Complexation with cell constituents

The principle behind the test method(s) is that antibodies are made of proteins that recognize and bind with foreign substances (antigens) that invade host animals. Synthetic antibodies have been developed to complex with petroleum constituents. The antibodies are immobilized on the walls of a special ceU or filter membrane. Water samples are added directly to the cell, while soils must be extracted before analysis. A known amount of labeled analyte (typically, an enzyme with an affinity for the antibody) is added after the sample. The sample analytes compete with the enzyme-labeled analytes for sites on the antibodies. After equilibrium is established, the cell is washed to remove any um-eacted sample or labeled enzyme. Color development reagents that react with the labeled enzyme are added. A solution that stops color development is added at a specified time, and the optical density (color intensity) is measured. Because the coloring agent reacts with the labeled enzyme, samples with high optical density contain low concentrations of analytes. Concentration is inversely proportional to optical density. 

Most of the DNA of animal cells is found in the nucleus, where DNA is the major constituent of the chromosomes. On the other hand, most of the RNA is located in the cytoplasm. Nuclear DNA exists as a thin, double helix only 2 nm wide. The double helix is folded and complexed with protein to form chromosomal strands approxim-ately 100 to 200 nm in diameter. Each chromosome contains a single DNA duplex. The human chromosomes vary in size the smallest contains approximately 4.6 X 10 base pairs of DNA, and the largest 2.4 X 10 base pairs. In contrast, the Escherichia coli chromosome has 4.5 x 106 base pairs. The DNA of die chromosomes is tightly packed and associated with both histone and nonhistone proteins. 

Formation of a chemical complex with other molecules in solution can change fluorescence behaviour, e.g. the presence of caffeine in solution reduces the fluorescence of riboflavin. This alteration of fluorescence upon binding is used to advantage when examining binding of fluorescent molecules to proteins or other constituents of cells. 

Metabolism involves a bewildering array of chemical reactions, many of them organized as complex cycles which may appear difficult to understand. Yet, there is logic and orderliness. With few exceptions, metabolic pathways can be regarded as sequences of the reactions considered in Chapters 12-16 (and summarized in the table inside the back cover) which are organized to accomplish specific chemical goals. In this chapter we will examine the chemical logic of the major pathways of catabolism of foods and of cell constituents as well as some reactions of biosynthesis (anabolism). A few of the sequences have already been discussed briefly in Chapter 10. 

Lignin is a complex phenolic cell wall polymer that is chemically cross-linked with hemicellulose and cell wall proteins. Most of the methods to determine lignin content are based on the removal of all other cell wall constituents, typically through acid hydrolysis, which will readily remove hemicellulose under mild conditions, and non-crystalline cellulose under more severe conditions. Several different methods will be discussed below. The different methods have also been extensively reviewed and compared by Hatfield et al. (1994), Brinkmann et al. (2002), Fukushima and Hatfield (2004), and Hatfield and Fukushima (2005). 

The mechanism of action for diesel fuels is not well characterized due to the complexity of its petroleum hydrocarbon mixture. The presence of additives that improve fuel combustion or prevent microbial growth may contribute to toxicity. Based on research conducted with individual components of diesel fuels, the primary mechanism of action for central nervous system (CNS) depression from diesel fuel is the reversible, physical interaction of the aromatic and aliphatic hydrocarbons with cell membranes. Renal toxicity is possibly attributed to oxidative metabolites of some of the aromatic constituents. Eye and skin injury are attributable to direct irritant action and the high lipid solubility that may dissolve protective skin oils and allow penetration into the skin tissue. The dermal carcinogenesis observed in rodents subjected to chronic dermal exposure to diesel may be attributed to the genotoxic activity of PAHs and the promoting activity of repeated dermal injury. 

A pollutant may combine with a cell constituent and form a complex. This often leads to impaired function. For example, carbon monoxide (CO) in the blood readily binds to hemoglobin (Hb), forming carboxyhemoglobin (COHb) as shown below ... 

Speciation of the pyrimidone complex 19 in blood as revealed by EPR and NMR, based on ref. 36a. Extra refers to blood serum, intra to the intracellular medium of red blood cells. 21a enters the cells by diffusion and vanadate through phosphate channels. VO + stands for any vanadyl complex formed with cytosolic constituents after intracellular reductive degradation of 21a or reduction of vanadate. 

Consider a typical eukaryotic cell, for instance, a muscle cell. By weight, the cell is about 75% water. However, this estimate fails to convey the truly aqueous nature of the cell a far more realistic description is in terms of mole ratios. Because of the low molecular weight of water, the nominal 75% water translates into a very large number of moles of water relative to the number of moles of other cell constituents. Thus, the aqueous nature of the cell is better illustrated by noting that for every 20,000 water molecules there are only about 75 lipid molecules, 100 sodium, potassium, and chloride ions (with at most a few hundred other small molecules or ions), and only one or two protein molecules. By sheer numbers water molecules totally dominate, and in this perspective life is merely some complex biochemistry in an extensive matrix of water, stabilized by a few lipids and macromolecules. The two dominating factors in cell biology are thus, simply, water molecules and interfaces. 

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