Acceleration of surface-dependent autocatalytic activation of blood coagulation factor XII by divalent metal ions

Zinc: An important cofactor in haemostasis and thrombosis

There is mounting evidence that zinc, the second most abundant transition metal in blood, is an important mediator of haemostasis and thrombosis. Prompted by the observation that zinc deficiency is associated with bleeding and clotting abnormalities, there now is evidence that zinc serves as an effector of coagulation, anticoagulation and fibrinolysis. Zinc binds numerous plasma proteins and modulates their structure and function. Because activated platelets secrete zinc into the local microenvironment, the concentration of zinc increases in the vicinity of a thrombus. Consequently, the role of zinc varies depending on the microenvironment; a feature that endows zinc with the capacity to spatially and temporally regulate haemostasis and thrombosis. This paper reviews the mechanisms by which zinc regulates coagulation, platelet aggregation, anticoagulation and fibrinolysis and outlines how zinc serves as a ubiquitous modulator of haemostasis and thrombosis.

Polyphosphate as a haemostatic modulator

Platelets are small anuclear cells that play a central role in haemostasis. Platelets become activated in response to various stimuli triggering release of their granular contents into the surrounding milieu. One of these types of granules, termed dense granules, have been found to contain polyphosphate (polyP) in addition to other inorganic biomolecules, such as serotonin, ADP, ATP, PPi. Individuals deficient in dense granules exhibit bleeding tendencies, emphasizing their importance in haemostasis. Platelet polyP is of a relatively defined size, approximately 60-100 phosphate monomers in length. These linear polymers act at various points in the coagulation and fibrinolytic systems thereby modulating the haemostatic response. Due to its highly anionic nature, polyP lends itself to being a natural activator of the contact system. The contact system functions in multiple pathways including coagulation, fibrinolysis, inflammation and complement. Activation of the contact system accelerates thrombin generation, the terminal enzyme in the coagulation cascade. PolyP also modulates factors further downstream in the coagulation cascade to augment thrombin generation. The net effect is increased fibrin formation and platelet activation resulting in faster clot formation. PolyP is incorporated into the forming clot thereby modifying the structure of the resulting fibrin network and its susceptibility to degradation by certain plasminogen activators. In conclusion, release of platelet polyP at the site of injury may facilitate clot formation and augment clot stability thereby promoting wound healing.

Polyphosphate colocalizes with factor XII on platelet-bound fibrin and augments its plasminogen activator activity

Activated factor XII (FXIIa) has plasminogen activator capacity but its relative contribution to fibrinolysis is considered marginal compared with urokinase and tissue plasminogen activator. Polyphosphate (polyP) is released from activated platelets and mediates FXII activation. Here, we investigate the contribution of polyP to the plasminogen activator function of αFXIIa. We show that both polyP₇₀, of the chain length found in platelets (60-100 mer), and platelet-derived polyP significantly augment the plasminogen activation capacity of αFXIIa. PolyP₇₀ stimulated the autoactivation of FXII and subsequent plasminogen activation, indicating that once activated, αFXIIa remains bound to polyP₇₀ Indeed, complex formation between polyP₇₀ and αFXIIa provides protection against autodegradation. Plasminogen activation by βFXIIa was minimal and not enhanced by polyP₇₀, highlighting the importance of the anion binding site. PolyP₇₀ did not modulate plasmin activity but stimulated activation of Glu and Lys forms of plasminogen by αFXIIa. Accordingly, polyP₇₀ was found to bind to FXII, αFXIIa, and plasminogen, but not βFXIIa. Fibrin and polyP₇₀ acted synergistically to enhance αFXIIa-mediated plasminogen activation. The plasminogen activator activity of the αFXIIa-polyP₇₀ complex was modulated by C1 inhibitor and histidine-rich glycoprotein, but not plasminogen activator inhibitors 1 and 2. Platelet polyP and FXII were found to colocalize on the activated platelet membrane in a fibrin-dependent manner and decorated fibrin strands extending from platelet aggregates. We show that in the presence of platelet polyP and the downstream substrate fibrin, αFXIIa is a highly efficient and favorable plasminogen activator. Our data are the first to document a profibrinolytic function of platelet polyP.

