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Plasmin is a protease that cuts fibrin into soluble products, helping dismantle a blood clot. It is produced by proteolytic activation of plasminogen, an inactive precursor. Activation of an existing precursor can switch on an enzymatic process without waiting for new protein synthesis.^[ref]

This supplies a useful contrast within clot biology. Thrombin helps form the fibrin network; plasmin helps remove it. In a simplified experiment, a fibrin gel can therefore be formed first and its subsequent breakdown measured after adding active plasmin. The observable loss of the network is a consequence of peptide bond cleavage, linking a molecular reaction to a macroscopic change in material structure.

The protein plasminogen is the inactive precursor of plasmin, a protease that helps break down fibrin in a blood clot. Activation requires cleavage of a particular peptide bond, converting an abundant circulating precursor into an active enzyme. Production of the precursor and activation of that precursor are therefore separate control points. ^[ref]

A useful measurement distinction follows: a sample can contain substantial plasminogen yet have little ongoing proteolysis. An assay that supplies an activator tests potential activity under those assay conditions; it does not directly report the original rate of clot breakdown. Local activation and inhibition help confine protease action to appropriate sites.

Tranexamic acid limits breakdown of fibrin by interfering with plasminogen binding. Stabilizing an existing clot is mechanistically distinct from directly producing new fibrin.^[ref]