Effect of non-enzymatic browning products on the activity of gastric proteases from the rainbow trout Oncorhynchus mykiss

• Browning heavy products from a glucose-glycine model reduce the activity of gastric proteases of the rainbow trout
• The activity reduction is dependent on the pH, ionic strength and the presence of divalent cations in the medium
• The mechanism of the activity reduction is probably based on the co-precipitation of gastric proteases and browning products

The products resulting from the non-enzymatic browning in processed diets are known to exert negative effects on the digestive enzymes of vertebrates. In addition, browning heavy products (BHP's) are known to co-precipitate with proteins depending on the pH and ionic strength of the medium and on the isoelectric point of the protein. As the manufacture of aquafeeds as well as aquafeed ingredients frequently implies heating processes, the effects of BHP's from a well-known model mixture (glucose + glycine) on the gastric proteases of the rainbow trout Oncorhynchus mykiss were investigated in the present work. The results support that BHP's interact with gastric proteases most probably to form a co-precipitate, thus potentially reducing the protease activity in the fish stomach. The maximal loss of activity found in the experiments was above 20%. The interaction is pH and ionic strength-dependent and relatively stable against mechanical perturbation. In the range of BHP concentration 500–1000 μg/mL, the interaction was dose-dependent at pH 3.5 with a maximum above 20%, and apparently independent of the dose at pH 4.0. Above an ionic strength of 100 mM due to monovalent ions (NaCl), the intensity of the interaction is reduced. On the other hand, divalent cations such as Ca2 + and Mg2 + in the range 30–80 mM disturb the interaction between BHP's and gastric proteases even at ionic strengths below 100 mM, thus alleviating the inactivating effect of BHP's.