Toxic molecules

Most of the organic pollutants described in the present text act at relatively low concentrations because they, or their active metabolites, have high affinity for their sites of action. If there is interaction with more than a critical proportion of active sites, disturbances will be caused to cellular processes, which will eventually be manifest as overt toxic symptoms in the animal or plant. Differences between species or strains in the affinity of a toxic molecule for the site of action are a common reason for selective toxicity. 

Apart from the oxidations just mentioned, cyclodienes are rather stable chemically. It should, however, be noted that dieldrin can undergo photochemical rearrangement under the influence of sunlight to the persistent and toxic molecule photo dieldrin, which occurs as a residue following the application of this insecticide in the field. 

Ferritin, an iron-binding protein, prevents ionized iron (Fe ) from reaching toxic levels within cells. Elemental iron stimulates ferritin synthesis by causing the release of a cytoplasmic protein that binds to a specific region in the 5 nontranslated region of ferritin mRNA. Disruption of this protein-mRNA interaction activates ferritin mRNA and results in its translation. This mechanism provides for rapid control of the synthesis of a protein that sequesters Fe +, a potentially toxic molecule. 

Ricin, an extremely toxic molecule isolated from the castor bean, inactivates eukaryotic 28S tibosomal RNA by providing the N-glycolytic cleavage or removal of a single adenine. 

In other chapters of this volume considerable attention is given to marine toxins whose cellular sites of action have been identified. For example, saxitoxin, brevetoxin, and sea anemone toxins are prototypes of toxic molecules whose chemical structure is known, and whose actions on ionic channels in the cell membrane have been elucidated. Recent additions to such toxins are the piscivorus cone... 

Molecular designers can exploit these rules and design chemicals that demonstrate characteristics that are likely to lead to decreased absorption and therefore minimize toxicity. Molecules that are permanently charged at physiological pH like the neurotoxin curare or... 

Politzer, P. 1988. Computational Approaches to the Identification of Suspect Toxic Molecules. Tox. Letts. 43, 257. 

The effect of MAPK activation on cellular processes that affect cell function and the resulting pharmacology has been delineated using modem techniques such as knock-out cells and animals [1,3,6]. Activation of MAPK in inflammatory cells such as T-cells, B-cells, macrophages and eosinophils leads to expression and/or activation of pro-inflammatory genes and mediators such as interleukin-1(3 (IL-1(3), TNFa, IL-6, chemokines [e.g., IL-8, macrophage inflammatory factor-1 a, (3 (MIP-la,[3)J, MMPs and toxic molecules such as free radicals and nitric oxide [1,3]. These pro-inflammatory mediators induce cellular proliferation, differentiation, survival, apoptosis and tissue degradation/destruction and help induce chronic inflammation. Inhibition of any one or more of the MAPK family... 

The anammox catabolism, an exceptionally slow process generating toxic intermediates (hydroxylamine and hydrazin), takes place in an intracytoplasmic compartment called the anammoxosome. A surrounding impermeable membrane protects the cytoplasm from the toxic molecules produced inside this organelle-like structure. Such a tight barrier against diffusion seems to be realised by four-membered aliphatic cyclobutane rings that have been found... 

Therefore, for infrared spectroscopic methods, the total petroleum hydrocarbons comprise any chemicals extracted by a solvent that are not removed by silica gel and can be detected by infrared spectroscopy at a specified wavelength. The primary advantage of the infrared-based methods is that they are simple and rapid. Detection limits (e.g., for EPA 418.1) are approximately 1 mg/L in water and 10 mg/kg in soil. However, the infrared method(s) often suffer from poor accuracy and precision, especially for heterogeneous soil samples. Also, the infrared methods give no information on the type of fuel present in the sample, and there is little, often no information about the presence or absence of toxic molecules, and no specific information about potential risk associated with the contamination. 

The contribution of oxysterols to the cytotoxicity of oxidized LDL is likely, but it is to be noted that oxysterol concentrations necessary to trigger apoptosis in cultured cells are much higher than that reported in toxic concentrations of oxidized LDL and in atherosclerotic plaques (Brown and Jessup, 1999). This could be due to a synergistic effect of the different toxic molecules brought by the oxLDL. 

The latter compound results from loss of two hydrogen atoms by metabolic oxidation at each of two of the carbon atoms of hexane, and their replacement by oxygen atoms. Schaumburg and Spencer not only demonstrated this, but also showed that the identical neurotoxic events result from direct exposure to compound II and also another common solvent called methyl-n-butyl ketone (III). Chemical III is, like hexane, readily metabolized to the active toxicant, molecule II. Because both I and III yield the same metabolite (II), and because this metabolite is the source of toxicity, then exposure to both of these chemicals produces the identical type of neuropathy. 

The brain is protected from trauma by the skull and spinal column and from potentially toxic molecules by the blood-brain barrier. 

By complexation with various components of the natural subsurface solution, the initial properties of a toxic molecule can be changed. These transformations involve adsorption on the subsurface solid phase, transport into the saturated or partially saturated zone, and contaminant half-life in the subsurface environment. 

Bispecific monoclonal antibodies (BsMAb) which combine the specificity of two different antibodies within one molecule and cross-link an effector cell or a toxic molecule with the target cell (Section 8.5.2). 

Another approach to selectively inducing tumour cell kilhng is by the use of bispecific monoclonal antibodies (BsMAb).They combine the specificity of two separate antibodies within one molecule and cross-hnk an effector killer cell or a toxic molecule with the target cell to be destroyed [75]. There are three major approaches for creating BsMAbs.They can be obtained by chemical cross-linking of two MAbs, by fusing two hybridomas [76], or by genetic... 

In addition to generation of toxic compounds in the frying oil, toxic molecules may be generated in foodstuff. In April 2002, Swedish scientists sounded an alarm when they discovered that certain cooked food, particularly potato chips and French fries, contained high levels of acrylamide, a chemical compound that is listed by the World Health Organization (WHO) as a probable human carcinogen (Mitka,... 

The shape of the front is given by the spreading of the front around the point S. The spreading depends mainly on toxic mass transfer kinetics between gas and solid phases. Thus numerous parameters can be considered air flow rate, size of the toxic molecule, pore sizes of the carbon, carbon grain diameter. With large pores and small grains, the mass transfer from gas to carbon is very fast the spreading... 

However, it does not necessarily follow that a natural molecule will have a favorable environmental profile [40b]. Many of the most toxic molecules known to science come from Nature, and natural molecules may not break down effectively in the absence of the organisms or conditions required for their biodegradation. 

In an aromatic cycle or in an unsaturated carbon chain, the doublets 11 of the double bonds may move and thus modify the reactivity of the molecule. For instance, this modification may specifically happen with acrolein - a corrosive (and toxic) molecule (Fig. 3.25) ... 

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