This manuscript reports changes in key enzymes and metabolites related to protein metabolism and nitrogen excretion in the liver of juveniles jundiá (Rhamdia quelen) fed on isocaloric diets containing 20%, 27%, 34% and 41% of crude protein. The hepatic activity of alanine aminotransferase (ALAT), aspartate aminotransferase (ASAT), glutamate dehydrogenase (GDH), and arginase (ARG) increased with the content of protein in the diet, and the ratios among the aminotransferases and GDH allowed evaluating metabolic preference. The concentration of free amino acids, ammonia and urea also rose with the dietary protein content. Increase of plasma urea and ammonia was the resultant effect of over amino acids catabolism as consequence of dietary protein surplus. Since the increase of protein in the diets resulted in weight gain, the rise in the hepatic activity of protein-metabolising enzymes in the fish fed high protein diets denoted effective use of dietary amino acids for growth and as a substrate for gluconeogenesis. Analysis of changes on metabolite levels and key enzyme activities in amino acid metabolism is proposed as a tool for assessing the proper balance of diet macronutrients.