The aim of my PhD project was to look at imaging of cell death in vivo using PET imaging. I have just uploaded the powerpoint slides for a talk that I gave for an interview a few months after my viva into google.docs. You can find it here (a bit scrappy, but you get the picture).
A very simple reaction that I performed numerous times during this study was the direct iodination of a protein with iodine-124.
The reaction takes place in the presence of an oxidising agent which removes electrons from the iodide ion so that you end up doing an electrophilic substition at the position ortho to the hydroxy group on the tyrosine residue.
You have to be slightly paranoid about this oxidising agent as your protein might be sensitive to oxidation itself. It turned out that annexin V was sensitive to stronger oxidising agents (like chloramine T), but seemed to retain its ability to bind to phosphatidylserine if the milder reagent iodogen was used.
We found that the radiolabeled proteins were actually quite stable (to dehalogenation) in vitro – even when incubated in plasma, but in vivo was a whole nother story. It seemed that most of the radioactivity was in the form of iodide ions within about a minute of being injected into mice. This is usually explained by the fact that the iodinated tyrosine residue is sufficiently similar to thryoxin to be deiodinated by the same enzymes. I still think that it dehalogenates amazingly fast to be explained by this alone.
Another moderately mysterious thing that we found was that the radioactive iodide seemed to accumulate not only in the urine, but in the stomach contents. Bear in mind that we injected the radiolabelled protein into the tail vein. At the time, the best explanation that we could come up with that had some support from the literature was that the iodide was somehow passing into the gut through the same channels as the chloride ions of HCl. Another theory is that the mice were injesting their urine. My pet rat, for example, does a behaviour where she wees on me and then proceeds to lick it up – I am sure that she is not the only one!