Talmudic Question #53A

In recent years, the use of radioactive isotopes has diminished considerably in microbiological re­search. What has been lost in the process?
 

Commentary by Elio
 

Source

This is a nostalgia trip. Molecular microbiologists of my generation re­lied on the use of radioactive isotopes for much of their research. In those days, the size of a de­partment's research effort could be mea­sured by the number of scintillation counters it had at its disposal. Ra­dioactivity was a convenient way of detecting and quan­titating very small amounts of cell constituents. This has now been superseded by fluorescence and other non-radioactive methods. However, there are some things that radioactive isotopes do best. Thus, they are still in use. Here are two such uses.

1. Determining which molecules are made anew. Adding radioactive precursors of macromolecules to a growing culture will result in selective labeling of the newly synthesized molecules. If the labeled compound is added for a short time ("pulse labeling"), the radioactivity is found in the portion of the molecule most recently made. Among many other things, such methods were used to elucidate the direction of synthesis of proteins (from N- to C-terminus) and of nucleic acids (5'- to 3'-terminus). Moreover, with this strategy you could estimate the speed with which such macromolecules are synthesized.
2. Measuring the stability of a molecule. Adding a labeled precursor for some period of time, then replacing it with its non-radioactive counterpart (“pulse-chase”) lets us measure the stability of synthesized molecules. If the molecules are stable, they will retain the radioactive label; if unstable, the label will be lost. This method enabled us to determine the half-life of high-turnover molecules such as mRNA and certain regulatory proteins.

These days, when I mention radioactive isotopes, the response I get tends to focus on the dangers of working with them. Well, in the past we were not only careful but also aware of what constitutes danger. Mostly, we worked under safe conditions. Or so we thought. And I'm still around, many millicuries of ³²P later.
 

 
 

 
 

Comments

[qetzal]

As a molecular biologist, I mostly miss the benefits of using 32P-end-labeling for detecting trace amounts of DNA in gels. Especially for things like gel shifts, there's nothing easier than 32P.

Well, nothing easier if you only consider the experiment itself. Once you factor in the regulatory, handling, & disposal requirements, things change.

In my limited experience, quantum dots are cool, but there are things you can do with 32P or other radionuclides that you can't possibly do with quantum dots.

As for lab safety, I think most molecular labs used small enough amounts that safety was not such a huge issue. Obviously, proper procedures are needed, but they're not really THAT hard to maintain. Safety for the folks who have to dispose of aggregated wastes from multiple labs may well be a bigger issue, though.