Digestive proteases of Penaeus vannamei Boone: Relationship between enzyme activity, size and diet

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The relationships between protein level, protein source, size, and digestive protease enzyme activities of the marine shrimp, Penaeus vannamei Boone, were investigated during three 30-day growth experiments. Three sizes of shrimp (4.0, 9.8, 20.8 g) were maintained in 2650-l indoor tanks and fed ad libitum with six isocaloric diets. The protein sources were varied by changing the animal to plant protein ratio ( a p ratio), 2:1 and 1:1, while each of these two series was composed of three diets having protein levels of 22, 30 and 38%. The following digestive enzyme activities were detected in crude extracts prepared from the digestive system of the shrimp; trypsin, carboxy-peptidase A, carboxypeptidase B, acid protease and general protease. The extracts lacked chymotryptic and peptic activities. The level of protein in the diet had a greater effect on the enzyme total activities (activity per g wet tissue) in the large shrimp (17–30 g) than in small shrimp (< 10 g). When the specific activities (activity per mg of protein in the extract) of the fed shrimp were evaluated, small shrimp (< 10 g) fed the 1:1 a p ratio diets displayed lower activities than those fed the 2:1 ratio diet. The protein level influenced the enzyme activities in shrimp of all sizes while the protein source had a greater effect on the enzyme activities in small shrimp (< 10 g). This differing proteolytic response to protein level and source as a function of size supports the formulation of specific diets for shrimp of different sizes, taking into consideration the changes in digestive physiology as the shrimp grow.