The specificity of lysosomal tripeptidyl peptidase-I determined by its action on angiotensin-II analogues

Tripeptidyl peptidase-I (TPP-I) is a lysosomal peptidase which cleaves trip eptides from the N-terminus of peptides, The function of the enzyme is uncl ear but its importance is demonstrated by the fact that mutations in TPP-I are responsible for late infantile neuronal ceroid lipofuscinosis, a lethal Lysosomal storage disease, As a step towards identifying its natural subst rates, we have used a series of synthetic peptides, based on angiotensin-II , to explore the effects of peptide chain length and the effects of amino a cid substitutions at the P-1 and P-1' positions on the rate of catalysis. W ith the exception of angiotensin-(1-8) (angiotensin-II), which is a relativ ely poor substrate for TPP-I, the rate of catalysis increases with increasi ng chain length. K-cat/K-m values increase 50-fold between angiotensin-(1-5 ) and angiotensin-(1-14), TPP-I shows little specificity for the nature of the amino acids in the P-1 and P-1' positions, K-cat/K-m values varying onl y 5-fold for a range of substitutions. However, Pro or Lys in the P-1 posit ion and Pro in the P-1' positions are incompatible with TPP-I activity, The se observations suggest that TPP-I is a non-specific, but essential, peptid ase involved in the latter stages of lysosomal protein degradation. (C) 200 1 Federation of European Biochemical Societies, Published by Elsevier Scien ce B.V. All rights reserved.