Cathepsin A-like activity is possibly the main acidic carboxypeptidase in human platelets.

Human platelets were investigated for activity of the acidic carboxypeptidases: cathepsin A, lysosomal carboxypeptidase B and prolyl-carboxypeptidase. It was found that the main acidic carboxypeptidase in human platelets had cathepsin A activity. No activity of lysosomal carboxypeptidase B and prolyl-carboxypeptidase in human platelets was detectable using their specific substrates. Human platelet cathepsin A-type enzyme hydrolyzed at the highest rate Cbz-Phe-Ala, Cbz-Phe-Met and Cbz-Phe-Leu, did not require sulfhydryl activator and was inhibited by serine protease inhibitors (DFP, DCI) and inhibitors that react with the SH group (mersalyl acid, PCMS, PCMB, HgCl2). Cbz-Phe-Ala and Cbz-Glu-Tyr were hydrolyzed at a broad pH range with optimum at pH 5.0-6.0. The chromatographic analysis on Con A-Sepharose and DEAE-Sephacel showed different forms of Cbz-Phe-Ala-hydrolyzing enzyme in human platelets. The multiple forms of this enzyme were probably due to heterogeneity of carbohydrate moiety.