Radiolabel choice

The turnover rate of a transmitter can be calculated from measurement of either the rate at which it is synthesised or the rate at which it is lost from the endogenous store. Transmitter synthesis can be monitored by administering [ H]- or [ " C]-labelled precursors in vivo these are eventually taken up by neurons and converted into radiolabelled product (the transmitter). The rate of accumulation of the radiolabelled transmitter can be used to estimate its synthesis rate. Obviously, the choice of precursor is determined by the rate-limiting step in the synthetic pathway for instance, when measuring catecholamine turnover, tyrosine must be used instead of /-DOPA which bypasses the rate-limiting enzyme, tyrosine hydroxylase. 

These factors make 125I the iodine label of choice for radiolabeling biological molecules. Its commercial availability from a number of suppliers at relatively low cost further adds to its popularity. Even though it has lower specific activity than 131I, iodine-125 still provides much greater sensitivity than 14C, 32P, 35S, or 3H in labeling biomolecules. In fact, the use of a radioactive iodine label can create probes that have 150-fold more sensitivity than tritiated molecules and as much as 35,000 times the detectability of 14C-labeled molecules (Bolton and Hunter, 1986). 

Careful choice of the label and the conjugation chemistry is a key issue for detection and characterization of targeters. Among the quantitative methods, radiolabeling techniques are distinguished by their unrivaled sensitivity, but... 

In the last decade, a large number of radiolabeled small biomolecules have been studied for their potential as radiopharmaceuticals for diagnosis and radiotherapy of various diseases. This review wiU focus on some fundamental aspects of receptor-based diagnostic radiopharmaceuticals, including radiopharmaceutical design, receptors and receptor imaging, choice of biomolecule, and modification of pharmacokinetics. 

From a medicinal chemist s perspective, nuclear magnetic resonance (NMR) was still the analytical tool of choice, whereas mass spectrometry, infrared (IR), and elemental analyses completed the necessary ensemble of analytical structure confirmation. Synthesis routines were capable of generating several milligrams of product, which is more than adequate for proton and carbon NMR experiments. For analyses that involved natural products, metabolites, or synthetic impurities, time-consuming and often painstaking isolation methods were necessary, followed by expensive scale-up procedures, to obtain the necessary amount of material for an NMR experiment. In situations that involved trace-mixture analysis, radiolabeling approaches were often used in conjunction with various formats of chromatographic separation. 

Choice of Radioligand. A 11+C radiolabel will probably exist for most pesticides which will be considered for radioimmunoassay development. Such an intrinsic radiolabel will prove very valuable in titering antisera and possibly in numerous other steps from antigen synthesis through assay development. Unfortunately, for the actual assay, the commonly available 11+C radiolabels may not be of high enough specific activity. The theoretical limit on the specific activity of a single carbon atom is 63 mCi/mmole, and few pesticides have a specific activity of over 50 mCi/mmole even when they are labeled in... 

The clinical use of chelate-tagged albumin in place of 125I-albumin has the fundamental advantage of not having the protein radiolabeled until just before it is used. This permits a choice among radionuclides so that the half-life, radiations, and energies can be suited to each particular clinical procedure, and it removes the radioactive half-life as a limitation on the shelf-life of the modified protein. 

Since radiolabeled molecules are hazardous and difficult to dispose, most researchers and clinical laboratories prefer to use nonisotopic methods of labeling molecules. For this reason, investigators have established (3-glucosidase assay conditions that allow the estimation of glucocerebrosidase content of human tissues when nonphysiological (3-glucosides serve as the substrate. The artificial substrate of choice,... 

The described method is suitable to compare up to 50 drugs in one experiment. The price for this is the limited quantification, as the staining is not strictly stoichiometric, and does not allow the distinction between matrix synthesis and degradation. For more detailed assessment, radiolabeling is the better choice. The limitation of these primary culture assays lies in the elaborate preparation and isolation of the chondrocytes. Several attempts to immortalize this differentiated mesenchymal cell type have so far resulted in the loss of certain cartilage-specific properties. 

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-... 

RIA) using 2 I-labeled antibodies and enzyme-linked immunosorbent assay (ELISA) using alkaline phosphatase conjugates, wll be described. It is assumed that in most cases, the second antibodies will be bought either as purified material for radiolabeling or already conjugated to fluorescein, biotin, or the enzyme of choice (seeref. 2 for additional methods). 

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