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Protein turnover can be measured by calculating the "decay" or loss of radio-labeled proteins in the blood, for example, but I am confused at how this calculation works. Wouldn't the radioactive isotope that you're using be decaying due to being unstable and not just due to protein turnover? Would you have to account for that in your calculations?

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  • $\begingroup$ Which calculation are you using? $\endgroup$ – Chris Dec 2 '17 at 9:48
  • $\begingroup$ A=Ao*e^-kt, where A = tagged protein at any time, Ao = tagged protein at zero time, t = time in days, and k = turnover rate as fraction of the protein pool turned over per day. By rearranging the equation you can get turnover rate (k). $\endgroup$ – strugglebus Dec 2 '17 at 15:42
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Technically yes, but practically it depends on what the half-life of your isotope is relative to the half-life of your protein. For example, tritium has a half-life of a little over 12 years, but most protein labeling experiments take place over only a few days at most (it depends on your protein). So while technically the tritium is decaying during your experiment, the amount of decay will be below your detection limit, and therefore is negligible over the course of the experiment.

So basically it works best to choose an isotope that will not appreciably decay over the time course of your experiment so you don't have to worry about it. If this is not possible, then yes, you would need to correct for this.

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    $\begingroup$ I've read some papers that use Iodine 131 to label proteins, which has a half life of 8 days. So I have another question: how do people detect these isotope labels? Do people rely on their decay to measure it (for example the release of beta-minus and gamma emissions? In that case, wouldn't you want to use something as radioactive as I-131? $\endgroup$ – strugglebus Dec 2 '17 at 15:39
  • $\begingroup$ Detection depends on the isotope used. Tritium is a low energy beta emitter, so it would need to be removed from the place you are measuring and put into scintillation fluid or something analogous.For something that is higher energy, like P-32, you could use film or a geiger counter. For imaging in live organisms, I think you do need something fairly strong like I-131, unless you are harvesting samples from the organism. For I-131 you can do a PET scan, since it is a gamma emitter. Typically, this does rely on the decay of the particle. 8 days is still plenty of time for most imaging though. $\endgroup$ – Hayden S Dec 2 '17 at 18:28

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