High specific activity

The ideal plant growth regulator should leave no harmful persistent residue in a finished product or crop and the paradigm compounds are ones that have high specific activity, are target specific, and are environmentally biodegradable. 

Direct quantitation of receptor concentrations and dmg—receptor interactions is possible by a variety of techniques, including fluorescence, nmr, and radioligand binding. The last is particularly versatile and has been appHed both to sophisticated receptor quantitation and to dmg screening and discovery protocols (50,51). The use of high specific activity, frequendy pH]- or p lj-labeled, dmgs bound to cmde or purified cellular materials, to whole cells, or to tissue shces, permits the determination not only of dmg—receptor saturation curves, but also of the receptor number, dmg affinity, and association and dissociation kinetics either direcdy or by competition. Complete theoretical and experimental details are available (50,51). 

Many applications ia tracer technology require products of high specific activity, ie, compounds having a high degree of substitution of specific atoms with radioisotopes. For many labeled compounds nearly 100% labeling can be achieved at one or more locations ia a molecule usiag... 

The introduction of tritium into molecules is most commonly achieved by reductive methods, including catalytic reduction by tritium gas, PH2], of olefins, catalytic reductive replacement of halogen (Cl, Br, or I) by H2, and metal pH] hydride reduction of carbonyl compounds, eg, ketones (qv) and some esters, to tritium-labeled alcohols (5). The use of tritium-labeled building blocks, eg, pH] methyl iodide and pH]-acetic anhydride, is an alternative route to the preparation of high specific activity, tritium-labeled compounds. The use of these techniques for the synthesis of radiolabeled receptor ligands, ie, dmgs and dmg analogues, has been described ia detail ia the Hterature (6,7). 

The small synthetic scale used for production of many labeled compounds creates special challenges for product purification. Eirst, because of the need for use of micro or semimicro synthetic procedures, the yield of many labeled products such as high specific activity tritiated compounds is often low. In addition, under such conditions, side reactions can generate the buildup of impurities, many of which have chemical and physical properties similar to the product of interest. Also, losses are often encountered in simply handling the small amounts of materials in a synthetic mixture. As a consequence of these considerations, along with the variety of tracer chemicals of interest, numerous separation techniques are used in purifying labeled compounds. 

Eor products having relatively low specific activity, such as some compounds labeled with and which are synthesized on the scale of several millimoles, classical organic chemical separation methods may be utilized, including extraction, precipitation, and crystallization. Eor separation of complex mixtures and for products having high specific activity, such as those labeled with tritium, etc, chromatographic methods utilizing paper, thin... 

A related mechanism of degradation involves the direct interaction of the radioactive emission with other tracer molecules in the preparation. This phenomenon is likely to occur in high specific activity compounds stored at high radiochemical concentrations in the absence of free-radical scavengers. 

Applications in agrochemicals [42, 43], pharmaceuticals [44,45], and positron emission tomography (PET) [46, 47, 48 49] have resulted in the resuscitation of the Wallach reaction The Wallach technique provides high-specific-activity F-radiolabeled aromatic fluoride for PET studies, in contrast to the low-specific-ac-tivity product by the Balz-Schiemann route... 

Rudie, N. G., Mulkerrin, M. G., and Wampler, J. E. (1981). Earthworm bioluminescence characterization of high specific activity Diplocardia longa luciferase and the reaction it catalyzes. Biochemistry 20 344-350. 

The only respect in which the hot atom chemistry of organometallic compounds has so far been applied to other fields of study is in the area of isotope enrichment. Much of this has been done for isolation of radioactive nuclides from other radioactive species for the purpose of nuclear chemical study, or for the preparation of high specific activity radioactive tracers. Some examples of these applications have been given in Table II. The most serious difficulty with preparation of carrier-free tracers by this method is that of radiolysis of the target compound, which can be severe under conditions suited to commercial isotope production, so that the radiolysis products dilute the enriched isotopes. A balance can be struck in some cases, however, between high yield and high specific activity (19, 7J),... 

K, which necessitates use of high specific activity radioactive toxin (at or above 10 Ci/mmol). Work in progress indicates that all type-1 brevetoxins inhibit tritiated PbTx-3 binding in a purely competitive manner, whereas the type-2 brevetoxins inhibit in a mixed competition manner at higher concentrations (Figure 7). 

Radioisotope-labeled nitrosamines have proven valuable in development of analytical methods and for demonstrating efficiency of recovery of nitrosamines from tobacco products and smoke (37-39). The very high specific activity required for low part-per-billion determinations has discouraged most analysts from using this approach. Unless a radiochromatographic detector with adequate sensitivity is available, samples must be counted independently of the final chromatographic determination, and one of the advantages of internal standardization, correction for variation in volume injected, is lost. 

Fe, and B SI, Tl, Th, and Ce). The transforaed materials exhibit high specific activity In methanation, ammonia synthesis, and ethylene hydrogenation reactions. The similarity between the industrial methanation catalysts and catalysts obtained by decomposition of various Intermetallics Is striking. Most catalysts obtained by decomposition of a binary alloy Involve an associative combination of... 

The enzyme was purified to 277 fold with very high specific activity, 1387 unit/mg protein by hydroxyapatite column chromatography (Figure 3). 

Fox BG, Wa Froland, JE Dege, JD Lipscomb (1989) Methane monooxygenase from Methylosinus trichospo-rium OB3b. Purification and properties of a three-component system with a high specific activity from a type II methanotroph. J Biol Chem 264 10023-10033. 

They have an exceedingly high specific activity per active site the turnover number y is as high as 10 to 10 s in certain enzyme reactions, while at ordinary electrocatalysts having a number of reaction sites on the order of 10 cm , yhas a value of about 1 s at a current density of lOmA/cm. Thus, the specific catalytic activity of tfie active sites of enzymes is many orders of magnitude fiigher tfian tfiat of all other known catalysts for electrochemical (and also chemical) processes. 

The high specific activity of enzymes and tfie tfieoretical possibility of using them to conduct electrochemical reactions are topics of great scientific interest. However, it is difficult to envisage prospects for a practical nse of enzymes for an acceleration and intensification of industrial electrode processes. The difficulty resides in the fact that enzymes are rather large molecnles, and on the surface of an enzyme electrode, fewer active sites are available than on other electrodes. Per unit snrface area, therefore, the effect expected from the nse of enzymes is somewhat rednced. 

High-specific activity D-P- H] panthothenic acid (Figure 10.8) was prepared from commercially available p-[3-3H]alanine using Escherichia coli strain DVl, which converted 85 to 90% of the input p-[3-3H]alanine to extracellular D-[3- H]panthoth-enate under appropriate growth conditions. The radiolabeled vitamin was purified... 

Gilmour CC, Riedel GS. 1995. Measurement of Hg methylation in sediments using high specific-activity 203Elg and ambient incubation. Water Air Soil Pollut 80 747-756. 

The radioligand should also have a high specific activity so that very small quantities of bound ligand can be accurately measured. The specific activity, simply defined as the amount of radioactivity, expressed in becquerels (Bq) or curies (Ci) per mole of ligand, is dependent on the half-life of the isotope used and on the number of radioactive atoms incorporated into the ligand molecule. A radioisotope with a short half-life decays rapidly so that many disintegrations occur in unit time,... 

Transfer of technetium from seawater to animals has been studied by laboratory experiments using 95mTc, The advantages of 95mTc over "Tc are 95mTc emits y-rays, thus whole-body counting techniques can be used, and it has a sufficiently high specific activity because of its relatively short half life (61 d). 

Chelating Systems for Preparation of High Specific Activity " Tc... 

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