D-dimer: An Overview of Hemostasis and Fibrinolysis, Assays, and Clinical Applications

D-dimer is the smallest fibrinolysis-specific degradation product found in the circulation. The origins, assays, and clinical use of D-dimer will be addressed. Hemostasis (platelet and vascular function, coagulation, fibrinolysis, hemostasis) is briefly reviewed. D-dimer assays are reviewed. The D-dimer is very sensitive to intravascular thrombus and may be markedly elevated in disseminated intravascular coagulation, acute aortic dissection, and pulmonary embolus. Because of its exquisite sensitivity, negative tests are useful in the exclusion venous thromboembolism. Elevations occur in normal pregnancy, rising two- to fourfold by delivery. D-dimer also rises with age, limiting its use in those >80 years old. There is a variable rise in D-dimer in active malignancy and indicates increased thrombosis risk in active disease. Elevated D-dimer following anticoagulation for a thrombotic event indicates increased risk of recurrent thrombosis. These and other issues are addressed.

Single-chain factor XII exhibits activity when complexed to polyphosphate

BACKGROUND

The mechanism underpinning factor XII autoactivation was originally characterized with non-physiological surfaces, such as dextran sulfate (DS), ellagic acid, and kaolin. Several 'natural' anionic activating surfaces, such as platelet polyphosphate (polyP), have now been identified.

OBJECTIVE

To analyze the autoactivation of FXII by polyP of a similar length to that found in platelets (polyP70 ).

METHODS AND RESULTS

PolyP70 showed similar efficacy to DS in stimulating autoactivation of FXII, as detected with amidolytic substrate. Western blotting revealed different forms of FXII with the two activating surfaces: two-chain αFXIIa was formed with DS, whereas single-chain FXII (scFXII; 80 kDa) was formed with polyP70 . Dissociation of scFXII from polyP70 abrogated amidolytic activity, suggesting reversible exposure of the active site. Activity of scFXII-polyP70 was enhanced by Zn(2+) and was sensitive to NaCl concentration. A bell-shaped concentration response to polyP70 was evident, as is typical of surface-mediated reactions. Reaction of scFXII-polyP70 with various concentrations of S2302 generated a sigmoidal curve, in contrast to a hyperbolic curve for αFXIIa, from which a Hill coefficient of 3.67 was derived, indicative of positive cooperative binding. scFXII-polyP70 was more sensitive to inhibition by H-d-Pro-Phe-Arg-chloromethylketone and corn trypsin inhibitor than αFXIIa, but inhibition profiles for C1-inhibitor were similar. Active scFXII-polyP70 was also able to cleave its physiological targets FXI and prekallikrein to their active forms.

CONCLUSIONS

Autoactivation of FXII by polyP, of the size found in platelets, proceeds via an active single-chain intermediate. scFXII-polyP70 shows activity towards physiological substrates, and may represent the primary event in initiating contact activation in vivo.

Dietary zinc depletion and repletion affects plasma proteins: an analysis of the plasma proteome

Zinc (Zn) deficiency is a problem world-wide. Current methods for assessing Zn status are limited to measuring plasma or serum Zn within populations suspected of deficiency. Despite the high prevalence of Zn deficiency in the human population there are no methods currently available for sensitively assessing Zn status among individuals. The purpose of this research was to utilize a proteomic approach using two-dimensional gel electrophoresis (2DE) and mass spectrometry to identify protein biomarkers that were sensitive to changes in dietary Zn levels in humans. Proteomic analysis was performed in human plasma samples (n = 6) obtained from healthy adult male subjects that completed a dietary Zn depletion/repletion protocol, current dietary zinc intake has a greater effect on fractional zinc absorption than does longer term zinc consumption in healthy adult men. Chung et al. (Am J Clin Nutr 87 (5):1224-1229, 2008). After a 13 day Zn acclimatization period where subjects consumed a Zn-adequate diet, the male subjects consumed a marginal Zn-depleted diet for 42 days followed by consumption of a Zn-repleted diet for 28 days. The samples at baseline, end of depletion and end of repletion were pre-fractionated through immuno-affinity columns to remove 14 highly abundant proteins, and each fraction separated by 2DE. Following staining by colloidal Coomassie blue and densitometric analysis, three proteins were identified by mass spectrometry as affected by changes in dietary Zn. Fibrin β and chain E, fragment double D were observed in the plasma protein fraction that remained bound to the immunoaffinity column. An unnamed protein that was related to immunoglobulins was observed in the immunodepleted plasma fraction. Fibrin β increased two-fold following the Zn depletion period and decreased to baseline values following the Zn repletion period; this protein may serve as a viable biomarker for Zn status in the future.

Immobilized transition metal ions stimulate contact activation and drive factor XII-mediated coagulation

BACKGROUND

Upon contact with an appropriate surface, factor XII (FXII) undergoes autoactivation or cleavage by kallikrein. Zn(2+) is known to facilitate binding of FXII and the cofactor, high molecular weight kininogen (HK), to anionic surfaces.

