Abstract: Proteases were extracted from shrimp heads as a byproduct from the processing of Pacific white shrimp (Penaeus vannamei). The types of enzymes contained in the extract and the effect of salt concentration on these protease activities were investigated. The extract mainly contained trypsin, cathepsins and aminopeptidases. At NaCl concentration of 25%, trypsin, cathepsin B+L and cathepsin K retained about 10% of their initial activity while the activity of phenylalanine aminopeptidase remained 30% of its initial level. High salinity (25%) had an activating effect on alanine aminopeptidase and methionine aminopeptidase, increasing them by 31% and 91%, respectively. The crude proteases were used to degrade waste proteins from soy sauce production. The degradation efficiency and the inhibitory activity of the resulting products against angiotensin I-converting enzyme (ACE) were evaluated. It was found that the proteases were still able to digest the proteins even at high salinity. After enzymatic hydrolysis, the polypeptide content increased from 96.16 to 1 049.55 μg/mL, the proportion of proteins with molecular mass higher than 5 kDa decreased from 43.78% to 16.85%, and the proportion of peptides with molecular mass lower than 1 kDa increased from 28.50% to 58.96%. The total amount of free amino acids increased from 3.01 to 97.16 mg/100 mL. The utilization rate of soybean proteins increased by about 11.82%. The IC50 value of the resulting hydrolysate for inhibition of ACE was 90.02 μg/mL. Our present study provides a theoretical basis for high-valued utilization of by-products from the processing of soy sauce and shrimp.

Key words: crude shrimp proteases, enzyme activity, waste proteins from soy sauce production, angiotensin I-converting enzyme inhibitory activity