Radioisotopes (Molecular Biology)

Isotopes are atomic species of the same atomic number (belonging to the same element) that have different mass numbers. The number of elements in the periodic table is about 110, and each one has more than one isotope; the total number of known isotopes is more than 1500. Each isotope of a given element has the same number of protons in its atomic nucleus, but differs in the number of neutrons in its nucleus. Isotopes of an element cannot be distinguished chemically because they have the same electronic structure and undergo the same chemical reactions.

Although some isotopes are stable, the nuclear configurations of radioisotopes (or radionuclides) are unstable, and they spontaneously undergo a radioactive transformation (or decay) to a more stable energy state (see Radioactivity). The half-life of each radioisotope is the time required for exactly one-half of the atoms to undergo radioactive transformation. Radioisotopes may decay to either stable or other radioactive species. Decay from one radioisotope to another is called a decay series.

Radioisotopes occur in small amounts in nature as the result of the decay of long-lived primordial materials (such as uranium-238). Atmospheric reactions with solar particles also produce radioactive species. Approximately 50 radionuclides occur naturally in the atmosphere, ocean, or the earth’s crust; these include carbon-14, potassium-40, radon-222, radium-226, and uranium-238.

Radioisotopes can be produced artificially by nuclear high energy reactions that combine atomic nuclei. The first human-made or artificial radioisotopes were made by Frederic and Irene Joliot-Curie in 1933, who irradiated a thin aluminum foil with alpha particles and observed tracks in a cloud chamber that diminished in intensity with a half-life of about 3 min, due to phosphorus-30 beta-plus decay. When they replaced aluminum with a boron foil, they found new activity, with a half-life of 14 min, due to nitrogen-13 beta-plus decay (1). In 1934, Lawrence produced small amounts of new radioisotopes at the Berkeley cyclotron using deuteron bombardment reactions on stable-element targets. Fermi produced heavier radioisotopes of the same element by neutron bombardment (or activation ), also in 1934 (1). Hevesey conducted the first biological studies with radioisotope tracers. These developments made it possible to discover, produce, and test a large number of scientifically significant radioisotopes during the decade that followed. Radioisotope production continues today. In general, neutron-rich radioisotopes are produced in nuclear reactors, whereas neutron-lean radioisotopes are produced in charged-particle accelerators. A carrier-free radioisotope is one that is not produced or mixed with any other isotope of the same element. The specific activity of a radioisotope preparation is the radioactivity (bequerels or curies) exhibited per unit mass or volume of the radioactive material.

Radioisotopes can be detected easily and identified using radiation detection instruments or photographic film (see Autoradiography and Fluorography). Therefore, they have numerous practical applications in the physical, chemical, and biomedical sciences. Among the most important applications in biomedical research are those that involve the tagging of a radioisotope to a biomolecule to permit tracking of the molecule in reaction processes and metabolism. Animal tissues containing radioisotopes are analyzed by nuclear radiation-detection techniques such as liquid scintillation counting, gamma spectroscopy, alpha spectrometry, and neutron activation analysis— depending on the relevant radiation emission.

In studies of life processes, the most important radioisotopes are those of hydrogen, carbon, sulfur, and phosphorous, because these elements are present in practically all cellular components essential to maintaining life. Some of the more common radionuclides used in biomedical research are given in Table 1, together with their mass number, physical half-life, beta-particle yield and energies, and associated gamma-ray energies.

Table 1. Common Radioisotopes Used in Biomedical Research, with Principal Radioactive Emissions, Yields, and Energies (2)

^a Atomic mass units. ^b Million electronvolts.

In molecular biology, the most important application of radioisotopes is the radioactive labeling of nucleic acids and proteins. Radioactively labeled cells, such as organelles and chromosomes, can be imaged on high speed X-ray film in a process called autoradiography.