Dietary protein concentration regulates the mRNA expression of chicken hepatic malic enzyme

Chicken hepatic malic enzyme activity varies with dietary protein content. The mechanisms responsible for this alteration in activity are unclear. In a series of four experiments, broiler chicks were allowed free access for 1.5, 3, 6 or 24 h to a low (13 g/100 g diet), basal (22 g/100 g diet) or high (40 g/100 g diet) protein diet. The diets were isocaloric and had equal concentrations of dietary fat. Hepatic malic enzyme mRNA expression and enzyme activity as well as total liver lipid concentration were examined for each experimental duration. There were no differences in the expression of the mRNA for malic enzyme at 1.5 h, but at 3, 6 and 24 h, malic enzyme mRNA expression was significantly (P < 0.05) reduced in chicks fed the high protein diet and significantly enhanced in chicks fed the low protein diet compared with chicks fed the basal diet. Hepatic malic enzyme activities and total lipid concentration were not different among the chicks fed the different diets at 1.5 and 3 h. At 6 and 24 h, malic enzyme activity and total liver lipid concentration were both significantly greater in birds fed the low protein diet compared with levels in the birds fed the other two diets. In birds fed the high protein diet, malic enzyme activity and total liver lipid concentration were significantly reduced at 24 h compared with birds fed the basal diet. In a final experiment, the observed differences in malic enzyme mRNA expression at 6 h were confirmed when chicks were given access to isocaloric diets with the same protein levels as the initial 4 experiments, but with the dietary concentration of carbohydrate held constant. The results suggest that previously observed alterations in the activity of malic enzyme, which were correlated with dietary protein intake, are due to rapid changes in the mRNA expression of this enzyme.