Page 284 - Radiochemistry and nuclear chemistry

268 Radiochemistry and Nuclear Chemistry

turnover in nutrients fed to animals can be determined; it was found that 20% of the
phosphorus in cow's milk comes directly from the feed, while 80 % is taken from the cow's
bone.
If a radioactively labeled compound such as an amino acid, a vitamin, or a drug is
administered to an animal, the substance is incorporated to varying degrees in different
organs (biological affinity). The substance undergoes chemical exchange with other
substances in the body, is broken down, and, finally, discharged from the body (i.e.
metabolizes). The radionuclide distribution in samples (cells, tissues, body fluids, etc)
removed from living species gives significant information about the (normal or abnormal)
physiology of that species. "C1. Biochemical analysis" in Table 9.3 lists these techniques,
which are discussed below.
(a) Autoradiography
Figure 9.12(a) shows the distribution of 14C-labeled p-aminosalicylic acid, PAS, the first
tuberculostatic agent developed, in a 20/~m thick section of a mouse. The autoradiograph
was obtained after 10 MBq 14C-PAS had been injected, and the mouse (weight 20 g) had
been killed by immersion into a CO2-acetone (-80~ mixture and sectioned. It is seen that
the PAS is concentrated mainly in the lung, where it is effective against tuberculosis, and
the kidney and intestine as it is excreted through these organs.
Figure 9.12(b) shows an autoradiograj~h of radioactively labeled cells. In the "pulse
labelling" (i.e. 10 - 30 rain exposure) to ~ only cells in the S-phase of the cell
cycle reacts with the thymidine and thus become labeled (the very dark spots). Both the cell
cycle times and number of cells in the cycle can be measured with this technique, which
is important in cancer cell research.
As an alternative to using the blackening of a photographic film for radiation detection
"instant imagers" based on semiconductor array detectors are commercially available.
(b) Radioimmunoassay (RIA)
Immunoassay is an application of the substoichiometric principle (w developed by
Yalow (Nobel laureate in 1977) for protein analysis. In the United States tens of millions
radioimmunoassays are made annually in hospitals to measure hormones, enzymes, viruses,
serum proteins, drugs, and so forth. Only a drop of the patient's blood is needed, reflecting
the versatility and sensitivity of this technique, which can be performed automatically.
Commercial RAST-kits (Radio Allergy Sorbent Tests) are used for rapid diagnosis of
allergic reactions.
In immunoassay, a known mass, w 0, of a labeled protein P* is allowed to react with a
much smaller (substoichiometric) mass of an antibody A, so that a complex P*A is formed.
The P*A is isolated and its radioactivity, R s, measured. Under the same conditions, an
identical mass, w 0, of labeled protein is mixed with an unknown mass, w u, of the identical
protein to be determined. This sample is also allowed to react with the same amount of anti-
body A as before; the complex P*A is again isolated (weight Wu+s) and its radioactivity,
Ru+ s, measured. The unknown weight, w u, is then calculated from (9.15).
(c) DNA-analysis
The chemical composition (the base sequence) of the DNA molecules, which make up the
chromosomes of the cell nucleus, is unique for each species and individual. The detailed
analysis of the DNA molecule provides important information about its host. This is used
in studies of the evolution of ~ies, in forensic science to identify criminals (e.g. from