Z-Gly-Pro-Arg-4MβNA is the cleavage of the substrate of thrombin to release free 4-methoxy-2-naphthylamine (4MβNA). Free 4MβNA can be captured by 5-nitrosalicylaldehyde to produce an insoluble yellow fluorescent and marks the site of thrombin activity[1].
References
[1] Bushi D, et al. A novel histochemical method for the visualization of thrombin activity in the nervous system. Neuroscience. 2016 Apr 21;320:93-104. DOI:10.1016/j.neuroscience.2016.01.065