title: Proteolytic properties of single-chain factor XII: a mechanism for triggering contact activation.
abstract: When blood is exposed to variety of artificial surfaces and biologic substances, the plasma proteins factor XII (FXII) and prekallikrein undergo reciprocal proteolytic conversion to the proteases αFXIIa and α-kallikrein by a process called contact activation. These enzymes contribute to host-defense responses including coagulation, inflammation, and fibrinolysis. The initiating event in contact activation is debated. To test the hypothesis that single-chain FXII expresses activity that could initiate contact activation, we prepared human FXII variants lacking the Arg353 cleavage site required for conversion to αFXIIa (FXII-R353A), or lacking the 3 known cleavage sites at Arg334, Arg343, and Arg353 (FXII-T, for "triple" mutant), and compared their properties to wild-type αFXIIa. In the absence of a surface, FXII-R353A and FXII-T activate prekallikrein and cleave the tripeptide S-2302, demonstrating proteolytic activity. The activity is several orders of magnitude weaker than that of αFXIIa. Polyphosphate, an inducer of contact activation, enhances PK activation by FXII-T, and facilitates FXII-T activation of FXII and FXI. In plasma, FXII-T and FXII-R353A, but not FXII lacking the active site serine residue (FXII-S544A), shortened the clotting time of FXII-deficient plasma and enhanced thrombin generation in a surface-dependent manner. The effect was not as strong as for wild-type FXII. Our results support a model for induction of contact activation in which activity intrinsic to single-chain FXII initiates αFXIIa and α-kallikrein formation on a surface. αFXIIa, with support from α-kallikrein, subsequently accelerates contact activation and is responsible for the full procoagulant activity of FXII.
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN.
authors.forename: Ivan
authors.initials: I
authors.lastname: Ivanov
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN.|Department of Bioengineering and Organic Chemistry, Tomsk Polytechnic University, Tomsk, Russia; and.
authors.forename: Anton
authors.initials: A
authors.lastname: Matafonov
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN.
authors.forename: Mao-Fu
authors.initials: MF
authors.lastname: Sun
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN.
authors.forename: Qiufang
authors.initials: Q
authors.lastname: Cheng
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN.
authors.forename: S Kent
authors.initials: SK
authors.lastname: Dickeson
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN.
authors.forename: Ingrid M
authors.initials: IM
authors.lastname: Verhamme
authors.affiliation: Centre for Biomolecular Sciences, School of Pharmacy, University of Nottingham, Nottingham, United Kingdom.
authors.forename: Jonas
authors.initials: J
authors.lastname: Emsley
authors.affiliation: Department of Pathology, Microbiology and Immunology, Vanderbilt University Medical Center, Nashville, TN.
authors.forename: David
authors.initials: D
authors.lastname: Gailani
mesh_terms: D001777:Blood Coagulation; D020134:Catalytic Domain / Q000235:genetics; D005174:Factor XII / Q000378:metabolism*; D015956:Factor XIIa / Q000378:metabolism; D006801:Humans; D007610:Kallikreins / Q000378:metabolism; D059748:Proteolysis; D013499:Surface Properties
chemical_list: D005174:Factor XII; D007610:Kallikreins; D015956:Factor XIIa