OBJECTIVES

To investigate whether transition metal ions immobilized on liposome surfaces can initiate coagulation via the contact pathway.

METHODS AND RESULTS

Liposomes containing a metal ion-chelating lipid, 1,2-dioleoyl-sn-glycero-3-{(N[5-amino-1-carboxypentyl]iminodiacetic acid)succinyl} ammonium salt (DOGS-NTA), were prepared by membrane extrusion (20% DOGS-NTA, 40% phosphatidylcholine, 10% phosphatidylserine, and 30% phosphatidylethanolamine). Ni(2+) immobilized on such liposomes accelerated clotting in normal plasma, but not factor XI (FXI)-deficient or FXII-deficient plasma. The results were similar to those obtained with a commercial activated partial thromboplastin time reagent. Charging such liposomes with other transition metal ions revealed differences in their procoagulant capacity, with Ni(2+) > Cu(2+) > Co(2+) and Zn(2+). Plasma could be depleted of FXI, FXII and HK by adsorption with Ni(2+) -containing beads, resulting in longer clot times. Consistent with this, FXI, FXII and HK bound to immobilized Ni(2+) or Cu(2+) with high affinity as determined by surface plasmon resonance. In the presence of Ni(2+) -bearing liposomes, K(m) and k(cat) values derived for autoactivation of FXII and prekallikrein, as well as for activation of FXII by kallikrein or prekallikrein by FXIIa, were similar to literature values obtained in the presence of dextran sulfate.

CONCLUSIONS

Immobilized Ni(2+) and Cu(2+) bind FXII, FXI and HK with high affinity and stimulate activation of the contact pathway, driving FXII-mediated coagulation. Activation of the contact system by immobilized transition metal ions may have implications during pathogenic infection or in individuals exposed to high levels of pollution.

Histidine-rich glycoprotein binds factor XIIa with high affinity and inhibits contact-initiated coagulation

Histidine-rich glycoprotein (HRG) circulates in plasma at a concentration of 2μM and binds plasminogen, fibrinogen, and thrombospondin. Despite these interactions, the physiologic role of HRG is unknown. Previous studies have shown that mice and humans deficient in HRG have shortened plasma clotting times. To better understand this phenomenon, we examined the effect of HRG on clotting tests. HRG prolongs the activated partial thromboplastin time in a concentration-dependent fashion but has no effect on tissue factor–induced clotting, localizing its effect to the contact pathway. Plasma immunodepleted of HRG exhibits a shortened activated partial thromboplastin time that is restored to baseline with HRG replenishment. To explore how HRG affects the contact pathway, we examined its binding to factors XII, XIIa, XI, and XIa. HRG binds factor XIIa with high affinity, an interaction that is enhanced in the presence of Zn2+, but does not bind factors XII, XI, or XIa. In addition, HRG inhibits autoactivation of factor XII and factor XIIa–mediated activation of factor XI. These results suggest that, by binding to factor XIIa, HRG modulates the intrinsic pathway of coagulation, particularly in the vicinity of a thrombus where platelet release of HRG and Zn2+ will promote this interaction.

Zinc modulates the interaction of protein C and activated protein C with endothelial cell protein C receptor

Zinc is an essential trace element for human nutrition and is critical to the structure, stability, and function of many proteins. Zinc ions were shown to enhance activation of the intrinsic pathway of coagulation but down-regulate the extrinsic pathway of coagulation. The protein C pathway plays a key role in blood coagulation and inflammation. At present there is no information on whether zinc modulates the protein C pathway. In the present study we found that Zn(2+) enhanced the binding of protein C/activated protein C (APC) to endothelial cell protein C receptor (EPCR) on endothelial cells. Binding kinetics revealed that Zn(2+) increased the binding affinities of protein C/APC to EPCR. Equilibrium dialysis with (65)Zn(2+) revealed that Zn(2+) bound to the Gla domain as well as sites outside of the Gla domain of protein C/APC. Intrinsic fluorescence measurements suggested that Zn(2+) binding induces conformational changes in protein C/APC. Zn(2+) binding to APC inhibited the amidolytic activity of APC, but the inhibition was reversed by Ca(2+). Zn(2+) increased the rate of APC generation on endothelial cells in the presence of physiological concentrations of Ca(2+) but did not further enhance increased APC generation obtained in the presence of physiological concentrations of Mg(2+) with Ca(2+). Zn(2+) had no effect on the anticoagulant activity of APC. Zn(2+) enhanced APC-mediated activation of protease activated receptor 1 and p44/42 MAPK. Overall, our data show that Zn(2+) binds to protein C/APC, which results in conformational changes in protein C/APC that favor their binding to EPCR.

