Human complement factor h is a reductase for large soluble von willebrand factor multimers-brief report

OBJECTIVE-: Ultralarge von Willebrand factor (vWF) strings are secreted by, and anchored to, stimulated human endothelial cells. A disintegrin and metalloprotease with thrombospondin domains-type 13 cleaves the ultralarge vWF strings into large soluble vWF multimers. Normal plasma contains a nonproteolytic reducing activity that subsequently rapidly diminishes the size of the large soluble vWF multimers. APPROACH AND RESULTS-: The vWF reductase activity was isolated from normal cryoprecipitate-poor plasma by chromatography and identified as the complement regulatory protein, factor H (FH), by mass spectroscopy, SDS-PAGE, and monospecific anti-FH antibody. Removal of FH from partially purified vWF reductase by immunoabsorption eliminated the reducing activity, and the activity was recovered in the eluates. Recombinant human FH reduced large soluble vWF multimers in a free thiol-dependent reaction that was not inhibited by a variety of protease inhibitors. CONCLUSIONS-: FH contributes to the reduction of large soluble vWF multimers.