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.
[Illustration: Figure 24 _A college chemistry major analyzing a sample
of radioactive materials with an instrument known as a proportional beta
counter._]