Cathepsin Off-target Effects – How do I ensure results are not due to Z-VAD-FMK inhibiting lysosomal enzymes?

At high concentrations (typically > 50 μM ), Z-VAD-FMK is known to inhibit certain Cathepsins (B, L, and S). To distinguish these effects:

• Concentration Control: Keep your Z-VAD-FMK working concentration as low as possible (e.g., $10-20 μM).
• Alternative Inhibitors: Use a more specific inhibitor like Q-VD-OPh alongside Z-VAD-FMK. Q-VD-OPh has significantly lower cross-reactivity with Cathepsins.
• Functional Rescue: If your phenotype is purely Caspase-dependent, adding a Cathepsin-specific inhibitor (like E-64) should not mimic the exact effect of Z-VAD-FMK.