Radioisotopes incorporation

Radioisotope-incorporation studies100 indicate that the polymers are products of an equilibrium process (see Section 11,1, p. 435). 

Cornacoff JB, House RV, Dean JH (1988) Comparison of a radioisotopic incorporation method and the mouse ear swelling test MEST) for contact sensitivity to weak sensitizers. Fund Appl Toxicol 10 40-44 Dunn BJ, Rusch GM, Siglin JC, Blaszcak DL (1990) Variability of a mouse ear swelling test (MEST) in prediction of weak and moderate contact sensitizers. Fundam Appl Toxicol 5 242-248... 

The synthesis of fatty acids and sterols in the liver cytosol depends upon a common pool of acetyl-CoA. This was demonstrated by Decker and Barth in a series of experiments utilizing perfused rat liver [10]. Lipid synthesis was measured by incorporation of tritium from [ H]H20. They used (- )-hydroxycitrate to inhibit ATP-dependent citrate lyase and measured radioisotope incorporation into fatty acids and sterols as a function of the concentration of this inhibitor. A parallel decrease in incorporation into these two products was found as the concentration of (- )-hydroxycitrate in the perfusate was increased. Contrastingly, if radioisotopic acetate was used as the substrate in the perfusing medium, this inhibitor had relatively little effect on the rate of sterologenesis, a result that would be expected if the natural source of acetate was from the action of the cytoplasmic citrate lyase. Their experiments also demonstrated that the ratio of fatty acid synthesis to sterol synthesis in the liver of fed rats is about 10 1. 

The subject of phospholipase assays has been concisely reviewed [1-5]. A standard approach employs phospholipids with radioisotopes incorporated into specific positions in the molecule. By the appropriate choice of labeling, the specificity of the enzymes can readily be established and as little as a few picomoles of product can be detected. The use of isotopes has been helpful in the measurement of phospholipase activity using the membranes of whole cells or isolated subcellular fractions previously labeled with radioactive phospholipid precursors. 

Although the results presented so far indicate an interdependency between the PN cycle and ricinine biosynthesis, the details concerning the order of intermediates in ricinine biosynthesis have not been elucidated. The conflicting evidence (Waller et aL, 1965 Hiles and Byerrum, 1969) in which the PN cycle intermediate is the immediate precursor of ricinine may best be explained by a complicated relationship between the cycle and ricinine biosynthesis. One possible relationship is the metabolic grid concept, which is defined as a series of pm allel reactions in which analogous transformations occur, but at different rates thus a compound may be converted to a product by several different parallel pathways (Bu Lock, 1965). The proposed metabolic grid, as shown in the structure. Figure 6.37, in which several of the PN cycle intermediates enter into the ricinine biosynthesis pathway, explains the similarity of radioisotopic incorporation results when each of the PN cycle intermediates ([ C]-labeled) were fed (Waller et al., 1965). It is presumed that these reactions are catalyzed by their respective enzymes. The report that a tenfold excess of NAD blocks... 

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,... 

Radioisotopes are continuously produced in the atmosphere, the stratospheric production being roughly twice the tropospheric one. As shown in figure 4 it is useful to distinguish between isotopes of noble gases, isotopes incorporated in molecules of gases or vapors, and isotopes which get attached to aerosols. 

Great care must always be exercised in handling radioisotopes. It is not only the powerful emitters that are dangerous but also weak emitters with long half-lives, e.g. tritium, carbon-14, which may be incorporated into the body and over a period of time can constitute a serious hazard. 

Much research has gone into raising the sensitivity and selectivity of immunosensors to the desired levels. Several labels have proved to ensure a high sensitivity, yet radioisotopic labels have essentially been avoided. Non-isotopic labels for immunosensors include various enzymes, catalysts, fluorophores, electrochemically active molecules and liposomes. Labelled immunosensors are basically designed so that immunochemical complexation takes place on the surface of the sensor matrix. There are several variants of the procedure used to form an immunocomplex on the matrix. In the final step, however, the label should always be incorporated into the immunocomplex for determination, as shown in Fig. 3.27.B. 

The probe species is often radioactively labeled, or it may carry a fluorescent tag, or some other chemical or enzymatic moiety to generate a positional signal. For radioactive labeling, a common choice of radioisotope is phosphorus-32 (or 32P), because it can be incorporated as phosphate into DNA or RNA relatively easily, and it emits energetic beta particles that are easy to detect. The radioactivity on the membrane can be used to expose an adjacent x-ray film in a pattern corresponding to the radioactive spots on the membrane. After a suitable exposure time, one develops the film and studies the location and intensity of the images of the radioactive spots to deduce the position and degree of probe hybridization on the membrane. 

Cell kinetics is defined as the measurement of time parameters m biological systems. Traditionally, this has involved the use of radioactive precursors of DNA, such as tritiated thymidine (3HTdR), and autoradiography to detect their incorporation into DNA. This technique has provided detailed knowledge of cell kinetics in both in vitro and in vivo experimental systems. The technique, however, is time consuming and arduous and is not readily applicable to human tumor research because of ethical problems involved in incorporation of a radioisotope into DNA. 

Radioisotopes that decay by spontaneous fission with the direct accompanying release of neutrons are usually associated with the natural elements of uranium and thorium and the manmade element plutonium. However, the rate of decay of these elements by fission is so slow that it is only by incorporating them into large nuclear piles or chain reactors that they can be utilized as intense neutron sources. In the US Dept of Energy National Transplutonium Program, small quantities of elements heavier than plutonium are produced for basic research studies and to discover new elements with useful properties. One of these new elements, californium-252 (2S2Cf), is unique in that it emits neutrons in copious quantities over a period of years by spontaneous fission... 

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