Substrate specificities of cathepsin A,L and A,S from pig kidney.

The substrate specificities of two different molecular sizes of cathepsin A, A,L (large form) and A,S (small form), for synthetic substrates were examined kinetically. Both enzymes showed a similar broad substrate specificity against various acyl dipeptides, amino acid esters, and amino acid amides. Z-Phe-Ala and Ac-Phe-OEt were good substrates. Peptides containing hydrophobic amino acids were hydrolyzed rapidly. The presence of hydrophobic amino acid residues, not only at the C-terminal position but also at the second position and probably the third position from the C-terminal, resulted in an increase in the rate of hydrolysis. Peptides containing glycine and proline were hydrolyzed slowly. Inhibition studies with Z-D-Phe-D-Ala and Z-Phe suggested that the peptidase and esterase activities of the enzymes are both catalyzed by the same site of the enzyme molecule, but it remains to be elucidated whether or not the binding sites for peptides and esters are the same.