THE INFLUENCE OF DIETARY-PROTEIN ON THE KINETICS OF NADPH PRODUCTION SYSTEMS IN VARIOUS TISSUES OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

The effects of a decrease in dietary protein level on the kinetic behaviour of the four NADPH production systems [glucose 6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), malic enzyme (ME), and NADP-isocitrate dehydrogenase (NADP-IDH)] were investigated in four different trout tissues. Typical hyperbolic saturation curves were always obtained for the activity of these enzymes. A low-protein diet caused a significant decrease in the maximum velocity values (V(max)) of all the hepatic enzyme systems studied. The decrease in hexose monophosphate dehydrogenases (G6PDH and 6PGDH) was about 35 and 50%, respectively, whereas for ME and NADP-IDH it was about 35 and 25%. No significant changes were found in the Michaelis constant. These kinetic characteristics are compatible with an increase in the catalytic efficiency of these enzymes without there being any changes in their activity ratio values. The kinetic parameters for these enzymes in the kidney, spleen, and gill tissue did not undergo any significant change.