protein extraction for use in ELISA

[James Munro]

Hello,
 
I am hoping for some suggestions for a protein that I am trying to extract for detection in an ELISA.
 
I have a protein that is 22 kDa which is soluble in sodium borate at either a low pH (<4) or high pH (>9). In its pure form I can detects it in a ELISA when using specific antibodies. When the protein is in commercial fish feeds I can not longer detect it. 
Has any one got an extraction method that I could try? 
 
Thanks
James