Fig. (4) Metal ion dependence of CIAP-catalysed hydrolysis of pNPP and bis-pNPP. Reaction mixtures (200 µl) containing 25 mM Tris- HCl (pH 8.5), 2.5 mM p-nitrophenyl phosphate (A), or bis-p-nitrophenyl phosphate (B), the indicated concentration of the metal ion (ZnCl₂ or MgCl₂), and 10 µM CIAP were incubated at 37 °C for 10 minutes. Reactions were initiated by the addition of the substrate to the reaction mixture and stopped by the addition of 1.0 ml 0.5 M NaOH. Activities are expressed as change in A₄₁₀ per minute for phosphomonoesterase activity and change in A₄₁₀ per hour for the phosphodiesterase activity.