Characterization of the proteases involved in hydrolyzing paddlefish (polyodon spathula) myosin

An extract from paddlefish surimi possessed activities of B, L, and H-like cathepsins. The optimal pH was around 5.0 for cathepsins B and L, and was between 6.0-6.5 for the H-like cathepsin. The enzyme activities were not impaired by heating at 40C for 20 min. However, the protease extract lost about 20% of its cathepsin B, 50% B+L, and 90% H-like cathepsin activities after heating at 50C for 20 min. The activity of H-like cathepsin was not inhibited by E-64, suggesting that it did not belong to the known cysteineprotease group. The protease extract was capable of hydrolyzing myosin heavy chain, producing a major fragment(s) around 140 kDa. Degradation of myosin by the protease extract was substantially reduced by protease inhibitors including E-64, a protease inhibitor mixture, and bovine plasma powder.