This chemical, known as Spectrozyme PCa or D-Lys(Z)-Pro-Arg-pNA diacetate, is a chromogenic peptide substrate specifically designed for the detection and quantification of activated protein C (APC) activity in biochemical assays. APC is a serine protease that plays a key role in anticoagulation by inactivating factors Va and VIIIa. Upon cleavage by APC, the substrate releases p-nitroaniline (pNA), generating a yellow color that can be quantified spectrophotometrically at 405 nm. This method allows precise measurement of APC levels in plasma and other biological samples, supporting research in hemostasis, thrombosis, sepsis, and the development of anticoagulant therapies. It may also be cleaved by other serine proteases like thrombin or factor Xa at higher concentrations, but its primary specificity is for APC, making it essential in coagulation studies, diagnostics, and pharmaceutical screening.