Biochemical studies on dipeptidyl peptidase I (cathepsin C) from germinated Vigna radiata seeds

• Plant dipeptidyl peptidase I (DPP-I) is a cysteine protease.
• It preferably cleaves Gly–Arg–4mβNA at pH 7.0.
• It is a monomer with a molecular weight of approximately 38 kDa.
• These findings can lead to further studies into the role of this enzyme in plant development.
• Plant DPP-I may be involved in the activation of other proteases and generation of bioactive peptides.

Dipeptidyl peptidases (DPPs) are widely distributed exopeptidases that hydrolyse the dipeptide moieties from the N-termini of oligopeptide chains. In the present study, DPP-I was purified from germinated moong bean seeds via acid and ammonium sulphate precipitation followed by successive chromatographies, that is, gel filtration (pH 7.4), cation exchange (pH 5.9) and anion exchange (pH 7.5). The purity of the enzyme was confirmed by native polyacrylamide gel electrophoresis (PAGE) and in situ gel assay. Purified plant DPP-I is a monomeric enzyme with a molecular weight of 38 kDa. It works optimally at pH 7.0 and 40 °C, and it exhibits stability at pH ranging from acidic to slightly alkaline. Plant DPP-I preferentially hydrolyses glycine–arginine–4-methoxy-β-naphthylamide and various other synthetic dipeptidyl substrates, but none of the studied endopeptidase and monopeptidase substrates. Inhibitory studies revealed the role of Cys and His amino acids in the catalytic mechanism. Functional studies of DPP-I revealed the significant role of this glycoproteinous enzyme in protein mobilization during germination.

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