Radionuclides
Radionuclides or radioisotopes are radioactive isotopes of elements that are extremely important tools in biochemistry and cell biology. Radionuclides allow scientists to tag specific molecules without altering the structure or function of the studied compounds. Radioactive isotopes of elements nor­mally found in biological systems include carbon 14, hydrogen 3 (tritium), sulfur 35, and phosphorous 32. These unstable atoms decay over time (from seconds to centuries), emitting radioactive particles that can be detected by laboratory instruments.
Because radioactive elements can be detected, the tagged molecule, such as a protein, nucleic acid, or sugar, can then be detected with great accu­racy and sensitivity, especially if only a small amount of the molecule is pre­sent.
In a type of experiment called a pulse-chase, a radionuclide-tagged amino acid or nucleotide is incorporated into a protein or nucleic acid, and the fate of the protein or nucleic acid is monitored over time. For in­stance, cells are fed a nutrient mixture (the “pulse”) containing a radioac­tively tagged amino acid such as methionine, containing sulfur 35. After a few minutes, the cells have incorporated the tagged amino acid into most of the proteins synthesized during exposure to the tagged amino acid. The tagged amino acid mixture is then removed and replaced with untagged amino acids (“the chase”) that are then incorporated into all newly synthe­sized proteins from that point on. As time passes, the radioactivity incor­porated into each protein will disappear due to degradation of the protein by the cell. This is a measure of protein longevity and will vary from pro­tein to protein.
Radionuclides can also be used to monitor the metabolic fate of a nu­trient. For instance, radionuclide-tagged sugars can be fed to cells or to a live animal and samples of the cells or waste products from the animal an­alyzed over time. The radionuclide tag will appear in new compounds as time passes. The first products to appear are the initial metabolic break­down products of the sugar. Later, other radioactive compounds will appear representing intermediates in the complete breakdown of the sugar. This procedure can be used to identify the metabolic pathway used to break down the sugar and derive energy. For instance, the carbon in C-14-tagged glu­cose will eventually be found in carbon dioxide, the final breakdown prod­uct.