Role of zinc in hemostasis: a review

Zinc is a multi-functional element that is found in almost 300 enzymes where it performs catalytic, co-catalytic, and/or structural functions. In 1982, Gordon et al. (Am J Clin Ntr 35:849-857, 1982) found that a low zinc diet caused poor platelet aggregation and increased bleeding tendency in adult males. This fact drew interest to the role of zinc in blood clotting. It has been shown that hyperzincemia predisposes to increased coagulability, and hypozincemia to poor platelet aggregation and increased bleeding time. The blood clotting disturbances can be regressed by appropriate zinc intake management. Considering the importance of zinc as an essential element, its participation in regulation of the equilibrium between pro- and anti-thrombotic factors originating in platelets and endothelium prompted further investigations.

Inhibition of contact activation by a kininogen peptide (HKH20) derived from domain 5

Contact activation can be initiated by interaction of Factor XII, prekallikrein (PK) and high molecular weight kininogen (HK) with inorganic negatively charged biologic macromolecules, or upon cell surfaces, or interaction with membrane protein derivatives such as aggregated beta amyloid. The latter two examples are zinc-dependent. The interaction with cells is dependent on peptides derived from HK domains 3 and 5 termed LDC27 and HKH20, respectively. We have tested the ability of each of these peptides to inhibit HK-dependent contact activation. HKH20 inhibited activation of prekallikrein when a mixture containing HK, prekallikrein and Factor XII was incubated with dextran sulfate, gC1qR, amyloid beta or endothelial cells. Comparable quantities of LDC27 had no effect. The binding of biotinylated HK or biotinylated Factor XII was inhibited in a dose response fashion by increasing concentrations of HKH20 while LDC27, again had no effect. The N-terminal region of HKH20 (amino acids 475-485) is of particular importance for binding and histidine 485 prominently enhances the reaction as assessed employing overlapping and deleted peptides. Since there is a role for HK heavy chain in binding to endothelial cells and LDC27 can be employed as an affinity ligand to isolate the binding proteins, we increased the LDC27 concentration from 10-fold to 250-fold to determine whether it is functional. Inhibition of endothelial cell-dependent prekallikrein activation required 100-fold greater concentration of LDC27 when compared to HKH20 to achieve significant inhibition. We conclude that the interactions of the light chain of HK via HKH20 is of particular importance for activation of the bradykinin forming cascade in zinc-dependent or independent reactions and is true for all "surface" initiators tested thus far.

Binding of Zn2+ to a Ca2+ loop allosterically attenuates the activity of factor VIIa and reduces its affinity for tissue factor

The protease domain of coagulation factor VIIa (FVIIa) is homologous to trypsin with a similar active site architecture. The catalytic function of FVIIa is regulated by allosteric modulations induced by binding of divalent metal ions and the cofactor tissue factor (TF). To further elucidate the mechanisms behind these transformations, the effects of Zn2+ binding to FVIIa in the free form and in complex with TF were investigated. Equilibrium dialysis suggested that two Zn2+ bind with high affinity to FVIIa outside the N-terminal gamma-carboxyglutamic acid (Gla) domain. Binding of Zn2+ to FVIIa, which was influenced by the presence of Ca2+, resulted in decreased amidolytic activity and slightly reduced affinity for TF. After binding to TF, FVIIa was less susceptible to zinc inhibition. Alanine substitutions for either of two histidine residues unique for FVIIa, His216, and His257, produced FVIIa variants with decreased sensitivity to Zn2+ inhibition. A search for putative Zn2+ binding sites in the crystal structure of the FVIIa protease domain was performed by Grid calculations. We identified a pair of Zn2+ binding sites in the Glu210-Glu220 Ca2+ binding loop adjacent to the so-called activation domain canonical to serine proteases. Based on our results, we propose a model that describes the conformational changes underlying the Zn2+-mediated allosteric down-regulation of FVIIa's activity.

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.

