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 This has now been superseded by fluorescence and other non-radioactive methods.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

[Mark O. Martin]

It's certainly true that people of my "era" in biology became pretty paranoid (or cavalier) about radioactivity. It's good, I think, that hot isotopes are not used in the undergraduate classroom much these days.

Nanodots---also called quantum dots---may be used in the future in the way hot isotopes were used in the Good Old Days:

Frasco, MF and N Chaniotakis. (2009). "Bioconjugated quantum dots as fluorescent probes for bioanalytical applications." Anal. Bioanal. Chem. ---- this just came out on August 28th and is available electronically; PUBMED does not give a good record yet.