Ca²⁺ causes a change in absorbance of ADAMTS13, but only in low-ionic-strength conditions. (A) The Abs₂₈₀ (± SD) of 1 μM of purified ADAMTS13 in 20 mM Tris-HCl (pH 7.8) with or without 10 mM CaCl₂, and with or without 150 mM NaCl, was measured at 0 minutes and 60 minutes after the addition of CaCl₂ and/or NaCl. (B) Change in Abs₂₈₀ of 1 μM ADAMTS13 in 20 mM Tris-HCl (pH 7.8) and 10 mM CaCl₂ over 60 minutes. (C) Change in Abs₂₈₀ of 1 μM ADAMTS13 in 20 mM Tris-HCl (pH 7.8) preincubated for 1 hour with 0 to 5 mM CaCl₂. From these data, the EC₅₀ was derived.