14 Radioisotope techniques
R. J. SLATER
14.1 Why use a radioisotope?
14.2 The nature of radioactivity
14.3 Detection and measurement of radioactivity
14.4 Other practical aspects of counting of radioactivity and analysis of data
14.5 Safety aspects
14.6 Suggestions for further reading
14.1 WHY USE A RADIOISOTOPE?
When researchers contemplate using a radioactive compound there are several things
they have to consider. First and foremost, they must ask the questions: is a radioisotope
necessary, is there another way to achieve our objectives? The reason for this is that
radioisotope use 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? There are very good reasons; here
are some of them. Firstly, it is possible to detect radioactivity with exquisite sensitivity.
This means that, for example, the progress of a chemical through a metabolic pathway
or in the body of a plant or animal can be followed relatively easily. In short, much less
of the chemical is needed, and the detection methods are simple. Secondly, 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
(e.g.^14 C-labelled sodium bicarbonate) and see what happens to it. Thirdly, it is possible
to trace what happens to individual atoms in a pathway. This is done for example by
creating compounds with^14 C in specific locations on the molecule. Fourthly, 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 DNA footprinting, a method for investigating
sequence-specific DNA-binding proteins).
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