Radiolabeled reagents

Obviously the circumstances are so varied that only the major possibilities can be considered here. The volumes by Murray and Williams (1958) remain a useful source of synthetic methods for small molecules. The major radiochemical suppliers now stock an impressive array of reagents 

In a dramatic example of radiochemical instability Bernier and Escher (1980) found that an analog of angiotensin containing 4 -azido-3, 5 -di-tritiophenylalanine (73 Ci/mmol ) decomposed completely in one day at 

Lability of the radiolabel during photolysis occurs in other types of molecules and the investigator should be aware of the peculiarities of those that he deals with. For example, 3H can be lost from photohydrates of pyrimidines (Wang, 1976). 

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Another technique which has much potential in analysis, but which to date has had only limited use, is the formation of radioactive derivatives of non-radioactive compounds for quantitation by radiocounting. A radiolabeled reagent is used to form the derivative. This approach has been of use in combination with chromatography. The advantage of this technique is that it avoids problems of sample background which are often associated with spectrophotometric methods. The 14C-methylation of carboxylic acids and the 14 C-acetylation of hydroxyl groups have been studied [39,40]. These methods are quantitative and the sensitivity is dependent on the activity of the radioactive group added to the molecules. The radioactive derivatization of lipids has been reviewed [41]. 

Chloramphenicol acetyl transferase (CAT). This bacterial enzyme was the first reporter protein used for studying the transcriptional activity of eukaryotic regulatory sequences (Gorman et al., 1982). CAT inactivates chloramphenicol, an inhibitor of prokaryotic protein synthesis, by converting it to the mono- or di-acetylated species. Measurement of CAT activity requires a 14C-radiolabeled chloramphenicol or acetyl-CoA and, therefore, an additional step is neccessary to separate the radio-labeled reactant from the product. Novel detection methods based on fluorescent substrates or ELISA assays, which do not use radiolabeled reagents, have been described more recently (Bullock and Gorman, 2000). 

Utility Radiolabeled Reagent for Characterizating Channels in Tissues... 

The preparation of tritium radiolabelled reagents using the reducing agent, LiEt3BT, (Supertritide), is described (4). 

Many derivatives of histidine are not stable to acid hydrolysis and are not discussed here (however, see 2.12.2 for those that occur naturally in proteins). Brief mention should be made of the iodination of histidyl residues by HOI ( 3.7.2). The mono- and diiodohistidines can be identified and distinguished from the iodinated tyrosines by high voltage paper electrophoresis in 1 M formic acid (Roholt and Pressman 1972) after complete enzymic hydrolysis of the protein or peptide ( 2.11 Roholt and Pressman 1972). Quantitation and identification are facilitated by the use of radiolabeled reagent. 

BPN, a potent opioid partial agonist , 277, has been labelled with carbon-11 at the 6-methoxy position, [ C PPN], using a precursor 278 synthesized in high yield (89%) from BPN 277 in two steps (equation Ilia) and employing [ C]iodomethane as the radiolabelling reagent (equation lib). 

Aliquots (10 pL) of each radiolabeled reagent are retained and counted with the tissue samples for subsequent calculation of percent injected dose per gram of the tissue samples. 

V. USE OF RADIOLABELED REAGENTS TO FOLLOW THE CHANGES IN RESIN FUNCTIONALITIES... 

An interesting approach for the analysis of the functionalization of soluble polymer supports could be the introduction of radiolabeled reagents. For a detailed description of the application of these methods see the respective chapters in [50]. 

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