Inhibition of AP-B (22 nM) by zinc (at 1, 2, 5, and 10 μM) was assessed by inverse Lineweaver-Burk plots [15], which indicated zinc as a mixed inhibitor. The Lineweaver-Burk plot also showed that AP-B (without zinc) has Km value of 27 μM Arg-MCA and Vmax of 1.2 μM/min. The Ki and Ki′ values for zinc inhibition of enzyme and enzyme/substrate complexes were calculated as 6 μM and 12 μM zinc, respectively.