Metabolic pathways constituents

Wilson and Madsen [152] used the metabolic pathway for bacterial naphthalene oxidation as a guide for selecting l,2-dihydroxy-l,2-dihydronaphthalene as a unique transient intermediary metabolite whose presence in samples from a contaminated field site would indicate active in situ naphthalene biodegradation (Fig. 26). Naphthalene is a component of a variety of pollutant mixtures. It is the major constituent of coal tar [345], the pure compound was commonly used as a moth repellant and insecticide [345], and it is a predominant constituent of the fraction of crude oil used to produce diesel and jet fuels [346]. Prior studies at a coal tar-contaminated field site have focused upon contaminant transport [10,347], the presence of naphthalene catabolic genes [348, 349], and non-metabolite-based in situ contaminant biodegradation [343]. 

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. 

The isotopic composition of the internal constituents of cells has occasionally been determined to understand metabolic pathways. Separation of N-labeled cellular constituents based on solubility in TCA was used by Wheeler et al. (1982) and GHbert and McCarthy (1984). Bronk and Gilbert (1991) also used TCA extraction, followed by ultrafiltration to extract the soluble LMW component of field samples. 

Differences in nutritional effects between PLs and TAGs can be caused by several factors not related to their fatty acid composition, such as the presence of a phosphate group and a nitrogen base (mainly chohne) that may interact in several metabolic pathways (82). Moreover, several glycerophospholipid preparations studied can contain other components such as cholesterol, cerebrosides, sphingomyelins also depending on their source, method of isolation, and purification. These components may also affect the nutritional properties. In this chapter, the metabolic fate of constituent fatty acids of PLs and TAGs will be compared. 

Aspartame is hydrolyzed entirely in the gastrointestinal tract to its constituent amino acids, aspartate and phenylalanine, and methanol. These are absorbed by the body and utilized via the same metabolic pathways as when these same constituents are derived from common foods they are found in common foods in much larger quantities than from aspartame in foods or beverages. 

Besides being fundamental constituents of proteins they are the parent substances from which powerful hormones are derived, for example, adrenaline (epinephrine), noradrenaline (norepinephrine), thyroxine and related substances, 5-hydroxytryptamine (enteramine, serotonin), and the plant hormone indoleacetic acid. Tryptophan is also the precursor of the B vitamin nicotinic acid and hence of part of the important pyridine nucleotides. All three aromatic amino acids are potential precursors of other substances having powerful physiological activity, for example, many of the alkaloids. Errors in the metabolism of the aromatic amino acids in man can give rise to sometimes serious, but fortunately comparatively rare, disorders such as alkaptonuria and phenylketonuria. The numerous metabolic pathways involved in aromatic amino acid metabolism therefore make an important as well as an interesting study. 

A substantial number of pharmaceutically and clinically related problems require the detection and determination of small amounts of metal ions and other inorganic constituents of biological and xenobiotic substances (1-3). Some obvious examples are the detections of heavy metals and lithium in biological fluids and tissue samples in cases of suspected intoxication and the determination of potassium for purposes of quality control in intravenous solutions to be given to cardiac patients. Trace amounts of nonmetals such as selenium and iodine, which are associated with the functions of coenzymes or hormones, also must be analyzed in order to determine their roles in metabolic pathways. 

Antimetabolites block or alter a metabolic pathway involved in DNA synthesis. They are analogues or antagonists of normal cell constituents. 

The C2 pool, which is also part of glycolytic breakdown, is the starting point for lipid synthesis. In contrast, amino acids have several precursors and they are connected to a range of pools and metabolic pathways. To understand the structural and chemical similarity and possible differences between organisms, the following biochemical groups are described more in detail carbohydrates, phenylpropanes and their associated derivatives, amino acids, lipids, and the major cell wall constituents. 

Substances foreign to the body, or xenobiotics, are metabolized by the same enzymatic pathways and transport systems that are utilized for dietary constituents. Xenobiotics to which humans are exposed include environmental pollutants, food additives, cosmetic products, agrochemicals, processed foods, and drugs. Many xenobiotics are lipophilic chemicals that, in the absence of metabolism, would not be efficiently eliminated and would accumulate in the body, possibly causing toxicity. Most xenobiotics are subjected to metabolic pathways that convert these hydrophobic chemicals into more hydrophilic derivatives that are readily eliminated in urine or bile. 

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