Biochemical use of isotopes

Radioactive isotopes provide a very convenient way of monitoring the fate or metabolism of compounds that contain the isotopes. When used in this way, the isotope is described as a tracer and compounds into which the radioactive atom has been introduced are said to be labelled or tagged. The labelled molecules need only comprise a very small proportion of the total amount of the unlabelled radioactive substance because they act in the same way as the non-radioactive substance but can be detected very much more easily. The varied applications of tracers in biochemistry range from studies of metabolism in whole animals or isolated organs to sensitive quantitative analytical techniques, such as radioimmunoassay. Phosphorus-32 is used in work with nucleic acids, particularly in DNA sequencing and hybridization techniques. In these instances the isotope is used as a means of visualizing DNA separations by autoradiographic techniques. 

While it is generally assumed that radioisotopes are metabolized in exactly the same way as the non-radioactive substances, occasionally some differences in reactivity are seen, due mainly to differences in the atomic weights of the two isotopes. The effect is most obvious for elements with the lowest atomic number, where the difference in mass between isotopes is the 

Ideally, molecules should be labelled by introducing a radioactive isotope in place of a normal atom, e.g. carbon-14 replacing a carbon-12 in a carbohydrate. This method of labelling involves the synthesis of the molecule either in vivo or in vitro and the use of enzymes often permits the isotope to be introduced in a particular position in the molecule. The position of the labelled atom should be indicated wherever possible as for example in glucose- 1-14C. 

Alternatively, it may be necessary to label a molecule by introducing an additional atom and in this case it has to be assumed that its presence does not alter the reactivity or metabolism of the molecule. Proteins, for instance, are often labelled with an isotope of iodine. It is very important that any labels used are firmly attached to the molecule otherwise invalid results will be obtained. 

The choice of an isotope for tracer studies requires an appreciation of not only the radiochemical properties of the element, but also the effects that they might have both biochemically and analytically. The isotope should have a half-life that is long enough for the analysis to be completed without any significant fall in its activity. Occasionally this might present a problem in that some elements only have radioactive isotopes with very short half-lives, e.g. fluorine-18 has a half-life of 111 min. Conversely, isotopes with very long half-lives should not be used for in vivo studies because accumulation in the tissues of the recipient is unacceptable. 

---