Identification of a cysteine proteinase from Jumbo squid (Dosidicus gigas) hepatopancreas as cathepsin L

A cysteine proteinase from Jumbo squid (Dosidicus gigas) hepatopancreas was partially purified by a two step procedure involving ammonium sulfate precipitation and gel filtration chromatography and further by SDS–PAGE. The molecular weight of the proteinase was 24 kDa determined by SDS–PAGE and 23.7 kDa with mass spectrometry. The activity had an optimum pH of 4.5 and optimum temperature of 55 °C under the assay for cathepsin L specific synthetic substrate Z-PAAFC. The cathepsin B and H specific synthetic substrates Z-AAAFC and H-AMC did not show any hydrolysis with the partially purified enzyme. Peptide mapping of trypsin digests of the 24 kDa band from SDS–PAGE showed the squid cysteine proteinase was homologous to cathepsin L from different animal sources. The activity of the partially purified fraction with the cathepsin L specific substrate Z-PAAFC was inhibited 75–89% by enzyme inhibitors specific for cysteine proteinases but was also significantly inhibited by serine and aspartate proteinase inhibitors.