There are a few different substrates that are hydrolyzed by plasmin. If I want to use as short incubation times as possible, and need a selective substrate for plasmin, which should I choose?

The substrates for plasmin include S-2251™, S-2302™, and S-2403™. While S-2251™ is a popular and suitable substrate for detection of plasmin, S-2403™ has a higher kcat/km value. It is a faster substrate, and incubation times can be shorter. S-2403™ is therefore the substrate of choice for this situation.

How is plasminogen converted to plasmin?

Plasminogen is activated by endogenous, natural activators, and exogenous activators. Activation by its natural activators, tPA and uPA, involves a bond cleavage at a specific site in the plg molecule, which gives rise to a two-chain molecule linked by two disulfide bonds. tPA binding to fibrin concentrates and correctly orientates the tPA and plasminogen, as well as inducing a conformational changes in the molecules that promote efficient clot lysis. uPA can only activate plasminogen in the presence of fibrin, but it does not bind to, nor is it activated by fibrin. Streptokinase is an exogenous plasminogen activator and functions by forming a 1:1 complex with human plg. This complex can function as an activator of other plg molecules. Complex formation is accompanied by a conformational change in the plg molecule, exposing the active site.

The activity of plasmin and plasminogen is expressed in CU. What is this, and what is the equivalent in nkat?

CU stands for Casein Units, and is a measure of the proteolytic activity on the substrate casein. For example, a plasminogen reagent when activated to plasmin with streptokinase can show an activity of 7.3 nkat (S-2251™) per CU. 1 mg of plasmin corresponds to 0.20 nkat (S-2251™), or to 0.024 CU.