Amino acids are major energy substrates for tissues of hybrid striped bass and zebrafish.

Fish generally have much higher requirements for dietary protein than mammals, and this long-standing puzzle remains unsolved. The present study was conducted with zebrafish (omnivores) and hybrid striped bass (HSB, carnivores) to test the hypothesis that AAs are oxidized at a higher rate than carbohydrates (e.g., glucose) and fatty acids (e.g., palmitate) to provide ATP for their tissues. Liver, proximal intestine, kidney, and skeletal muscle isolated from zebrafish and HSB were incubated at 28.5 °C (zebrafish) or 26 °C (HSB) for 2 h in oxygenated Krebs-Henseleit bicarbonate buffer (pH 7.4, with 5 mM D-glucose) containing 2 mM L-[U-14C]glutamine, L-[U-14C]glutamate, L-[U-14C]leucine, or L-[U-14C]palmitate, or a trace amount of D-[U-14C]glucose. In parallel experiments, tissues were incubated with a tracer and  a mixture of unlabeled substrates [glutamine, glutamate, leucine, and palmitate (2 mM each) plus 5 mM D-glucose]. 14CO2 was collected to calculate the rates of substrate oxidation. In the presence of glucose or a mixture of substrates, the rates of oxidation of glutamate and ATP production from this AA by the proximal intestine, liver, and kidney of HSB   were much higher than those for glucose and palmitate. This was also true for glutamate in the skeletal muscle and glutamine in the liver of both species, glutamine in the HSB kidney, and leucine in the zebrafish muscle, in the presence of a mixture of substrates. We conclude that glutamate plus glutamine plus leucine contribute to ~80% of ATP production in the liver, proximal intestine, kidney, and skeletal muscle of zebrafish and HSB. Our findings provide the first direct evidence that the major tissues of fish use AAs (mainly glutamate and glutamine) as primary energy sources instead of carbohydrates or lipids.