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, S-2390 (discontinued in the 25 mg quantities), 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.