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

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Chapter | 10 Hemostasis
that is present in unactivated endothelial cells and monocytes,
as well as free in plasma ( Monroe and Key, 2007 ). TF
is recognized to contribute to many biological processes in
addition to coagulation, which include thrombus propagation,
migration and proliferation of vascular smooth muscle
cells, development of embryonic blood vessels, tumor neovascularization
and metastasis, and induction of the proinflammatory
response ( Monroe and Key, 2007 ). Following
cellular activation by vascular trauma or an inflammatory
stimulus, TF becomes exposed on the plasma membrane
where it interacts with circulating FVII, or its activated form,
FVIIa, to form the enzymatically reactive TF-FVIIa complex
( Gentry, 2004 ). About 99% of FVII circulates in the inactive
zymogen form ( Monroe and Key, 2007 ). Cell-associated
TF binds to either FVII or FVIIa in order to either promote
FVII activation or enhance catalysis, respectively. TF and
FVII/FVIIa bind together over a large area at multiple sites
in a Ca -dependent manner to form the binary complex.
The stabilization of the structure of FVIIa is crucial for its
procoagulant activity. Unique from tenase and prothrombinase
complex formation, TF-FVIIa complex formation is
less dependent on a phospholipid surface for expression of
its procoagulant activity ( Monroe and Key, 2007 ). This complex
functions primarily to convert FX to its activated form,
FXa, but it also activates circulating FIX to FIXa. It is this
FXa formation that results in initial production of thrombin
(see Section II.C.3).
b. Tenase Complex
Factor IX in plasma is proteolytically activated to the
serine protease factor IXa by the TF-VIIa complex (see
Section II.C.2.a) (see Fig. 10-2 ). FVIII normally circulates
in a complex with von Willebrand factor (vWF), which
effectively extends the plasma half-life of FVIII because
it is protected from proteolytic degradation in the complex
form ( Gentry, 2004 ). Initial thrombin formation not only
results in dissociation of FVIII from vWF, but it also converts
it to a more potent cofactor, FVIIIa. Factor IXa and
factor VIIIa then assemble on phosphatidyl-L-serine-containing
phospholipid membranes in the presence of Ca 2
to form an enzymatic complex known as the tenase complex
( Blostein et al ., 2003 ). The tenase complex converts
factor X to factor Xa, the enzyme that converts prothrombin
to thrombin leading to the conversion of fibrinogen to
fibrin and the formation of a fibrin clot. This tenase complex
cleaves FX at the same reactive site as that cleaved
by the TF-FVIIa complex and hence produces the same
FXa product. Factor X activation by the tenase complex is
the rate-limiting step for thrombin generation during tissue
factor-dependent coagulation ( Blostein et al ., 2003 ).
c. Prothrombinase Complex
The generation of thrombin requires the formation of the
prothrombinase complex, which consists of FXa, phospholipid,
calcium, and a protein cofactor, FV ( Gentry, 2004 )
(see Fig. 10-2 ). The complex catalyzes two cleavages in
prothrombin, at Arg320 (to produce meizothrombin) and at
Arg271, leading to the formation of thrombin ( Autin et al .,
2006 ). The first few molecules of thrombin generated by
this prothrombinase complex initiate several positive-feedback
reactions that sustain its own formation and facilitate
the rapid growth of the blood clot or thrombus around the
area of vascular damage. For example, thrombin can convert
FXI to its proteolytically active form, FXIa, which, in
turn, converts FIX to FIXa. The thrombin-induced conversion
of FV to FVa, along with the increased availability
of FXa, greatly enhances the rate and extent of thrombin
formation by the prothrombinase complex. This is a crucial
reaction for normal blood coagulation. FXa alone can
slowly activate PT, but the rate of thrombin formation is
enhanced several orders of magnitude by the presence of
FVa in the PTase complex ( Autin et al ., 2006 ). Another
positive feedback response is the increased availability of
phospholipids on the surface on thrombin-activated platelets
that accumulate at sites of vascular damage ( Gentry,
2004 ).
3. Mechanisms of Thrombin Formation
Minimal activation of coagulation proteins is needed to initiate
further propagation and amplification of the cascade
of reactions that ultimately leads to the formation of thrombin,
which then converts fibrinogen to fibrin and forms the
stable clot at a site of vascular injury (see Fig. 10-2 ). Each
complex that participates in thrombin generation is simply
composed of a serine protease interacting with a cofactor
or receptor on an activated cell membrane surface ( Mann,
2003 ). Coagulation is dependent on vitamin K because
adequate concentration of this vitamin is required for activation
of procoagulants, FVII, FIX, FX, prothrombin, and
for anticoagulants, protein C, and protein S. The clinical
consequences of antagonism or deficiency of vitamin K
demonstrate how important it is to proper hemostasis (see
Sections IV.B.5) .
The release of tissue thromboplastin or TF from damaged
cells or its enhanced expression on cell membranes
initiates in vivo coagulation. This occurs when TF binds
with and activates FVII to FVIIa (see Section II.C.2.a).
The TF-FVIIa complex then has the ability to activate
subsequent zymogens FIX and FX in the presence of calcium
ions and a negatively charged phospholipid surface
provided by activated platelets (see Section II.B.2), resulting
in the formation of their respective activated forms,
FIXa and FXa. The latter is the more efficient substrate
in the early phase of thrombin generation. Once some Xa
has been formed, it actually facilitates further activation
of FIX ( Lawson and Mann, 1991 ) . The TF-FVIIa activation
process has historically been referred to as the extrinsic
coagulation pathway and is currently often referred to