Protease activity assay using pNA-peptide as substrates - Can anyone help me? (Oct/24/2008 )

I want to use N-CBZ-Gly-Gly-Leu-pNA(Sigma,C-0322) as substrate peptide to test protease activity because this peptide contains the potential cleavage site possible recognized by my protease.The peptide was dissolved in Methanol(100mM) as stock .When i use my assay buffer(50mM Tris.Hcl,pH8.0) to dillute to 1mM or 10mM , i found the white precipitate formation and the liquid is cloudy.I just don't know whether it is normal or not.Does anyone have the same experience or anyone can help me to solve the peptide solubility issue?
Thanks a lot in advance.

when you dilute the substrate that much then you shock it into crystallizing. you may want to try diluting the substrate more slowly (serially, 1+1 then 1+4 then 1+9) so that you don't dilute it too fast.

Thanks a lot.
I will try it.