Isotopic labelling with radioactive carbon

The heavy carbon-isotope and the radioactive carbon isotope were used to trace the pathway of carbon atoms through the citric acid cycle. One such experiment created a great controversy over whether or not citric acid was the first tricarboxylic acid in the cycle. In this experiment, acetate labeled in the carboxyl carbon (CHj—C ooi was incubated aerobically with a tissue preparation. After incubation, a-ketoglutaric acid was isolated from the tissue preparation. This compound was then degraded through known chemical reactions in order to establish the positions of the radioactive carbon derived from the labeled acetate. In TCA cycle. 

When compounds enriched in the heavy-carbon isotope 13C and the radioactive carbon isotopes nC and 14C became available about 60 years ago, they were soon put to use in tracing the pathway of carbon atoms through the citric acid cycle. One such experiment initiated the controversy over the role of citrate. Acetate labeled in the carboxyl group (designated [1-14C] acetate) was incubated aerobically with an animal tissue preparation. Acetate is enzymatically converted to acetyl-CoA in animal tissues, and the pathway of the... 

A radioactive tracer method is described for the quantitative assay of HMX in mixts of HMX and RDX. The method is an isotope dilution technique in which a known amt of pure HMX, labeled with carbon-14, is added to the mixt, and the radioactivity of a fraction of... 

In a specimen of a monomer labelled with carbon-14 or other radioactive isotope, only a small fraction of the molecules actually contain an atom of the isotope if only part of the labelled monomer is polymerized, the specific activity of the part which has reacted may be a little different from that of the residual monomer because of a kinetic isotope effect. The polymerization of a labelled monomer might therefore be regarded as a co-polymerization with a drift in the composition of the feed. This effect has been examined by comparing the polymerizations of unlabelled styrene, a-14C-styrene and /3-14C-styrene in both radical and cationic reactions (18). 

Commonly used radiopharmaceuticals are carbon 14 (ti/2 30 years), cobalt 57 (ti/2 271 days), cobalt-58 (ti/2 70.8 days), gold-198 (ti/2 2.7 days), iodine-123 (ti/2 12.3 hours), iodine-125 O1/2 60 hours), iodine-131 (ti/2 8.04 days), and tritium (ti/2 12.3 years). The iodine radioisotope is used to study thyroid function and is used in the treatment of hyperthyroidism and thyroid carcinoma. Various monoclonal antibodies labeled with iodine-171 are used for the detection of malignant neoplasms. Genetic damage is a dangerous side effect of radioactive isotopes prior to and during the reproductive period. Exposure to large doses leads to leukopenia, anemia, skin inflammation, radiation sickness, and neoplasm. 

Nucleic acids can be labeled with a radioactive isotope in vitro by heating them in a mixture containing radioactive iodine and thallic trichloride (TICI3). The reaction is rapid and simple. It results in the formation of a stable covalent bond between the radioactive iodine atom and carbon atom 5 of cytosine in the nucleic acid. The biological properties of the nucleic acid are not significantly affected by this procedure. 

Direct measurements of active centre concentrations have been made by terminating the polymerization with some reagent such as iodine [86], carbon dioxide [115] or an alcohol [116], labelled with an isotope which introduces radioactivity into the polymer e.g. 

By using cyanide ions labeled with the radioactive isotope carbon-14, chemists have shown that [Ni(CN)4] undergoes ligand exchange very rapidly in solution. The following equilibrium is established almost as soon as the species are mixed ... 

The most sensitive method for measuring carbon dioxide production by histidine decarboxylase is that in which histidine labelled with carbon-14 in the carboxyl group is used as substrate. The C02 evolved is trapped in a suitable absorbent medium, and its radioactivity is then determined directly in a scintillation spectrometer. Several variations of this procedure have been described - - - - , and these compare favourably in speed and sensitivity with alternative isotopic methods in which the production of radioactive histamine from ring- C-labelled histidine is measured (see following section). 

In the use of radioactive tracers it is assumed that the radioactive isotopes studied are identical in chemical behavior to the nonradioactive isotopes. The first experiments that used radioactive tracers were carried out in 1913 in Germany and were designed to measure the solubility of lead salts via the use of a radioactive isotope of lead. In industry, radionuclides have been used for analytical purposes, for measurements of flow in pipes, and as part of many other apphcations. Another example of an important tracer study has been the investigation of photosynthesis of carbohydrates from atmospheric CO2 in the presence of light and chlorophyll. Scientists used eC, 15P, and iH to identify the intermediate steps involved in the photosynthesis of carbohydrates in plants that had been placed in an atmosphere composed of fyC-labeled CO2 and had been irradiated with hght. The presence of the radioactive carbon in the synthesized carbohydrate was evidence that O2 was involved in the synthesis. 

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