Tripeptide-specific aminopeptidase from Escherichia coli AJ005.

A tripeptidase, TP, from the ribosome-free fraction of Escherichia coli AJ005, a peptidase-deficient mutant of strain K-12, has been obtained using gel electrophoresis and chromatography on DEAE-Sephadex A-50, hydroxylapatite, and Sephadex G-200. Characterization studies on tripeptidase TP, freed of other detectable peptidases, indicate that this enzyme is capable of cleaving an amino-terminal leucine, lysine, methionine, or phenylalanine residue from certain tripeptides. Only one band of activity toward several tripeptides (and no activity toward dipeptides) was detected following gel electrophoresis of this preparation. Tripeptidase TP, the only strain AJ005 peptidase known to attack trilysine, was inactive toward all dipeptides, peptide amides, substituted peptides, esters, and tetrapeptides tested as substrates. Trilysine cleavage is optimal at about pH 8.5, as determined in Tris, borate, or phosphate buffers. Tripeptidase TP activity tested under a number of conditions was not inhibited by soybean trypsin inhibitor (3 mg/mL), phenylmethanesulfonyl fluoride (25 micrometer), or iodoacetate (9 mM). p-Mercuribenzoate (10 micrometer), divalent copper, cobalt, calcium (2.5 mM), zinc (25 micrometer), and mercury (10 micrometer) are inhibitory. Based on Sephadex G-200 chromatography tripeptidase TP has a particle weight of approximately 80 000 daltons. An apparent Km of 5.3 mM was determined for methionylglycylglycine cleavage.