What reagent or drug can be used as proteinase inhibitor in vivo - (Mar/15/2010 )

We found a transcription factor being reduced at protein level up DNA damage but not changed at mRNA level. So the protein might be reduced by degradation. I wonder what reagents or drugs can be used to block protein degradation in general so that I can treat my cells with it along with DNA damage to see if the protein level can be maintained or less reduced.

MG132 will block the 18s proteasome, but there are more proteases than just that in cells.