Effect of temperature on the association step in thrombin-fibrinogen interaction

Extending the working properties of liquid platelet-rich fibrin using chemically modified PET tubes and the Bio-Cool device

OBJECTIVES

Platelet-rich fibrin (PRF) has been utilized in regenerative medicine as a concentration of autologous platelets and growth factors that stimulates tissue regeneration. More recently, liquid-PRF (also called injectable-PRF; i-PRF) has been brought to market utilizing PET plastic tubes. Due to new advances made in tube technology, the first aim of the present study was to investigate the liquid consistency of liquid-PRF utilizing both standard and chemically modified PET plastic tubes. Furthermore, it is well known that the conversion of PRF into a fibrin matrix is derived from the temperature-controlled enzymatic process that converts liquid fibrinogen and thrombin to solid fibrin. This study also investigated for the first time the use of a cooling device (Bio-Cool) to extend the liquid working properties of liquid-PRF.

MATERIALS AND METHODS

In total, 30 participants enrolled in this study. From each patient, four tubes of liquid-PRF were drawn, two standard white Vacuette tubes and two blue chemically modified hydrophobic tubes. Following centrifugation at 700 RCF-max for 8 min in a Bio-PRF horizontal centrifuge, one white and one blue tube were kept upright at room temperature, while the other white and blue tube were placed within the cooling device. Thereafter, the liquid-PRF layers were monitored over time until clotting occurred. Patient gender, age, and altitude above sea level (+ 5000 ft) were recorded and compared for clotting times.

RESULTS

The findings from the present study demonstrated that the chemically modified PET tubes performed 37% better than the control tubes (extended the working properties of liquid-PRF by over 20 min). Most surprisingly, tubes kept in the cooling device demonstrated an average of 90 min greater working time (270% improvement). While patients living at altitude did significantly improve the clotting ability of liquid-PRF, no differences were observed when comparing male vs female or younger vs older patients in liquid-PRF clotting times.

CONCLUSIONS

Cooling of blood following centrifugation represented a 270% improvement in working properties of liquid-PRF. Optimization of liquid-PRF tubes utilizing chemically modified hydrophobic PET tubes also delayed the clotting process by 37%. Patient gender and age had little relevance on liquid-PRF.

CLINICAL RELEVANCE

The present findings demonstrate for the first time that cooling of liquid-PRF is able to extend the working properties of liquid-PRF by over 90 min. Thus for clinicians performing longer clinical procedures, the cooling of blood may represent a viable strategy to improve the working time of liquid-PRF in clinical practice.

Strategies to reduce hemostatic activation during cardiopulmonary bypass

INTRODUCTION

We evaluated whether a modified protocol for cardiopulmonary bypass (CPB) could reduced the systemic hemostatic activation associated with this procedure.

MATERIALS AND METHODS

The in vivo rates of thrombin, fibrin, plasmin and D-dimer generation were determined in each subject during CPB using measured levels of hemostatic factors combined with a computer model of the cardiovascular and hemostatic systems. A standard CPB group using uncoated circuits, standard heparin levels and direct shed blood reinfusion (n=9) was compared to a modified CPB group using heparin-coated circuits, shed blood collection, washing and reinfusion post-operatively, lower heparin levels and epsilon-amino-caproic acid (n=10).

RESULTS AND CONCLUSIONS

Standard CPB increased average thrombin generation 9-fold, decreased fibrin generation 2-fold, increased plasmin generation 11-fold and increased fibrin degradation and D-dimer generation 19-fold. During CPB in the modified group thrombin generation was not increased beyond surgical levels, lower heparin concentrations allowed each thrombin to make more fibrin prior to inhibition, while fibrin degradation was suppressed by epsilon-amino-caproic acid. At baseline for every 100 fibrins formed only 1-2 were degraded to D-dimer. During standard CPB for every 100 fibrins generated on average 34 fibrins were degraded with some subjects showing a net fibrin loss. In contrast, in the modified CPB group for every 100 fibrins formed only 4 fibrins were degraded (p<0.0001 vs. standard group). Kinetic modeling of hemostasis in individual patients showed that a modified CPB protocol could reduce excessive thrombin generation during CPB and suppress fibrin degradation moving hemostatic regulation back towards normal.

Effects of Hemodilution, Blood Loss, and Consumption on Hemostatic Factor Levels During Cardiopulmonary Bypass

OBJECTIVES

The purpose of this study was to determine quantitatively the effects of consumption, hemodilution, and blood loss on coagulation and fibrinolytic factor levels during cardiopulmonary bypass.

DESIGN

A combination of measured levels of prothrombin, antithrombin, fibrinogen, plasminogen, and antiplasmin along with their activation markers F1.2, thrombin-antithrombin complex, fibrinopeptide A, plasmin-antiplasmin complex, and D-dimer were used with a computer model of each patient's vascular and hemostatic systems to estimate the cardiopulmonary bypass-associated loss of each factor because of hemodilution, blood loss, and consumption.

