Radioisotopes in Biochemical Research

This chapter will explore the origin, properties, detection, biochemical applications and safety concerns of radioisotopes. 

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The stability of a nucleus depends on the ratio of neutrons to protons. Some nuclei are unstable and undergo spontaneous nuclear disintegration accompanied by emission of particles. Unstable isotopes of this type are called radioisotopes. Three main types of radiation are emitted during nuclear decay a particles, j8 particles, and y rays. The a particle, a helium nucleus, is emitted only by elements of mass number greater than 140. These elements are seldom used in biochemical research. 

The properties of several radioisotopes that are important in biochemical research are listed in Table 6.1. Note that many of the isotopes are /3 emitters however, a few are y emitters. 

The half-life, defined in the previous section and listed for each isotope in Table 6.1, is an important property when designing experiments using radioisotopes. Using an isotope with a short half-life (for example, 24Na with ty2 = 15 hr) is difficult because the radioactivity lost during the course of the experiment is significant. Quantitative measurements made before and after the experiment must be corrected for this loss of activity. Radioactive phosphorus, 32P, an isotope of significant value in biochemical research, has a relatively short half-life (14 days), so if quantitative measurements are made they must be corrected as described in Equations 6.7 and 6.8. More information about the choice of a radioisotope in an experiment, the detec-... 

Since most of the radioisotopes used in biochemical research are j8 emitters, only methods that detect and measure /3 particles will be emphasized. Two counting techniques are in current use, scintillation counting and Geiger-Miiller counting. 

CHAPTER 7 Contents XVii Study Problems 1 87 Further Reading 1 87 Radioisotopes on the Web 1 88 Centrifugation in Biochemical Research 1B9... 

If a radioisotope is substituted for a nonradioactive isotope of the same element in a chemical reaction, aU compounds formed from that element in a series of steps will also be radioactive. That makes it possible to follow the reaction pathway using instruments that can detect radiation. In this way, the series of steps involved in many important reactions has been studied, as shown in Figure 21.17. Tracers containing radioactive phosphorus-32 have also been used in biochemical research to help clarify complicated metabolic pathways. Tracers are also used to test structural weaknesses in mechanical equipment and to follow the pathways taken by pollutants. 

Samples for liquid scintillation counting consist of three components viz. (1) the radioactive material, (2) a solvent, usually aromatic, in which the radioactive substances is dissolved or suspended, and (3) one or more organic fluorescent substances. Components (2) and (3) make up the liquid scintillation system. The P particles emitted from the radioactive sample (most of the radioisotopes used in biochemical research are P emitters) interact with the scintillation system, producing small flashes of light or scintillations. The light flashes are detected by a photomultiplier tube (PMT). Electronic pulses from the PMT are amplified and registered by a counting device called a scaler. 

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

Durbin, P. W., Assling, C. W., Johnson, M. E., Hamilton, J. G. and Williams, M. H. (1956a). The metabolism of the lanthanons in the rat. II. Live studies of the tissue deposition of intravenously administered radioisotopes, page 171 in Rare Earths in Biochemical and Medical Research A... 

Since the advent of the atomic age radioisotopes have played a variety of important roles in what is broadly be termed the life sciences, including agricultural and soil sciences. The benefits cannot be argued. Various research activities have over the years employed almost every conceivable radioisotope, but probably the most commonly used isotopes are those employed for specific medical treatments or as tracers in biochemical investigations (Table 1 ). Of particular value are the isotopes that can be attached to particular sites in cellular matter to trace chemical processes in living tissue H, and Other isotopes are used so routinely... 

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