Page 140

Radio-Isotopes in Biochemistry

  1. Those commonly used as research tools as β--emitters:

    Common
    name    		Decay equationHalf-LifeLimit Energy
    in MeV
    Tritium 1H3 → 2He3 + β- + anti-neutrino12¼ y0.019
    C-14 6C14 → 7N14 + β- + anti-neutrino5,730 y0.16
    S-35 16S35 → 17Cl35 + β- + anti-neutrino88 d0.17
    I-131 33I131 → 34Xe131 + β- + anti-neutrino8.0 d0.61
    P-32 15P32 → 16S32 + β- + anti-neutrino14 d1.71

  2. The typical energy spectrum of the emitted β-particles.

  3. The Paradox of Quantum Mechanics and a Continuous Spectrum Resolved

  4. Half-Life Mathematics

  5. Uses

    1. Everything has H- or C-atoms, so use tritium or C-14 to monitor biomass
    2. Only Proteins have S-atoms in them, so use S-35 to monitor protein content.
    3. Nucleic acids have most of the P-atoms in living systems
    4. Thyroxin and the thyroid gland have I-atoms. (Unlike the first three isotopes, this is a medicinal use rather than a research use.)

  6. Examples

  7. Pulse-Labelling

    1. Grow culture in tritiated-thymidine. All DNA becomes tritiated at an equilibrium concentration.

      At one point add P-32 phosphate. Newly forming RNA and DNA will incorporate the P-32. After a minute, add a thousand-times as much P-31 phosphate. From then one newly forming RNA and DNA will not be radioactive.

      If we isolate RNA from the culture a minute after adding the P-31, any RNA that was made during the time before adding P-31 will be radioactive (but none after the "quenching"). Suppose we have a good amount of radioactivity associated with the macromolecular fraction.

      We allow our culture to grow two more minutes, and again isolate RNA. But this time there is no macromolecular P-32. Where did it go?

      Next, let's take a look at the DNA made before, during, and after the P-32 "pulse."

    2. Suppose we infect E.coli with phage T4, and at the same time add P-32 phosphate. Soon the E.coli burst releasing hordes of T4, which we harvest, and store in the refrigerator.

      Immediately, we use some of those new T4s to infect E.coli and they have a high efficiency of infection.

      The next day we infect another batch of E.coli, but this time the efficiency has dropped more than a thousand fold. What happened in the refrigerator?

      Would it have made any difference had we used tritiated thymidine rather that P-32?

  8. Example of a discovery: the SOS-response (akin to the "hot-shock" response)