Dietary protein differentially regulates the kinetic behaviour of serine dehydratase and tyrosine aminotransferase of liver and white muscle of rainbow trout (Oncorhynchus mykiss).

We have determined the kinetic behaviour of serine dehydratase (SerDH) and tyrosine aminotransferase (TyrAT) in the liver and white muscle of juvenile rainbow trout (Oncorhynchus mykiss) fed on a low-protein/high-fat (LP/HF) and control diets. The relationship between the kinetic parameters and tissue-DNA concentration has also been determined. SerDH and TyrAT showed hyperbolic kinetics in all cases. The partial replacement of protein with fat significantly increased hepatic SerDH-specific activity, maximum velocity (V(max)) and Michaelis constant (K(m)) while no significant changes were detected in the values of these parameters in white muscle. Nevertheless, hepatic and white-muscle TyrAT specific activity and V(max) decreased in trout fed on LP/HF diet with respect to control. The K(m) of hepatic TyrAT was also lower in trout fed on LP/HF diet than in control. The TyrAT kinetic parameters expressed per cell unit also changed in the same sense as previously described. These results show that, in trout, SerDH and TyrAT are two enzymes regulated by the partial replacement of dietary protein by fat.