Sar-Pro-Arg p-nitroanilide dihydrochloride is primarily used as a synthetic substrate in biochemical research, particularly in the study of enzyme kinetics and proteolytic activity. It is commonly employed to assay the activity of serine proteases, such as thrombin and trypsin, due to its ability to release p-nitroaniline upon enzymatic cleavage. This release can be easily monitored spectrophotometrically, providing a quantitative measure of enzyme activity. Its application is crucial in understanding the mechanisms of blood coagulation, fibrinolysis, and other protease-mediated processes. Additionally, it serves as a valuable tool in drug discovery and development, where it aids in screening and characterizing inhibitors of proteolytic enzymes.