1. 1. Highly purified rat liver cathepsin A1 was shown to be a carboxypeptidase with a high degree of specificity for aromatic and hydrophobic amino acids. 2. 2. In lysosomal soluble fractions, cathepsin A1 was partially resposible for the hydrolysis of the model substrates, N-benzyloxycarbonyl (Cbz)-Gly-Phe, Cbz-Glu-Phe, and Ac-Phe-Tyr. Dipeptides and tripeptides were not hydrolyzed, but N-blocked dipeptides that contain an aromatic or hydrophobic amino acid were hydrolyzed. 3. 3. The optimum hydrolysis of N-blocked dipeptides by cathepsin A1 was at pH 5.0-5.8, with Km values in the range of 1.2-54 mM. 4. 4. The hexapeptide Leu-Trp-Met-Arg-Phe-Ala, glucagon, and insulin B chain were partially hydrolyzed by cathepsin A1 with the release of amino acids from the C-terminal end. These peptides all contain hydrophobic portions near the C-terminal end. Less hydrophobic peptides, such as insulin A chain and His-Ser-Gln-Gly-Thr-Phe, were not hydrolyzed. Very little hydrolysis of denatured hemoglobin was found. 5. 5. Cathepsin A1 was activated by halide ions and inhibited by Ag+ and Hg2+.
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