Chemical half-life

CHEMICAL HALF-LIFE IN TOTAL SYSTEM = 186 DAYS... 

The only difference between a chemical and a radioactive half-life is that the former reflects the rate of a chemical reaction and the latter reflects the rate of radioactive (i.e. nuclear) decay. Some values of radioactive half-lives are given in the Table 8.2 to demonstrate the huge range of values t j2 can take. The difference between chemical and radioactive toxicity is mentioned in the Aside box on p. 382. A chemical half-life is the time required for half the material to have been consumed chemically, and a radioactive half-life is the time required for half of a radioactive substance to disappear by nuclear disintegration. 

Superoxide has a chemical half-life measured in microseconds, but in even this short time serious damage can be caused to all types of biological macromolecules. Peroxidation of membrane lipids could cause haemolysis but the oxidation of ferrous (Fe2+) to ferric (Fe3+) iron in haemoglobin due to free radical action is a more immediate cause for concern within the red cell (Figure 5.17). 

The nitrosoureas are alkylating agents that are highly lipid soluble and share similar pharmacological and clinical properties. Carmustine (BCNU), lomustine (CCNU), and semustine (methyl-CCNU) are chemically unstable, forming highly reactive decomposition products. The chemical half-life of these drugs in plasma is only 5 to 15 minutes. Their marked lipid solubility facilitates distribution into the brain and cerebrospinal fluid (CSF). 

Case Example Pharmacokinetic Calculations to Interpret Phthalate Urinary Biomarker Data. The previous descriptions focused on blood or adipose biomarker concentrations that were converted to body burden to yield estimates of daily dose based on chemical half-life. A modified form of that is conversion of urinary biomarker data to daily exposure dose via simple model calculations as described for phthalates. 

The chemical half-life of DMSP in seawater is >8 years (Dacey and Blough 1987), which results in high abiotic stability under natural conditions (moderate temperatures and pH). Therefore, most of the DMSP removal is through enzymatic processes. In the microbial food web, dissolved DMSP has many fates and several recent reviews on the microbial pathways and involved mechanisms have been published (Bentley and Chasteen 2004 Kiene et al. 2000 Lomans et al. 2002 Yoch 2002). They all show that DMSP can be readily used in a complex network of enzymatic conversions. This versatility indicates that this single compound is of major importance for the nutrition of the bacterial community. Indeed, several studies have shown that DMSP alone can contribute 1 to 15% of the total bacterial carbon demand in surface waters. Moreover, DMSP assimilation can satisfy most, if not all the, sulphur demand of marine bacteria (Kiene and Linn 2000 Simo et al. 2002 Zubkov et al. 2001). Since the focal point of this section is the quantification of DMSP removal, only the overall effects of the main pathways originating from DMSP (Fig. 1) will be discussed here. 

PMSF is a hazardous chemical. Half-life time in aqueous solution is 35 min. PMSF is usually stored as 10 mM or 100 mM stock solution (1.74 or 17.4 mg/ml in isopropanol) at - 20° C. 

Most soil-boimd pesticides are less hkely to volatihze or to leach through the soil. They are also less easily taken up by plants. In terms of environmental fate, bound residues can be significant and may result in the underestimation of chemical half-life. A nine-year outdoor study showed that soil residues contained as high as 50-60% of the initial apphed radioactivity of C-atrazine [38,39], and some transformation products of atrazine persisted for 9 years in outdoor conditions [39]. Most of the boimd residues were the hydroxy analogues of atrazine and their dealkylated products. 

If the chemical reactions of the photoactivated intermediate were completely nonspecific, the thermodynamic dissociation constant, Kt, would be the primary consideration (assuming that the reactive intermediate has the same Ki), Since there may be highly reactive entities outside the site, such as other macromolecules or water, the rate of dissociation and the chemical half-life of the intermediate are also important. Further, noncovalently bound reaction products can prevent stoichiometric labeling by their occupancy of the ligand binding site. 

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