Lens exopeptidases

Abstract

Exopeptidases identified as dipeptidyl peptidases II and III were found in the bovine, human normal and human cataractous lenses. Arylaminopeptidase activities identified in extracts of these tissues were able to hydrolyze phenylalanine, leucine, methionine, lysine and alanine arylamidase substrates. Lysosomal DAP II and the cytosol DAP III were recognized by their marked preferences for the release of Lys-Ala from Lys-Ala-2-NNap (at pH 5·5) and Arg-Arg from Arg-Arg-2-NNap (at pH 9·0), respectively. DAP III in human lens extracts exhibited a pH optimum of 9·5 at 37°C. The K m for the DAP III substrate was estimated to be 4·64 × 10 −5 m for normal lens extracts, 2·74 × 10 −5 m for human cataractous lens extracts, and 2·83 × 10 −5 m for bovine lens extracts. Lens DAP III was strongly inhibited by PCMS, EDTA, and to a lesser degree by puromycin and DFP. Arylaminopeptidase activities (from human and bovine lenses) assayed on Leu-2-NNap were strongly inhibited by PCMS and puromycin. Negligible effects were obtained with EDTA and DFP. Only traces of leucyl aminopeptidase (assayed on leucinamide) could be detected in human lenses; however, aminopeptidase activities measured at pH 7·0 on the β-naphthylamides of leucine, methionine, lysine, phenylalanine and alanine were plentiful in both human and bovine lenses, and served to indicate the possible presence of other aminopeptidases such as alanine aminopeptidase and aminopeptidase B. Of particular interest was the finding that the specific activities of Met-, Lys-, and Ala-2-NNap were 2–5-fold higher in cataractous human lenses compared to normal lenses.