Blood inhibitory capacity toward exogenous plasmin

Area of Science:

• Biochemistry
• Hematology
• Pharmacology

Background:

• Stabilized, active plasmin is a potential thrombolytic agent for direct clot delivery.
• Plasma protease inhibitors are known to inhibit plasmin, but the exact inhibitory capacity of plasma and blood is not well-defined.
• Accurate determination of free plasmin activity is challenging due to interference from plasmin complexes.

Purpose of the Study:

• To quantify the inhibitory capacity of human plasma and whole blood against exogenously added plasmin.
• To identify the specific inhibitors and mechanisms involved in plasmin inactivation in vitro.
• To determine the threshold concentration of free plasmin detectable in plasma and blood.

Main Methods:

• Utilized size-exclusion chromatography to separate free plasmin from plasmin-inhibitor complexes.
• Incubated exogenous plasmin with human plasma and whole blood at 37°C for 5 minutes.
• Measured free plasmin activity using chromogenic substrates, accounting for potential false-positive signals.

Main Results:

• Exogenous plasmin was effectively inhibited in human plasma up to 2.4 micromol/L.
• Initially, alpha2-antiplasmin consumed plasmin up to 1.2 micromol/L, followed by alpha2-macroglobulin up to 2.4 micromol/L.
• Whole human blood demonstrated higher inhibitory capacity, with free plasmin detectable only above 3.8 micromol/L.

Conclusions:

• Human plasma and blood possess significant inhibitory mechanisms against exogenous plasmin.
• Alpha2-antiplasmin and alpha2-macroglobulin are key plasma inhibitors, while blood cells also contribute to plasmin inactivation.
• In vitro findings suggest that therapeutic doses of plasmin (up to ~12 mg/kg) could be completely inactivated in vivo.