Plasminogen

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.