When researchers contemplate using a radioactive compound there are several things they have to consider. First and foremost, they must ask the question: is a radioisotope necessary or is there another way to achieve the objectives? The reason for this is that use of radioisotopes is governed by very strict legislation. The rules are based on the premise that radioactivity is potentially unsafe (if handled incorrectly) and should therefore only be used if there are no alternatives. Then, once it is decided that there is no alternative, the safest way of carrying out the work needs to be planned. Essentially this means using the safest isotope and the smallest amount possible. But why do we use radioisotopes in the first place? First, it is possible to detect radioactivity with exquisite sensitivity. This means that, for example, the progress of an element through a metabolic pathway or in the body of a plant or animal can be followed relatively easily. Very small amounts of a radioactive molecule are needed, and detection methods are well established. Second, it is possible to follow what happens in time. Imagine a metabolic pathway such as carbon dioxide fixation (the Calvin cycle). All the metabolites in the cycle are present simultaneously; so a good way to establish the order of the metabolism is to add a radioactive molecule (in this example, 14C-labelled carbon dioxide in the form of sodium bicarbonate) and see what happens to it by extracting the metabolites from the plant and identifying the radioactive ones. Third, it is possible to trace what happens to individual atoms in a pathway. This is done, for example, by creating compounds with a particular isotope in specific locations in the molecule. Fourth, we can identify a part or end of a molecule, and follow reactions very precisely. This has been very useful in molecular biology, where it is often necessary to label one end of a DNA molecule (e.g. for techniques such as gel mobility shift assay or DNA footprinting, methods for investigating sequence-specific DNA protein binding), or immunochemical diagnostics). Fifth, γ-ray emitters (60Co or 137Cs) are employed in irradiators that may be used for a variety of purposes in research and industry, including sterilisation of medical and pharmaceutical supplies, preservation of foodstuffs, eradication of insects through sterile male release programs and calibration of thermoluminescent dosimeters. Finally, there is a use that sometimes seems too obvious for it to be considered. In chemistry and biochemistry, we are used to chemical reactions where one compound is turned into another. We can identify and measure (‘assay’) the reactants and products and learn something about the atomic rearrangement during the reaction. A prominent example in this context is DNA replication where the use of radioisotopes provides a method for detecting particular products of the reaction.
The use of radioactivity in biochemical research has made a very significant contribution to knowledge, for example with the above-mentioned use of carbon-14 in the 1940s to study photosynthesis, through to modern drug discovery methods and DNA sequencing. It is fair to say, however, that many techniques that began with the use of a radioisotope have been replaced by other forms of detection system such as fluorescence, mainly due to the special safety provisions required when working with radioisotopes. Despite these developments, there are still some recent research approaches that rely on radioactivity (e.g. the scintillation proximity assay as used in the pharmaceutical industry).
Having understood the principles behind why radioisotopes are useful as described above, we now need to understand what radioactivity is and how to use it.
