Table 3. Specific Activities of Cathepsin L, Cathepsin V, PC1/3, and PC2^a.

protease	substrate	specific activity (pmol AMC/ng/min)
cathepsin L	Z-Phe-Arg-AMC	8.5
	Z-Tyr-Lys-Arg-AMC	3.2
cathepsin V	Z-Phe-Arg-AMC	0.88
	Z-Tyr-Lys-Arg-AMC	0.14
PC1/3	pGlu-Arg-Thr-Lys-Arg-AMC	0.132
	Z-Tyr-Lys-Arg-AMC	0.463
PC2	pGlu-Arg-Thr-Lys-Arg-AMC	0.0067
	Z-Tyr-Lys-Arg-AMC	0.0023

^aSpecific activities were measured using Z-Phe-Arg-AMC for human cathepsin L (0.04 ng/μL) and human cathepsin V (0.2 ng/μL), and using pGlu-Arg-Thr-Lys-Arg-AMC for PC1/3 (0.9 ng/μL) and PC2 (4 ng/μL) as standard fluorogenic peptide substrates commonly used to assay for these proteases.³⁵⁻³⁷ Also, specific activities with the substrate Z-Tyr-Lys-Arg-AMC of this study were used to compare these four enzymes. Cathepsin L and cathepsin V activity with Z-Tyr-Lys-Arg-AMC required the use of cathepsin H in a coupled assay to generate AMC fluorescence (from Figure 5). PC1/3 and PC2 activities were the same when conducted without or without cathepsin H (from Figure 6) and, therefore, their activities without cathepsin H is provided for this table.