (Si/ ,CS )-81 in the racemic substrate]. 

Oxidations with Intact Microbial Cells 1201 References 1201... 

Stereoselective hydrolyses of (l-acetoxyalkyl)silanes have also been performed with intact microbial cells. An example of this is the highly enantioselective transformation of rac-(S,R/R,S)-235 into the optically active silanes (R,S)-226 using growing cells of the yeast Pichia pijperi (ATCC 20127)19 285 288. The product (R,S)-226 could be isolated with a yield of 80% [related to (R,S)-235] and an enantiomeric purity of >96%ee. 

Biotransformations involve the use of isolated enzymes or intact microbial cells for the highly selective transformations of organic molecules for cutting edge preparative organic syntheses. Ideally, the use of biocatalysts (i.e. enzymes or whole cells) in the industrial preparation of useful compounds would be economical and environmentally friendly. Readers wishing a more thorough discussion on this topic are referred to a recently published article [85]. 

Naumann, D., Keller, S., Helm, D., Schultz, C. Schrader, B. (1995) FT-IR spectroscopy and FT-Raman spectroscopy are powerful analytical tools for the noninvasive characterization of intact microbial cells. Journal of Molecular Structure 347, 399-405. 

As far as we know, transformations of steroids, carried out with intact microbial cells, occur within the cell and not in the medium surroundii the cell. To enter the cell the steroid being transformed must dissolve to some extent in the medium so that it can diffuse through the cell wall and into the enzyme-rich interior. The practical implication of this requirement is that solubility and rate of diffusion may become the rate-limiting factors for transformation. Most steroid substrates ordinarily employed have modest, though measurable, solubilities in water and in the aqueous media used for microbial culture. To ensure saturation of the mediiun and to minimize this rate-limiting effect, steroids are often introduced into reactions in micronized form or, more conveniently, in solution in a water-miscible solvent from which precipitation in very fine particles occurs upon dilution with the aqueous medium containing the microorganism. 

Mammalian cell culture is more technically complex and more expensive than microbial cell fermentation. Therefore, it is usually only used in the manufacture of therapeutic proteins that show extensive and essential post-translational modifications. In practice, this usually refers to glycosylation, and the use of animal cell culture would be appropriate where the carbohydrate content and pattern are essential to the protein s biological activity, its stability or serum half-life. Therapeutic proteins falling into this category include EPO (Chapter 10), the gonadotrophins (Chapter 11), some cytokines (Chapters 8-10) and intact monoclonal antibodies (Chapter 13). 

In essence, an enzyme-catalyzed equivalent exists for almost every type of chemically catalyzed reaction, and thousands of these have been documented in comprehensive monographs and reviews (9-26). Many reactions have been observed in relatively specialized areas, particularly with groups of organic compounds such as the steroids, other terpenoids, antibiotics, aromatics, and alkaloids. Specific chemical reactions have been accomplished with intact and growing microbial cells, with plant and mammalian tissue preparations, and with... 

The immobilization of microbial cells under conditions where an activity or set of enzymic activities remains intact, but the normal metabolic processes cease, represents a novel technique for enzyme immobilization. Moreover, immobilized cells might enable the standard fermentation methods to be replaced by immobilized -cell -based continuous processes. 

Ideally microbial cells should be consumable directly as food or food ingredients. However, because of their nucleic acid content the presence of undesirable physiologically active components the deleterious effects of cell wall material on protein bioavailability and the lack of requisite and discrete functional properties, rupture of cells and extraction of the protein is a necessary step. Importantly, for many food uses (particularly as a functional protein ingredient) an undenatured protein is required. For these reasons and for many potential applications of yeast protein(s) it is very desirable to separate cell wall material and RNA from the protein(s) for food applications. Much research is needed to develop a practical method for isolation of intact, undenatured yeast proteins from the yeast cell wall material to ensure the requisite nutritional and functional properties. 

Intact dried microbial cells have limited functional properties. Labuza et al. (58) reported that spray drying of yeast cells at higher temperatures improved the functional performance of yeast cells in bread making but they were still inferior to control samples. Spray drying of yeast cells at 75 and 124°C... 

The enormous interest in augmenting the world s supply of protein from microbial sources (135) has focussed much attention on this source of protein as a nutrient for animals as well as humans. However, there is some evidence that if intact yeast cells are included in a diet, the tough polysaccharide-containing cell wall may constitute at least a partial barrier in the effective utilization of the protein of the cytoplasm. One reason for this is the absence of enzymes in the digestive tract of humans and most warm-blooded animals capable of hydrolyzing the microfibrillar / -glucan component of the cell wall (135,136). Other factors may include the inability of the proteolytic enzymes of the digestive track to make effective contact with the cells protein. 

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