| Summary: | 碩士 === 國立成功大學 === 生物化學研究所 === 86 === Streptokinase (SK) is a secretory single-peptide protein of 414
or 415 amino acid residues produced by various strains of
b-hemolytic streptococcus. It is one of the plasminogen activators
that are clinicallly used as thrombolytic agents in the treatment
of myocardial infarction. Streptokinase is generally regarded
as a fibrin-nonspecific plasminogen activator, and it does not
distinguish a thrombus from a hemostatic plug. Certain drawbacks
such as systemic bleeding and plasminemia have limited its overall
usefulness. Our investigations focus on designing lytic agents
with greater affinity for thrombus-bound plasminogen while
reducing activation of circulating zymogen.
Since arterial thrombi are very rich in activated platelets, it
might be useful to design a lytic agent with specific affinity
for the activated platelets. The g-chain peptide sequence
(HHLGGAKQAGDV) and RGD sequence of fibrinogen are resposible
for the binding to glycoprotein IIbIIIa on activated platelets.
In order to target the SK molecule to platelet-rich thrombus,
g-chain and RGD peptide sequences were incorporated into the
SK molecule.
The recombinant SK genes, SK-K59E-g, SK-K59E-g-Glyn-RGD(n=4 or
7), SK-K59E-RGD, g-SK-K59E, and RGD-Gly7-g-SK-K59E, were prepared
by PCR techniques using SK-K59E as a template. The fusion proteins
were expressed in pET expression system and purified by anion
exchange chromatography on High Q support. The purified SK-K59E,
SK-K59E-g, SK-K59E-g-Glyn-RGD(n=4 or 7), and SK-K59E-RGD proteins
could activate plasminogen as efficiently as native SK, whereas
g-SK-K59E, and RGD-Gly7-g-SK-K59E had lower plasminogen activator
activities. SK-K59E-g-Gly4-RGD, SK-K59E-g-Gly7-RGD, and
RGD-Gly7-g-SK-K59E could specifically inhibit platelet aggregation.
The clot lysis rates of the fusion proteins increased when the tests
were performed with platelet-rich blood clots.
In conclusion, the blood-clot specificity of SK as a thrombolytic agent
in vitro might be improved by attachment of a platelet-specific ligand
to SK molecule. Whether the fusion proteins have better clot specificity
in vivo still has to be tested.
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