The effect of protein and methionine intake on glutamate dehydrogenase and alanine aminotransferase activities in juvenile black tiger shrimp Penaeus monodon

This study evaluates the influence of both dietary protein and methionine on amino acid trans- and deamination (alanine aminotransferase, ALAT and glutamate dehydrogenase, GDH) in three tissues (muscle, hepatopancreas, gills) of the marine black tiger shrimp (Penaeus monodon). Shrimp (2.4 g) were fed one of the six semi-purified diets containing 14, 34 or 54% crude protein (% dry matter) with two levels of methionine (normal or 30% reduced) for 6 weeks. Both ALAT and GDH activities were the highest in the muscle. ALAT activity in muscle significantly decreased when feeding the low vs. high protein diets. Compared to those fed the intermediate protein level, GDH activity in muscle decreased (by 35%) when fed the low and increased (by 26%) when fed the high protein diets (P < 0.05). A significant interaction between dietary protein and methionine was observed on GDH activity in gills which, due to the relative methionine deficiency, increased 4-fold at the intermediate protein level. In summary, our results demonstrate for the first time the capacity of up and downregulation of enzyme activity by dietary protein levels in the muscle of P. monodon, and the active role played by branchial tissue in ammoniogenesis in response to a relative indispensable amino acid (methionine) deficiency.