Carbohydrates Mammalian cells

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). 

Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

A special example of cell-cell interaction is the adherence of group B streptococci to canine, epithelial cells that are infected with influenza A virus. However, this capacity was blocked in the presence of tunicamycin, and this result supports the concept that adherence of streptococci to mammalian cells involves recognition of viral hemagglutinin, or its carbohydrate complement, the synthesis of which is blocked by tunicamycin.556... 

Various studies have shown the toxicity of various free monosaccharides towards mammalian cells. L-Fucose was found to inhibit the growth of some cells in culture, but only at high doses,292 and to inhibit the growth of implanted, mammary tumors in rats.293 In an examination of the effect of 93 carbohydrates on cell proliferation,294 D-fucose was one of the 42 that were toxic or growth-inhibitory, but the effect of L-fucose was not reported. The inhibitory effect of L-fucose in implanted, mammary tumors293 was considered to be accompanied by its incorporation into serum glycoproteins, and this would offer a possible explanation for some of its biological properties. 

There are many glycosylation mutants of cultured mammalian cells and yeast. They have been selected as rare survivors of treatments that kill cells expressing a particular carbohydrate or glycoprotein at the cell surface. For example, plants produce a variety of proteins, called lectins, which bind to cell surface oligosaccharides. Lectins are toxic to mammalian cells. They can be used to select for mutants that no longer bind the lectin because they lack a particular carbohydrate at the cell surface. Such glycosylation mutants have low amounts of glycoproteins that require carbohydrates for stable expression. 

The medium (mainly a carbohydrate source) to cultivate plant cells is well defined and inexpensive, while mammalian cells require very expensive serum as their medium. 

Stanley, P. "Surface Carbohydrate Alterations of Mutant Mammalian Cells Selected for Resistance to Plant Lectins". Lennarz, W.J. Ed. In "Biochemistry of Proteoglycans and Glycoproteins", Plenum Publishing Co., New York, (in press). 

While the main thrust of this book remains centered on the phosphorus-containing lipids of mammalian cell membrane, it would be remiss on the author s part not to discuss the sphingogiycolipids. These phosphorus-free, sphingosine-containing lipids coexist with the phospholipids in cellular membranes. In place of a phosphocholine moiety (as in sphingomyelin), a carbohydrate is substituted. These complex lipids were found to be present in significant amounts in brain some 60 years ago, and their potential association with... 

In addition to the cerebrosides, which contain only one carbohydrate residue, there are other glycosphingolipids in mammalian cells that contain more than one sugar component. These oligosaccharide derivatives are called globosides. For example, lactosyl ceramide (1-O-lactosyl-A acylsphingosine) is a constituent of the erythrocyte membrane. Ceramide trihexoside accumulates in the kidneys of patients with Fabry s disease, due to the lack of a lysosomal a-galactosidase A activity. 

In mammalian cells, glucose is the most abundant carbohydrate energy source. It is metabolized in all cells as a glycolytic fuel and is stored in liver and muscle as the polymer glycogen. But certain cells have the enzymes to catalyze the synthesis of glucose under certain conditions. The requirements are (1) the availability of specific carbon skeletons (carbon backbone structures of various types), (2) energy, in the form of ATP, necessary to accomplish the sequence of reactions, and (3) the enzymes to catalyze reactions of the sequence. 

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