Purification and characterization of a new enzyme dipeptidase from human lens

A new enzyme dipeptidase has been purified to homogeneity from human lens tissue adopting isoelectric focusing, preparative electrophoresis and gel filtration HPLC. The purified enzyme hydrolyses a wide variety of dipeptides containing aliphatic as well as aromatic amino acids but does not act on tripeptides and proteins. The identity of this enzyme as a dipeptidase has been confirmed by the use of dipeptides with modified amino or carboxyl groups. The optimum temperature and pH for this enzyme are 25 +/- 2 degrees C and 5.5 respectively and pI is 6.5. The Km for different dipeptides varied from 0.04 mM to 4.2 mM. The molecular weight of the native enzyme as determined by gel permeation HPLC is 52 kDa. Preparative electrophoresis, followed by HPLC gave two active proteins, with molecular weights of 52 kDa and 13 kDa. That with the molecular weight of 52 kDa was found to be the tetramer of the other by SDS-PAGE, and peptide mapping of tryptic digests. Properties of this enzyme have been compared with those reported for other proteinases and peptidases of the lens and dipeptidases of Escherichia coli and mouse tumour cells and they render additional support to the finding that this is a new enzyme. The physiological function of this enzyme is also discussed.