In recent years many metabolic studies have been carried out with isotopically labeled amino acids. The heavy isotope of carbon, of hydrogen and of nitrogen as well as the radioactive isotope of sulfur have been useful as markers of particular amino acids. These isotopic "tags" serve, figuratively speaking, as little red lanterns that permit the tracing of their bearers in the animal body. At the same time the markers do not alter the chemical reactivity of a compound containing them, for they are isotopes of the atoms which they replace. Isotopic studies have permitted the elucidation of many obscure details of particular phases of protein metabolism and have, moreover, forced a revision of the old concept of the rather static condition of body protein.
Investigations with marked amino acids have proved to be extremely useful in the identification of the precursors of certain physiologically active compounds. Thus, it has been
ISOTOPIC STUDIES AND PROTEIN METABOLISM. JAMA. 1943;123(17):1118–1119. doi:10.1001/jama.1943.02840520034011
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