Characterization of a Rat Brain Catheptic Carboxypeptidase (Cathepsin A) Inactivating Angiotensin‐ii

Abstract —Catheptic carboxypeptidase (cathepsin A) is present in lysosomal‐enriched fractions of rat brain at levels approximately 5‐fold that of cathepsin B1 and of the classical carboxypeptidase A but lower than that of cathepsin C and carboxypeptidase B. Cathepsin A was purified 40‐fold by extraction of calf brain with an acetate buffer containing 0.5% desoxycholate followed by heat treatment, salt precipitation and chromatography on DEAE‐Sephadex. Purification revealed the presence of two distinct isoenzymatic forms of high mol. wt that were very stable when frozen in the presence of sucrose and KCl. Among N‐protected dipeptides used as substrates the highest activity was given by Z‐Glu‐Tyr and Z‐Phe‐Tyr at a pH optimum of 5.5, Km1.1 mm and Vmax 0.5 μmol (Tyr)/mg protein per min. Brain cathepsin A was completely inhibited by low concentrations of DFP and PCMB but unaffected by thiols, EDTA and specific inhibitors of other cathepsins (pepstatin and chymostatin). The carboxypeptidase A‐like specificity of cathepsin A was confirmed by breakdown of Ile5‐angiotensin with release of C‐terminal Phe. Cathepsin A may play a role in the turnover of selected hormonal peptides containing C‐terminal neutral amino acids and in the sequential breakdown of proteins associated with degenerative conditions such as demyelination.