Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

II. Mechanisms of Hemostasis
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as the tissue factor pathway. There appears to be a small
proportion (1% to 2%) of constitutively activated FVII in
plasma ( Mann et al ., 2003 ). However, it does not exhibit
proteolytic activity unless bound to TF. Once the TF-FVIIa
complex forms, the rate of FXa formation increased fourfold
( Mann et al ., 2003 ). During the initiation stage, any
membrane-bound Xa produced is able to begin converting
a small amount of prothrombin to thrombin. This small
amount of thrombin is crucial for further propagation of
the coagulation cascade, as it activates platelets and converts
FV to FVa and FVIII to FVIIIa ( Brummel et al .,
2002 ).
The ability of the TF-FVIIa complex to continue the
generation of FIXa is compromised by the presence in the
circulation of tissue factor pathway inhibitor (TFPI) (see
Section II.C.5.a). Alternative activation pathways exist
for the activation of FIX so that the tenase complex can
be formed at a sufficient rate and adequate level to sustain
thrombin formation until a clot has formed and wound
healing is initiated. In a direct positive feedback reaction,
thrombin can convert FIX to the same enzymatically
active form of FIXa as produced by the TF-FVIIa complex
( Minnema et al ., 1999 ). In an indirect feedback pathway,
thrombin can activate FXI to a proteolytically active
enzyme (FXIa), which can, in turn, convert FIX to FIXa
( Gailani, 2000 ; Gailani and Broze, 1991 ). FXI is one of the
proteins involved in the coagulation pathway historically
known as the intrinsic pathway that is now referred to as
the contact activation system. The other components of this
system are FXII, prekallikrein (PK), and high-molecularweight
kininogen (HK) ( Table 10-5 ). This pathway is initiated
when the zymogen FXII comes into contact with a
negatively charged surface (e.g., damaged endothelium
or an endotoxin contact) and undergoes a conformational
change to become an active serine protease (FXIIa). FXIIa
then propagates coagulation by activating the next protein
in this system, FXI, in a reaction that is accelerated
by the presence of PK and HK ( Wachtfogel et al ., 1993 ).
In plasma, HK circulates as biomolecular complexes with
FXI and PK, respectively. The initial molecules of FXIIa
generated on damaged endothelium convert HK to HKa.
HKa has a high surface binding affinity and thus brings
large amounts of FXI and PK to the charged surface in
close proximity to the already adherent FXIIa. The spatial
configuration of these complexes permits FXIIa to rapidly
convert FXI to FXIa and convert PK to kallikrein. It is now
generally accepted that activation of the contact activation
system is not the predominant in vivo mechanism of thrombin
generation ( Gentry, 2004 ; Rojkjaer and Schmaier,
1999 ). However, it may serve as a link between fibrin formation,
fibrinolysis, and inflammation because kallikrein
can function as a weak activator of the fibrinolytic system
(see Section II.E) and as stimulator of both chemotaxis and
degranulation in neutrophils attracted to a site of endothelial
damage ( Jiminez and Fernandez, 2000 ).
After the coagulation cascade is initiated, further
amplification ensures that sufficient thrombin is produced
to cleave fibrinogen and form insoluble fibrin. Once FIXa
is formed, it complexes with calcium ions (Ca) and platelet
PSs and FVIIIa to form the FIXa-FVIIIa-Ca-PS (tenase
or Xase) complex, a potent stimulator of further FX activation.
Large amounts of FXa are created by the tenase
complex, which is approximately 50-fold more efficient
than the TF-FVIIa complex at performing the same process
( Ahmed et al ., 1992 ). Once formed, FXa converts
prothrombin (factor II) to thrombin (FIIa). This reaction is
additionally dependent on the formation of the prothrombinase
complex, composed of FVa, FXa, platelet phospholipid
surface, and calcium ions (FXa-FVa-Ca-PS) and is
300,000-fold more potent than Xa at catalyzing prothrombin
conversion ( Mann et al ., 2003 ).
Thrombin (FIIa) converts fibrinogen to fibrin and additionally
activates factor XIII, which is important for crosslinking
and stabilizing the fibrin clot (see Section II.C.4).
4. Fibrin Clot Formation
Ultimately, repair of a vascular lesion requires development
of a more permanent clot than a platelet plug can provide.
A stable fibrin clot is the end point of the conversion of
fibrinogen to fibrin and involves three phases: proteolysis,
polymerization, and stabilization. Proteolysis occurs when
thrombin cleaves fibrinogen to form fibrin monomers and
the secondary by-product fragments fibrinopeptides A and B
(FPA and FPB). These fibrinopeptide molecules are cleaved
from the N-terminal regions of the A α and B β chains
(Sidelmann et al ., 2000 ). It is proposed that removal of
FPA permits end-to-end polymerization, whereas removal
of FPB promotes side-to-side polymerization ( Mosesson,
1992 ). The cleaved fibrinogen molecules can then form
complexes with other fibrinogen and fibrin molecules to
form what is referred to as soluble fibrin or fibrin monomers
( Sidelmann et al ., 2000 ). Fibrin monomers then polymerize
at their ends via hydrogen bonding, and the polymers
are then stabilized covalently via peptide bond formation
under the influence of the transglutaminase FXIIIa
(which is activated by thrombin) to form cross-linked
fibrin strands.
Thrombin cleaves a peptide from each of two alpha
chains on FXIII, which results in the formation of an inactive
intermediate. In the presence of Ca, this intermediate
dissociates and produces the activated protein. FXIIIa then
induces adjacent γ -chains to quickly cross-link to form
γ -dimers, whereas α -chain cross-linking occurs more slowly.
Even native fibrinogen can be cross-linked by FXIIIa, following
the same pattern of initial γ-chain dimerization
followed by A α -chains to form high-molecular-weight
polymers.
The cross-linking that occurs between fibrin polymers
makes the fibrin “ gel ” more resistant to lysis by fibrinolytic