Labeling an Amino Acid with a Radioactive Isotope
Suppose we have the amino acid leucine labeled with ¹⁴C and we inject a solution containing it into an experimental animal. Since leucine is incorporated into proteins, if we isolate the proteins and determine both the amount of proteins and the amount of radioactivity, we can measure fairly accurately the rate of protein synthesis. Autoradiography, by the way, is of little help in studying most protein synthesis because all cells are always synthesizing proteins and so are all labeled after a single exposure to a radioactive amino acid. With RNA precursors autoradiography at least told us where RNA was being made, but with amino acids we do not even get this information because proteins are synthesized both in the nucleus and in the cytoplasm.
Under these circumstances radiochemical methods are better for studying protein synthesis. Proteins are isolated from the residue left after a nucleic-acid extraction process similar to that described previously, and the amount of protein is determined by a simple colorimetric analysis based on comparison of the color of the solution with a standard color. The amount of radioactivity (remember that we are now using a precursor labeled with ¹⁴C) can be determined with a gas-flow counter, which is probably more widely used at present than any other instrument for counting beta emitters, chiefly because of its reliability and low cost.
Figure 24 A college chemistry major analyzing a sample of radioactive materials with an instrument known as a proportional beta counter.