Protein purification: iodoacetic acid -does it stuff up cysteines

Warren Gallin gal-1 at bones
Tue Feb 21 09:57:55 EST 1995

On 21 Feb 1995, Orak wrote:

> I have been using iodoacetic acid (IAA) as a protease inhibitor for purifying
> recombinant transcription factors.  However, someone recently told me
> that IAA works by modifying cysteine residues.  Can someone confirm this
> for me?  Exactly how does it work?
> This could be a problem for me since ETS transcription factors are
> reliant on spaced cyteines for DNA binding activity.
Iodoacetamide and iodoacetic acid are both excellent alkylation agents 
for blocking cysteines specifically.  The sulfur on the reduced cysteine 
displaces the iodine, leaving you with carboxymethylcysteine.  It is used 
as a protease inhibitor because it will irreversibly inhibit proteases 
with active site cysteines.  It sounds to me from what you say that you 
shoudl avoid this like the plague for your project, although it shouldn't 
be as bad at acidic pH.

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