Variations in the kinetic behaviour of the NADPH-production systems in different tissues of the trout when fed on an amino-acid-based diet at different frequencies

We have studied the effects of feeding an amino-acid-based diet (ABD) at di fferent frequencies upon growth and several NADPH-production systems in the rainbow trout (Oncorhynchus mykiss). The kinetic behavior of glucose 6-phosphate dehydrogenase (G6PDH), 6-phosph ogluconate dehydrogenase (6PGDH), melic enzyme (ME) and NADP-linked isocitr ate dehydrogenase (NADP-IDH) was followed in the liver, kidney and adipose tissue. The kinetic parameters of NADP-IDH alone remained unaltered by eith er ABD or changes in feeding frequency. Maximum-velocity and catalytic-effi ciency values of hepatic G6PDH and ME increased significantly when fed four times a day compared to twice a day with both the control diet and ABD, al though these parameters for ME were significantly lower with ABD than with the control diet at both frequencies. In the kidney the activity and cataly tic efficiency of G6PDH and 6PGDH increased significantly with high-frequen cy feeding on ABD. The activities of these enzymes in adipose tissue were m uch lower than in hepatic tissue. In the liver, maximum velocity and the ca talytic efficiency of G6PDH, 6PGDH and ME increased significantly with the control diet at high-frequency feeding whereas they decreased significantly with ABD, especially with high-frequency feeding. Neither the Michaelis co nstant nor the activity ratios varied. Both feeding frequency and free amino acid altered the activity of the most important cytosolic NADPH-production systems. The varying response to nutr itional stimuli of NADP-linked enzymes in fish tissues shows that they have independent physiological and metabolic roles and that their regulatory me chanisms respond to changes in nutritional and metabolic factors. (C) 1999 Elsevier Science Ltd. All rights reserved.