Zinc ions promote the binding of factor XII/factor XIIA to acidic phospholipids but have no effect on the binding of high-Mr kininogen

Identification and characterization of plasma kallikrein-kinin system inhibitors from salivary glands of the blood-sucking insectTriatoma infestans

Two plasma kallikrein-kinin system inhibitors in the salivary glands of the kissing bug Triatoma infestans, designated triafestin-1 and triafestin-2, have been identified and characterized. Reconstitution experiments showed that triafestin-1 and triafestin-2 inhibit the activation of the kallikrein-kinin system by inhibiting the reciprocal activation of factor XII and prekallikrein, and subsequent release of bradykinin. Binding analyses showed that triafestin-1 and triafestin-2 specifically interact with factor XII and high molecular weight kininogen in a Zn2+-dependent manner, suggesting that they specifically recognize Zn2+-induced conformational changes in factor XII and high molecular weight kininogen. Triafestin-1 and triafestin-2 also inhibit factor XII and high molecular weight kininogen binding to negatively charged surfaces. Furthermore, they interact with both the N-terminus of factor XII and domain D5 of high molecular weight kininogen, which are the binding domains for biological activating surfaces. These results suggest that triafestin-1 and triafestin-2 inhibit activation of the kallikrein-kinin system by interfering with the association of factor XII and high molecular weight kininogen with biological activating surfaces, resulting in the inhibition of bradykinin release in an animal host during insect blood-feeding.

A mosquito salivary protein inhibits activation of the plasma contact system by binding to factor XII and high molecular weight kininogen

The salivary glands of female mosquitoes contain a variety of bioactive substances that assist their blood-feeding behavior. Here, we report a salivary protein of the malarial vector mosquito, Anopheles stephensi, that inhibits activation of the plasma contact system. This factor, named hamadarin, is a 16-kDa protein and a major component of the saliva of this mosquito. Assays using human plasma showed that hamadarin dose-dependently inhibits activation of the plasma contact system and subsequent release of bradykinin, a primary mediator of inflammatory reactions. Reconstitution experiments showed that hamadarin inhibits activation of the plasma contact system by inhibition of the reciprocal activation of factor XII and kallikrein. Direct binding assays demonstrated that this inhibitory effect is due to hamadarin binding to both factor XII and high molecular weight kininogen and interference in their association with the activating surface. The assays also showed that hamadarin binding to these proteins depends on Zn(2+) ions, suggesting that hamadarin binds to these contact factors by recognizing their conformational change induced by Zn(2+) binding. We propose that hamadarin may attenuate the host's acute inflammatory responses to the mosquito's bites by inhibition of bradykinin release and thus enable mosquitoes to take a blood meal efficiently and safely.

Partial identification of the Zn2+-binding sites in factor XII and its activation derivatives

With the purpose of identifying the Zn2+-binding sites in factor XII, the effect of chemical modification of His, Glu and Asp residues, amino acids known to participate in the catalytic coordination binding of Zn2+ in a number of Zn2+-binding proteins, was analysed. The number of modifiable His residues in factor XII and alpha-factor XIIa was 16.0+/-0.7 and 17.3+/-0.7, respectively. When factor XII/alpha-factor XIIa was incubated with saturating concentrations of Zn2+ before the diethylpyrocarbonate modification of the His residues, these numbers were reduced to 6.3+/-0.1 and 8.21+/-0.5, indicating that ten and nine His residues, respectively, are involved in the binding. Analysis of the Zn2+-binding capacity of factor XII, alpha-factor XIIa and beta-factor XIIa showed that while factor XII contains four Zn2+-binding sites, alpha-factor XIIa had only three and beta-factor XIIa had none. Modification of the His residues resulted in a complete loss of Zn2+-binding while Asp/Glu modification resulted in loss of two and one Zn2+-binding sites in factor XII and alpha-factor XIIa, respectively. This suggests that two of the four sites in factor XII contain His residues, exclusively, while the two others are comprised of two His residues and one Asp/Glu residue. One of the latter is lost when factor XII is activated to alpha-factor XIIa. Two of the sites are suggested to be located at positions His40-His44 and His78-His82. The location of the remaining two sites are reduced to four possible positions.

Influence of Zn2+ on the kinetic events that contribute to the 500-kDa dextran-sulfate-dependent activation of factor XII (Hageman factor)

The effect of zinc ions on kinetic and equilibrium steps that may contribute to the activation and subsequent autoactivation of human blood coagulation factor XII in the presence of 500-kDa dextran sulfate was studied. To analyze the results, the expression of the overall autoactivation constant that had been established from the model presented in a previous study was used. Comparison of the kinetics obtained at different levels of zinc, which included amounts lower than the residual concentration introduced by NaCl in the incubation mixture, suggested that the addition of Zn2+ up to 5 microM lowered the mean number of sites available for the binding of factor XII to the surface from 220 to 172 and increased the rate of the first-order activation of factor XII by one order of magnitude, from (1.6 +/- 0.4) x 10(-4) s(-1) to (8.0 +/- 0.4) x 10(-4) s(-1) in the presence of 550 nM dextran sulfate. Neither the factor XII/surface dissociation constant (1 microM), the apparent catalytic constant, nor the apparent Michaelis-Menten constant associated with the postulated multi-stage kinetic sequence were affected by the presence of zinc. Most experimental trends induced by the presence of zinc could be successfully interpreted by using the model, thus reinforcing its reliability under different conditions.

The surface-dependent autoactivation mechanism of factor XII

The dependency of concentrations of Zn2+ and the negatively charged surfaces, phosphatidylinositol phosphate (PtdInsP), sulfatide and dextran sulfate, on the autoactivation of human factor XII, has been studied. While the autoactivation induced by sulfatide, and low concentrations of dextran sulfate, was unaffected by the presence of Zn2+, that induced by PtdInsP and higher concentrations of dextran sulfate was completely dependent on Zn2+: the excess of Zn2+ needed to induce maximal activity with PtdInsP was 12-fold the concentration of factor XII, while with dextran sulfate it was 40-fold. Determination of the Zn2+-binding properties of factor XII revealed that a total of four zinc ions could bind to each factor XII molecule. The first bound zinc ions (Kd 0.1 microM) induced an increase in the intrinsic tryptophan fluorescence of factor XII, while further titration up to a 40-fold surplus resulted in a quenching of the fluorescence. Binding of the zinc ions that caused the quenching had an average Kd of approximately 1 microM, independent of whether it was determined from the fluorescence changes or by equilibrium filtration. Low concentrations of both sulfatide and PtdInsP induced a fluorescence increase similar to that at low concentrations of Zn2+ but, in contrast to sulfatide, higher concentrations of PtdInsP did not induce a quenching in fluorescence. As the Zn2+-independent activating surface (sulfatide) induced quenching in the fluorescence intensity, while the Zn2+-dependent activating surface (PtdInsP) did not, the quenching, whether it was caused by sulfatide or zinc ions, was assigned to a change in the conformation which resulted in a molecular structure of factor XII that could be autoactivated. Association of factor XII in this conformation on the activating surface was suggested to be responsible for the autoactivation.

Interaction of phospholipids with proteins and peptides. New advances III

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.