Cell membranes definitions

Fielding CJ, Fielding PE (2003) Relationship between cholesterol trafficking and signaling in rafts and caveolae. Bio-chim Biophys Acta 1610 219-228 Fine JD, Griffith RD (1985) A specific defect in glycosylation of epidermal cell membranes. Definition in skin from patients with epidermolysis bullosa simplex. Arch Dermatol 121 1292-1296... 

The invented biocatalysts based on R. rhodochrous strain ATCC No. 53968 and on B. sphaericus strain ATCC No. 53969, were protected not only as whole cell biocatalysts, but also their derivatives. Biocatalyst definition includes in addition to whole cells cell membranes, cell extracts and enzymes from those microorganisms. It should be noted that the first six patents are actually sets of similar patents with the first one providing coverage in Europe and the second one in US. This strategy involves coverage in US as well as Europe (the total number of patents is higher than the number of inventions) however, the allowed claims in US were always smaller than that allowed in Europe. IGT s last patent (May 1996) was filed in July 1994, when they already had begun... 

Drugs cross biological membranes most readily in the unionised state. The unionised drug is 1000-10000 times more lipid-soluble than the ionised form and thus is able to penetrate the cell membrane more easily. Chemical compounds in solution are acids, bases or neutral. The Bronsted-Lowry definition of an acid is a species that donates protons (H+ ions) while bases are proton acceptors. Strong acids and bases in solution dissociate almost completely into their conjugate base and H+. Weak acids and weak bases do not completely dissociate in solution, and exist in both ionised and unionised states. Most drugs are either weak acids or weak bases. For an acid, dissociation in solution is represented by ... 

Nonreceptor-Mediated Drug Action. At least one important class of drugs, the general anesthetics, has been assumed not to owe its therapeutic activities to a specific receptor process. Anesthetic potency shows an excellent linear correlation with partition coefficient and this has been extrapolated to a definition of action at a lipid site. The phospholipids of cell membranes, particularly nerve cells, have been considered as principal targets for general anesthetic action. It has been hypothesized... 

It is apparent that at this stage of development definitive conclusions are premature, and that this aspect of amino acid and lipide metabolism will be pursued vigorously in the near future. It is of considerable interest to us that biotin and pantothenic acid deficiencies affect amino acid transport in L. arabinosus, since both vitamins are known to play a prominent role in lipide biosynthesis. We are currently reexamining the turnover of lipide fractions in nutritionally normal and vitamin-deficient cell types to determine whether there is some relation between this aspect of metabolism and amino acid transport. In any case, the nature of the catalytic steps involved in amino acid transport is still unknown to us. They probably occur in the peripheral cell membrane, but even this elementary and widely accepted belief will require additional study before it can be accepted beyond doubt as an established fact. 

The isolation of lipids from cells or tissues is not as simple and straightforward as one might desire, but is essentially an important adjunct to characterization of membranes (composition, lipid-to-protein ratio, structure proof, definition, new lipids, etc.). While this is recognized by many investigators in the field, it is difficult for the novice in this area to become aware of some of the potential problems in extraction procedures and the reasons for particular approaches. Thus it seems fitting at this point in time to comment on some of the nuances of the approaches used in isolation, purification, and identification of lipids present in cell membranes. These topics are subdivided into areas which are considered to be of major import to a successful consideration of the extraction procedure. 

The definitions of effective diffusivity tensors are key parameters in the solution of the transport equations above. For an isotropic medium, the effective diffusivity is insensitive to the detailed geometric structure, and the volume fraction of the phases A and B influences the effective diffusivity. When the resistance to mass transfer across the cell membrane is negligible, the isotropic effective diffusivity, Ds e = Dg eI may be obtained from Maxwell s equation... 

An important question is how to achieve the most efficient delivery of PNA. This problem does not usually have a definite answer and many different factors must be considered. Table 1 reviews the main types of PNA-peptide conjugates and indicates the most important issues in each case. The internalization routes are schematically drawn in Fig. 2. Depending on the peptide attached, the PNA oligomer is internalized via endocytosis or directly delivered across the cell membrane. For nuclear localization signals (NLS) and tetralysine (K4) the delivery route is not clear yet. The same holds true for nuclear delivery with CPPs. Some CPPs (e.g., transportan) have been shown to accumulate in the nucleus, while others have not. 

Lipids are naturally occurring organic molecules that have limited solub.ility in water and can be isolated from organi.sms by extraction with nonpolar organic solvents. 1-atS oils, waxes, many vitamins and hormones, and most nonprotein cell-membrane components are examples. Note that this definition differs from the sort used for carbohydrates and proteins in that lipids are defined by a physical property (solubility) rather than by structure. Of the many kinds of lipids, we ll be concerned in this chapter only with a few triacylglycerols, eicosanoids, terpenoids, and steroids. 

Ammonification is another process that can result in N release. The simplest definition of the process is the release ofNH4 from organic matter (e.g., Herbert, 1999). It can occur by a number of different processes including remineralization by bacteria in the water column and sediments. Photochemical ammonification occurs abiotically when NH4+ is released from organic matter as a result of exposure to UV radiation (reviewed in Bronk, 2002 and Chapter 10 by Gryz-bowski and Tranvik, this volume). Ammonium efflux from cells has also been observed following urea uptake in a number of culture experiments (e.g., Price and Harrison, 1988 Rees and Bekheet, 1982 Uchida, 1976). The release may be due to passive diffusion through the cell membrane and is likely unavoidable because NH3 is lipid soluble. 

Glycosylphosphatidyl inositol anchors constitute a class of glycolipids that link proteins and glycoproteins via their C-terminus to eukaryotic cell membranes. The first structure of a GPI anchor, that of Trypanosoma brucei, was published by Ferguson et al. [262]. Since then quite a few examples of GPI anchors were described, allowing the definition of the core structure depicted inO Scheme 37 [263]. 

In recent years, there has been increasing awareness regarding the importance of transporters in the absorption and disposition of NMEs. While the major portion of NMEs or marketed drugs traverse cell membranes by passive diffusion, there are numerous examples where the involvement of specialized transport mechanisms has been demonstrated. Examples include the role of oligopeptide transporters in the intestinal absorption of P-lactam antibiotics, angiotensinconverting enzyme (ACE) inhibitors, and novel NMEs as well as the role of P-glycoprotein (P-gp) in the secretion of molecules into the intestine [11,77—79]. Transfection of cells with the transporter protein of interest has permitted the evaluation of precise cellular mechanisms of uptake and transport of NMEs. Transfected cell lines by definition are tailor-made to overexpress the protein of... 

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