When human plasma is applied to a dermatan sulfate column, amidase activity is detected in the bound fraction and complement factor H is cleaved [A. Saito, H. Munakata, Factor H is a dermatan sulfate-binding protein: identification of a dermatan sulfate-mediated protease that cleaves factor H, J. Biochem. 137 (2005) 225-233]. Here, the amidase-active fraction was purified by sequential gel filtration and hydroxyapatite chromatography, and the amidlase-active protein was identified to be plasma kallikrein by mass spectrometry. The activation of plasma kallikrein was further investigated by Western blotting using plasma deficient in prekallikrein or coagulation factor XII. The dermatan sulfate column-bound fraction of the prekallikrein- and factor XII-deficient plasmas did not show any amidase activity and factor H remained intact. Addition of kallikrein, but not activated factor XII, to factor H purified from plasma resulted in cleavage of factor H. Thus, dermatan sulfate induces contact activation and activates kallikrein-mediated cleavage of FH. (c) 2008 Elsevier Inc. All rights reserved.