Aminopeptidase Play a Critical Role in the Accumulation of Free Amino Acids in Abalone (Haliotis discus hannai) During Cold Storage

Abalones reveal unique taste after processing, mainly because of their abundant free amino acids (FAAs) and nucleotides. FAAs are nutrition components that can contribute to the unique taste. However, which factor(s) is responsible for the accumulation of FAAs still need further studies. To analyze the production of FAAs, we studied the variation of FAAs during 7 days of storage at 4°C. The content of taste-active amino acids, including Asp, Glu, Ser, and Gly increased by 1.7-fold, 2.0-fold, 3.0-fold, and 8.4-fold, respectively. The relative activity of cathepsin L and aminopeptidase (AP) increased significantly during the cold storage period. To identify AP in abalone and its function in mediating the production of FAAs, an aminopeptidase with wide substrate specificity was then extracted and purified from abalone muscle to homogeneity. Purified AP with a molecular mass of 100 kDa exhibited its maximum activity at 30°C, pH 7.5, and was further confirmed by LC-MS. Bestatin specifically inhibited the activity of AP, and metalloproteinase inhibitors EDTA, EGTA and 1, 10-phenanthroline also suppressed its activity to different degrees. Based on its highest activity to substrate Leu-MCA and its peptide sequences, the purified enzyme was identified as leucine aminopeptidase (LAP). Our present study indicated the essential role of AP for FAAs accumulation during cold storage of abalone.