SETTING

University hospital.

PARTICIPANTS

Nine patients undergoing coronary artery bypass graft surgery.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

At baseline, it was estimated that on average 2%, 3%, and 25%, respectively, of the baseline liver secretion of plasminogen, prothrombin,and fibrinogen were consumed by activation of these proteins. During cardiopulmonary bypass, thrombin and plasmin generation were increased, whereas fibrin generation was decreased because of heparin. Compared with baseline, hemodilution during cardiopulmonary bypass resulted in an average 35% +/- 7% decrease in the concentration of coagulation and fibrinolytic proteins, whereas blood loss was responsible for an average 6% +/- 5% decrease in these proteins. Blood loss varied substantially among patients, resulting in <1% to 14% decreases in hemostatic protein levels. On average, consumption because of activation resulted in less than a 1% drop in the concentration of coagulation and fibrinolytic factors during cardiopulmonary bypass.

CONCLUSIONS

Hemodilution is the primary cause of the drop in coagulation and fibrinolytic proteins during routine cardiopulmonary bypass, followed by blood loss, whereas consumption accounts for less than a 1% drop in most patients.

Molecular and functional characterization of a natural homozygous Arg67His mutation in the prothrombin gene of a patient with a severe procoagulant defect contrasting with a mild hemorrhagic phenotype

In a patient who presented with a severe coagulation deficiency in plasma contrasting with a very mild hemorrhagic diathesis a homozygous Arg67His mutation was identified in the prothrombin gene. Wild-type (factor IIa [FIIa]-WT) and mutant Arg67His thrombin (FIIa-MT67) had similar amidolytic activity. By contrast, the k(cat)/K(m) value of fibrinopeptide A hydrolysis by FIIa-WT and FIIa-MT67 was equal to 2.1 x 10(7) M(-1)s(-1) and 9 x 10(5) M(-1)s(-1). Decreased activation of protein C (PC) correlated with the 33-fold decreased binding affinity for thrombomodulin (TM; K(d) = 65.3 nM vs 2.1 nM, in FIIa-MT67 and in FIIa-WT, respectively). In contrast, hydrolysis of PC in the absence of TM was normal. The Arg67His mutation had a dramatic effect on the cleavage of protease-activated G protein-coupled receptor 1 (PAR-1) 38-60 peptide (k(cat/)K(m) = 4 x 10(7) M(-1)s(-1) to 1.2 x 10(6) M(-1)s(-1)). FIIa-MT67 showed a weaker platelet activating capacity, attributed to a defective PAR-1 interaction, whereas the interaction with glycoprotein Ib was normal. A drastic decrease (up to 500-fold) of the second-order rate constant pertaining to heparin cofactor II (HCII) interaction, especially in the presence of dermatan sulfate, was found for the FIIa-MT67 compared with FIIa-WT, suggesting a severe impairment of thrombin inhibition by HCII in vivo. Finally, the Arg67His mutation was associated with a 5-fold decrease of prothrombin activation by the factor Xa-factor Va complex, perhaps through impairment of the prothrombin-factor Va interaction. These experiments show that the Arg67His substitution affects drastically both the procoagulant and the anticoagulant functions of thrombin as well as its inhibition by HCII. The mild hemorrhagic phenotype might be explained by abnormalities that ultimately counterbalance each other.

Substrate-assisted catalysis of the PAR1 thrombin receptor. Enhancement of macromolecular association and cleavage

Platelet activation and aggregation are mediated by thrombin cleavage of the exodomain of the PAR1 receptor. The specificity of thrombin for PAR1 is enhanced by binding to a hirudin-like region (Hir) located in the receptor exodomain. Here, we examine the mechanism of thrombin-PAR1 recognition and cleavage by steady-state kinetic measurements using soluble PAR1 N-terminal exodomains. We determined that the primary role of the PAR1 Hir sequence is to reduce the kinetic barriers to formation of the docked thrombin-PAR1 complex rather than to form high affinity ground-state interactions. In addition, the exosite I-bound Hir motif facilitates the productive interaction of the PAR1 (38)LDPR/SFL(44) sequence with the active site of thrombin. This locking process is the most energetically unfavorable step of the overall reaction. The subsequent irreversible steps of peptide bond cleavage are rapid and allosterically enhanced by the presence of the docked Hir sequence. Furthermore, the C-terminal exodomain product of thrombin cleavage, corresponding to the activated receptor, binds tightly to thrombin. This would suggest that an additional role of the Hir sequence in the thrombin-activated receptor is to sequester thrombin to the platelet surface and modulate cleavage of other platelet receptors such as the PAR4 thrombin receptor, which lacks a functional Hir sequence.

