Caspase 3 is a member of the cysteine-aspartic acid protease (caspase) family. Sequential activation of caspases plays a central role in the execution-phase of cell apoptosis. Caspases exist as inactive proenzymes that undergo proteolytic processing at conserved aspartic residues to produce two subunits, large and small, that dimerize to form the active enzyme. This protein cleaves and activates caspase 6 and 7; and the protein itself is processed and activated by caspases 8, 9, and 10. Caspase 3 can hydrolyze the peptide substrate acetyl-Asp-Glu-Val-Asp-p-nitroanilide (Ac-DEVD-pNA) resulting in the release of the p-nitroaniline (pNA) moiety. p-Nitroaniline has a high absorbance at 405 nm. Thus the activity of recombinant mouse caspase 3 can be measured by calculating the concentration of the pNA released from the substrate. The reaction was performed in adding 50 μl 2×buffer (50mM HEPES,100mM NaCl,10mM DTT, 2mM EDTA, 10% glycerol) to 96 well plates, then add 50 μl various concentrations of caspase 3 (diluted by 1×buffer, 25mM HEPES, 50mM NaCl, 5mM DTT, 1mM EDTA, 5% glycerol, pH 7.4) to each well, finally, add 5 μl 4mM Ac-DEVD-pNA to each well. Cover the 96 well plates and incubate at 37 ℃ for 1h. p-Nitroaniline (pNA) Standard curve prepare by double dilute 200 μM pNA with 1×buffer and record the OD value at 405 nm. The specific activity of recombinant mouse caspase3 is >3000 pmol/min/μg.