unable to ppt. protein using ammonium sulphate

[S.A. Benson Lab]

Hello!
I have a problem trying to precipitate a purified protein, I have 
used almost 60% of ammonium sulphate and actually the solution is 
saturated and I still don't see any cloudiness. I'm sure that the 
concentration of the protein is at least about 15ug/ml and I can see 
a band on SDS gel when I loaded 10ul of the sample.  I heard that 
usually 45% of it will do the job! and I don't know what the story is
about my situation. Would someone who has an idea of this
situation gives me some advices please? and thanks in advance!

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Benson Laboratory
Department of Microbiology
Microbiology Building
University of Maryland
   at College Park
College Park, M.D. 20742
Email  sablab at microb.umd.edu
Phone  301-405-5441
Fax    301-314-9489
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