Radiolabeled substances

In the petroleum industry, the size of an underground oil deposit is deterrnined by the injection of radiolabeled substances into a well head. The occurrence of radioactivity in the oil—water mixture, which is pumped out of adjoining wells, gives an indication of the pocket size of the oil deposit (see Petroleum). 

Immunoradiometric assays (IRMAs) are like RIAs in that a radiolabeled substance is used in an antibody-antigen reaction, except that the radioactive label is attached to the antibody instead of the hormone. Furthermore, excess of antibody, rather than limited quantity, is present in the assay. All the unknown antigen becomes bound in an IRMA rather than just a portion, as in a RIA IRMAs are more sensitive. In the one-site assay, the excess antibody that is not bound to the sample is removed by addition of a precipitating binder. In a two-site assay, a molecule with at least two antibody-binding sites is adsorbed onto a solid phase, to which one of the antibodies is attached. After binding to this antibody is completed, a second antibody labeled with 125I is added to the assay. This antibody reacts with the second antibody-binding site to form a sandwich, composed of antibody-hormone-labeled antibody. The amount of hormone present is proportional to the amount of radioactivity measured in the assay. 

Be properly dressed e.g., safety eyeglasses, long pants, labcoat, closed-toe shoes, gloves (these must be appropriate for the chemical properties of radiolabeled substances), lead apron (if recommended), and wrist guards. Dosimeters must be worn in recommended locations (wrist, finger, lapel, etc.). 

An analogous assay using a radiolabeled soft nucleophile would also be required to complement the hard nucleophile radiolabeled cyanide trapping assay. Investigations into radiolabeled glutathione have proved unsuccessful since the material is unstable due to cross-reactions induced by beta radiation. Alternate soft nucleophiles such as cysteine, N-acetyl cysteine and P-mercaptoethanol all have promise as radiolabeled substances for quantitative trapping experiments since they are more stable than GSH and equally nucleophilic, although clearly these would not be substrates for GST. 

Utilizing the Grubbs alkene metathesis reaction, Katzenellenbogen et al. 164 described the synthesis of the ten-membered-ring lactam p-turn mimetic 129 (Scheme 47, some experimental details given below) and prepared a mimetic 130 of the neuropeptide substance P, and in particular of the four C-terminal amino acid sequence -Phe8-Gly-Leu-Metn-. Compound 130 was unable to inhibit the binding of radiolabeled substance P. 

The applied analytical method depends on the binding assay used, and on the phase of drag development when the protein binding is performed respectively, on the availability of radiolabeled substance. Generally the highest purity of the compound is preferred, to avoid interferences with contaminants or degradation products. 

In the stage of drag discovery, LC/MS-MS analytics is the method of choice to quantify the unbound drag concentration. The sensitivity can be increased by the use of radiolabeled substance. But, the radiochemical purity, isotope decay, if not 14C-label is used as well a sufficient specific activity must be taken into consideration (Wright et al. 1996). The concentrations of radioactivity in bound and unbound fraction are measured by liquid scintillation counting. The use of radiolabeled material allows easily examination of the potential of adsorption. However, the identity of the drag in unbound fraction should additionally be veri-... 

As the radiolabeled substances emerge from the laboratory to the clinics, there will be a need for scaling up the batch size of the product. This can be done by increasing either the total volume of the produced batches or the specific activity of the product or both. When doing this, the following aspects should be considered ... 

A9.5.2.3.9.2 When using radiolabelled substances, the labelling is most often placed in the stable part of the molecule, for which reason the measured BCF value includes the BCF of the metabolites. For some substances it is the metabolite which is the most toxic and which has the highest bioconcentration potential. Measurements of the parent substance as well as the metabolites may thus be important for the interpretation of the aquatic hazard (including the bioconcentration potential) of such substances. 

Reactive metabolites [4] Elucidation of a potential metabolic prerequisite for toxicity of a chemical In vitro trapping studies available cellular systems developed only for radiolabelled substances Causality between reactive metabolite formation and toxicity difficult to prove... 

Figure 2. Effect of 1 (xM AFM on efflux of various radiolabeled substances from cucumber cotyledons ( ). At time zero, cotyledons were exposed to 600 fiE/m s (PAR) light and herbicide. A, Efflux of Cl. B, Efflux of C, Efflux of...

Since monkeys are expensive and special facilities are necessary to house and maintain them while using radiolabeled substances, only a limited number are likely to be utilized in these types of studies. 

Oldendorf, W.H., 1970. Measurement of brain uptake of radiolabeled substances using a tritiated water internal standard. Brain Research, 24(2), pp. 372-376. 

Barylko et al, 2001 Damke et al, 2001). The ratio of [GDP]/([GDP] + [GTP]) is determined for each sample to obtain an accurate measure of GTP hydrolysis that is independent of possible variations in the sample volumes spotted on TLC. This method, however, is prone to error if nonreacting, radiolabeled substances comigrate with GTP. Indeed, by allowing reactions to go to completion, we found that some P GTP batches had materials that did not react but had the same retention time as GTP. The presence of these nonreacting materials lowered measured values for GTP hydrolysis up to 50% for basal GTPase rates when compared to those obtained using the colorimetric method. 

In this classical RIA. a variable amount of unlabeled analyte (hapten) competes with a constant amount of a radiolabeled substance for a limited number of antibody binding sites ... 

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