Which is the best method to recover a precipitated protein back into its normally folded state?

Don't really understand the question.  Is that a list of possible answer choices or are they the different situations that caused the protein to precipitate?

Getting a precipitated protein to refold is a multi-step procedure.  I'm not familiar with using any of those chemicals.  Maybe if I give you my general reasoning process, it will help you figure it out.

The first thing I would do is resolubilize the precipitate.  I would try a solution with a high concentration of urea first.  If that didn't work, I would try guanidine.  If that didn't work, I don't know what I would use.  Detergents might get the protein solubilized again, but they are a royal pain in the ass to get rid of afterwards, so I would only consider them as a last resort.  I might try some sort of organic solvent, but that is starting to get a little crazy.  You could add a little heat as well.  You might need a reducing agent if there are disulfide bridges present.

After the protein is solubilized in its denatured form, the next step is to get it to refold.  I would dialyze the solution in multiple steps, headed towards a final buffer that did not have any denaturant in it.  The choice of buffer would require a lot of trial and error to get the right pH, ionic strength, metal ions, +/- glycerol, etc.  If there are disulfide bridges, you may need a redox active buffer of mixed oxidized and reduced glutathione.

Addendum: I don't believe it.  Yahoo censors language?  I typed three letters there, not three asterixes.


Another addendum: Maybe I am still misunderstanding.  Is the situation that you *want* to precipitate the protein in order to separate it from everything else?  If that is the case, I would go with ammonium sulfate, although I am confused by why it is listed twice.  If the solubility of the protein is not already known, then you would titrate in solid ammonium sulfate until the protein salted out.  Is albumin the protein you are trying to separate (either you didn't type it or it was cutoff)?  If so, 25 % ammonium sulfate is approximately correct (see my source--albumin stays in solution, while other serum globulins salt out).  Ammonium sulfate is probably the most commonly used reagent for this purpose.  Increasing the ionic strength with a non-chaotropic salt is probably the gentlest approach that you have listed.

Alcohols have variable effects on the solubility of different proteins, but I wouldn't use them as general reagents (good for nucleic acids though).  TCA will precipitate *most* of your proteins out, so it isn't helpful for separating things.  I am not familiar with any of the others.  I found one protocol that used perchloroacetic acid for precipitating muscle proteins, but I suspect it might have the same problem as TCA.  Sodium tungstate does make proteins precipitate, but I don't know enough about it to say whether it is helpful in a fractional prep.  I have no idea how uranyl acetate might help you.  As far as I know, it's primary use is for staining for electron microscopy.  I think barium hydroxide might be helpful for precipitating out other non-protein stuff and keeping your protein in solution, but using it with a sulfate might cause some problems.  Methinks the only precipitate you will get is barium sulfate--but that may not matter if you are trying to keep your protein *in solution*.  Again, not very familiar with it.