Contact activation of the plasma coagulation cascade. III. Biophysical aspects of thrombin-binding anticoagulants

A biophysical model linking fibrin polymerization kinetics (following release from a thrombin-fibrinogen complex), coagulation time, and competitive inhibition of thrombin illustrates the utility of thrombin-binding ligands as anticoagulants in blood collection applications. The resulting mathematical relationship connecting fibrinogen, ligand, and thrombin concentrations was tested against experimentally observed anticoagulation of whole, platelet-poor porcine plasma induced by short, single-stranded DNA oligonucleotides originally found to bind thrombin by screening combinatorial libraries. The thrombin-fibrinogen dissociation constant Ks served as the single adjustable parameter in a least-squares fitting of the model to experimental anticoagulation data. Best-fit Ks values corroborated microM values measured in plasma-free systems, and application of the model to a ligand challenge to the intrinsic pathway of plasma coagulation corroborated nM endogenous thrombin concentrations measured in porcine blood activated by endotoxin insult in vivo. The model fit to data suggests that only about 20% conversion of blood fibrinogen to fibrin is required to coagulate (gel) porcine plasma. This prediction is consistent with the common clinical laboratory observation of latent fibrin formation in "serum" separated from blood before fibrinogen is fully converted to fibrin. It was concluded that the thrombin-binding anticoagulation model was a reasonable simulation of the situation in which an initial bolus of either exogenous or endogenous thrombin is rapidly partitioned between fibrinogen-bound and ligand-bound forms with little or no additional free thrombin created over time.

Inhibitory mechanism of serpins. Interaction of thrombin with antithrombin and protease nexin 1

The mechanism for the inhibition of thrombin by the serpins antithrombin and protease nexin 1 has been investigated using several kinetic techniques at pH 7.9 and 37 degrees C with an ionic strength of 0.3 M. Rapid kinetic studies demonstrated that a two-step mechanism for the formation of the stable thrombin-serpin complex applied to both serpins. The inhibition constant for the initial thrombin-antithrombin complex was 265 microM, and the rate constant for the conversion of this complex to the final one was 3.9 s-1; the corresponding values for PN1 were 3.4 microM and 6.0 s-1. By using slow-binding kinetics, it was possible to obtain estimates of the second-order rate constants for the formation of the stable thrombin-serpin complexes (1.2 x 10(4) and 1.5 x 10(6) M-1 s-1 for antithrombin and protease nexin 1, respectively) and the dissociation constants for these complexes (< 1 nM for both serpins). The influence of viscosity on the reactions indicated that the rate of interaction of both serpins with thrombin was diffusion-controlled. Moreover, the results indicated that the initial complex reacted more rapidly to form the stable complex than it dissociated to free enzyme and inhibitor; i.e., the behavior of the serpins was analogous to that of "sticky" substrates. By using the results from slow-binding, viscosity, and rapid kinetic studies, it was possible to set values for all of the rate constants for the interactions of antithrombin and protease nexin 1 with thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of protons on the thrombin-fibrinogen interaction

Amidase activity of human alpha-thrombin toward the synthetic substrate Tosyl-Gly-Pro-Arg-NH-Ph and fibrinogen has been studied as a function of pH at t = 25 degrees C, under steady-state conditions. A viscosity-perturbation method allowed us to compute the equilibrium binding constant along with the rate constants for the acylation and deacylation reactions. The ionization constants for the groups affecting binding and hydrolysis of the synthetic substrate were measured by application of linkage thermodynamics principles. The binding of the synthetic substrate is controlled by two ionizable groups having pKa values of 7.5 and 8.7 in the free enzyme and 6.3 and 9.8 in the Michaelis adduct. These two groups were found to control the acylation process as well. Thrombin-fibrinogen interaction has been studied by measurements of steady-state hydrolysis of the synthetic substrate Phe-pipecolyl-Arg-NH-Ph in the presence of fibrinogen, used as a competitive inhibitor. This method allowed us to measure the Km of thrombin-fibrinogen interaction. The values of Km computed at different solution viscosities were used in order to calculate the equilibrium dissociation constant and both k2/k3 and k2/k-1 ratios. The same residues that were found to control binding of Tosyl-Gly-Pro-Arg-NH-Ph to alpha-thrombin, do modulate binding of fibrinogen as well. These residues shift their pKa values upon the formation of the Michaelis adduct from 7.5 to 5.7 and from 8.7 to 9.7, respectively. Furthermore the ratio kcat/Km as a function of pH has been obtained by HPLC measurements of fibrinopeptides release. The kcat/Km values along with the ratio k2/k-1, derived from viscometric experiments, allowed us to calculate the forward-rate constant, k+1, for the thrombin-fibrinogen interaction. The association process was found to depend on pH, namely in the alkaline region. The results for Tosyl-Gly-Pro-Arg-NH-Ph and fibrinogen are compared and discussed on the basis of the structural elements which differentiate the interactions of these substrates with human alpha-thrombin.