Starch digestion in tropical fishes: isolation, structural studies and inhibition kinetics of α-amylases from two tilapias Oreochromis niloticus and Sarotherodon melanotheron

α-Amylases from the intestinal cavity of two tilapia species, Oreochromis niloticus (ONI-AMY) and Sarotherodon melanotheron (SME-AMY), were purified using ammonium sulfate precipitation, affinity chromatography and chromatofocusing procedures. The purification was approximately 100-fold. The amylolytic activity, specific activity, product distribution, pH and temperature profile of ONI-AMY and SME-AMY are quite similar. The molecular mass differs slightly: 56 600 (ONI-AMY) vs. 55 500 (SME-AMY). As shown by isoelectric focusing analysis, both amylases contain two isoforms A and B with distinct p I: 7.2 (A) and 7.8 (B), vs. 8.3 (A) and 8.8 (B), respectively. It was not possible to isolate B, since B converts into A with time. The kinetics of the inhibition of ONI-AMY and SME-AMY activity by α-, β- and γ-cyclodextrin (α-, β- and γ-CD) were investigated using amylose as the substrate. Statistical analysis of the kinetic data expressed using a general velocity equation and assuming rapid equilibrium showed that the inhibition is of the mixed noncompetitive type. Similar results were obtained with ONI-AMY and SME-AMY. β- and γ-CD are stronger inhibitors than α-CD. ONI-AMY and SME-AMY are then closely related and show the general features common to the members of the α-amylase class (family 13). They enable ONI and SME tilapias to digest starch in food.