Hi Barbara, are you not seeing precipitation when you add BAPNA to the cell lysates?

The arginine (guanidine group) in BAPNA can precipitate SDS in the RIPA buffer. But then you should see that when you add BAPNA to the lysate.

Dear Barbara

Mast cell tryptase, the most abundant product of the human mast cell, is emerging as a key mediator of allergic inflammation and as a potential target for therapeutic intervention. For your experiment how was the BAPNA concentration? and trypase sample volume?

So to avoid cloudy results please try the following conditions: You have to monitor spectrophotometrically in a microplate, adding preparations of 90 μ l tryptase assay buffer (100 mm Tris base, 1 m glycerol, pH 8·0) containing 1 mm BAPNA to 10 μ l tryptase sample. A unit (U) of tryptase activity was defined as that required to hydrolyse 1 μ mol of BAPNA per min at 25°C. 

And yet you must  verify the purity of  trypase used also measure specific activities to range it  2·5–3·0 mU/μ g of protein and endotoxin with concentrations as

Thank you both. I did try 100 uls of mast cell lysate lysed in PBS (spun down after lysis) and 2 mM BAPNA in 100 mM tris pH 8.0 and 1 M glycerol. It still was cloudy but not nearly as much as when the lysates were in RIPA. I did try using less lysate and it did help. It was less cloudy than using the 100 uls of lysate. I will look up the papers you suggested. I will also try to lyse in RIPA minus the SDS and see how that goes. Since I am using lysates and not pure tryptase, I thought that maybe I was adding too much protein. That's why I cut down to 50 uls but I did QC up to 200 uls with buffer( either RIPA or PBS). Thank you again.

OK, make the PBS lysate 2 mM in EDTA before spinning and see if that helps