Platelet multielemental composition, lability, and subcellular localization

Diagnostic X-ray spectrometry (DXS), based on X-ray fluorescence, was used to quantitate directly the multiple elemental composition of washed, intact human platelets (n = 16), with the following results: K = 3.08 +/- 1.00 mg/g, Ca = 1.18 +/- 0.29 mg/g, Zn = 35 +/- 9 micrograms/g. These values show that washed platelets contain significant pools of K, Ca, and Zn, the latter some 30-60-fold higher than plasma levels. Dialysis of whole platelets against cation exchange resin (Chelex-100) did not extract Ca(II) and Zn(II) sequestered within whole cells. To identify the subcellular locale of the elements, platelet lysate was subjected to 30-70% sucrose gradient ultracentrifugation and subcellular enriched fractions were obtained. Fractions were analyzed by DXS (for elements), electron microscopy (for dense granules), and subcellular markers fibrinogen and von Willebrand factor. In contrast to Ca and K, which accumulate in the dense granules and the cytoplasm, respectively, Zn appears to be distributed in the alpha-granules (40%) and the cytoplasm (60%). The subcellular distribution of Zn(II) is discussed within the context of the sensitivity of platelet response to the availability of Zn(II) and the platelet release reactions following stimulation.

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

Binding of high-Mr kininogen and factor XII/factor XIIa to phospholipids coated on to polystyrene microtiter plates was investigated by ELISA. Both high-Mr kininogen and factor XII/factor XIIa bound specifically to the phospholipid surface. Binding was observed to negatively charged phospholipids only. The binding of high-Mr kininogen was not affected by the presence of zinc ions. At a surface concentration of 20% phosphatidylinositol phosphate in phosphatidylcholine a dissociation constant (kD) of 10 nM for the binding of high-Mr kininogen was calculated. The amount of bound purified alpha-factor XIIa could be increased 4-5-fold in the presence of zinc ions. The lowest zinc ion concentration giving maximal binding was 0.1 mM. The binding of alpha-factor XIIa was inhibited by high-Mr kininogen. Independent of the presence of zinc ions or high-Mr kininogen, a kD of 7.9 nM was calculated for alpha-factor XIIa binding. The binding of prekallikrein was dependent upon the presence and the concentration of high-Mr kininogen. In plasma containing aprotinin, the binding of high-Mr kininogen was apparently inhibited in the presence of zinc ions, which was a prerequisite for the binding of factor XII. This apparently inhibitory effect of zinc ions on the binding of high-Mr kininogen was probably due to the increased binding of factor XII, which displaced high-Mr kininogen.

The inositol-phospholipid-accelerated activation of prekallikrein by activated factor XII at physiological ionic strength requires zinc ions and high-Mr kininogen

In a system consisting of purified proteins inositol-phospholipid-accelerated activation of prekallikrein by alpha-factor XIIa was determined by measuring the appearance of kallikrein amidolytic activity towards the chromogenic substrate, H-D-Pro-Phe-Arg-NH-PhNO2 (PhNO2, 4-nitrophenyl). The activation reaction was ionic-strength dependent. In the absence of high-Mr kininogen optimal activity was recorded at I = 50 mM. Searching for conditions, which could change this optimum towards physiological values, high-Mr kininogen was added. This resulted in an inhibition of the activity, with no change in ionic strength optimum. If, however, Zn2+ were added concomitant with high-Mr kininogen, the inhibition was abolished and optimal activity recorded at physiological ionic strength. The optimal Zn2+ concentration was found to be 0.1 mM. Kinetic analysis of the reaction demonstrated that the kcat/Km was 1.2 x 10(5) M-1 s-1 in the absence and 1.1 x 10(6) M-1 s-1 in the presence of Zn2+. Zn2+ were also required for inositol-phospholipid-accelerated initiation of the contact activation in whole plasma.

Multinuclear magnetic resonance studies on the calcium (II) binding site in trypsin, chymotrypsin, and subtilisin

Two different nuclear magnetic resonance experiments were conducted to elucidate the properties of the Ca(II) binding locus on serine proteases in solution. Trypsin, alpha-chymotrypsin, and subtilisin were inactivated with diisopropyl fluorophosphate, and the distance of the phosphorus from Gd(III) in place of Ca(II) was determined from the lanthanide-induced relaxation on the 31P resonance. The distances found (between 20 and 21 A) were in excellent agreement with those reported in the X-ray crystallographic structures of trypsin and subtilisin, demonstrating that the method has wide applicability to systems for which no X-ray structure is available. Subsequently, the 113Cd spectra [in place of Ca(II)] were examined in the presence of the native enzymes. At ambient temperatures only a single 113Cd resonance could be observed, presumably representing the weighted average of the variously weakly bound ions and the free ion. At 280 K for trypsin and chymotrypsin, and at 268 K for subtilisin there was observed a resonance at ca. 65-70 ppm higher field than the previous averaged resonance that could be attributed to tightly bound Cd. The chemical shift of the resonance was consistent with its assignment to an octahedral environment around Cd with oxygen ligands.