Cathepsin D from Atlantic cod (Gadus morhua L.) liver. Isolation and comparative studies.

The isolated cathepsin D-like enzyme from Atlantic cod (Gadus morhua L.) liver was shown to be a monomer with a molecular mass of approximately 40 kDa. It was inhibited by Pepstatin A and had an optimum for degradation of haemoglobin at pH 3.0. The purified enzyme had lower temperature stability than bovine cathepsin D. Antibodies raised against the purified enzyme and against two C-terminal peptides of cod cathepsin D recognized a 40 kDa protein in immunoblotting of the samples from the purification process. Both antisera showed cross reactivity with a similar sized protein in liver from cod, saithe (Pollachius virens L.), Atlantic herring (Clupea harengus L.) and Atlantic salmon (Salmo salar L.). A protein of same size was detected in wolffish (Anarhichas lupus L.) liver with the antibody directed against the purified enzyme. This antibody also recognized the native enzyme and detected the presence of cathepsin D in muscle of cod, saithe, herring and salmon. These antibodies may be useful in understanding the mechanisms of post mortem muscle degradation in fish by comparing immunohistochemical localization and enzyme activity, in particular in cod with different rate of muscle degradation. They may also be used for comparing muscle degradation in different fish species.