The effect of dietary fat on the activity,content, rates of synthesis,and degradation and translation of messenger RNA coding for malic enzyme in mouse liver

The specific activity (units activity/mg cytosolic protein) of malic enzyme was found to be three-fold higher in the livers of mice fed a semipurified diet containing 50% ($\text{w}\text{w}$) glucose and 15% ($\text{w}\text{w}$) saturated and monounsaturated but no polyunsaturated fat (hydrogenated cottonseed oil) over an 11-day period than in the livers of mice fed a standard laboratory mouse chow (Purina) diet. In contrast, when other lab chow-fed mice were fed an isocaloric diet containing 15% ($\text{w}\text{w}$) polyunsaturated fat (corn oil), no change in the specific activity of malic enzyme occurred over a similar period of time. Rocket immunoelectrophoresis performed on cytosols from both dietary groups demonstrated that the livers of mice consuming the hydrogenated cottonseed oil diet contained approximately three times more malic enzyme protein than did the livers from the corn oil-fed animals. In mice pulse-labeled with l-4,5-³H]leucine, the rate of hepatic malic enzyme synthesis (relative to that for total protein) was approximately twofold greater in the hydrogenated cottonseed oil-fed mice than in their corn oil-fed counterparts whereas the rate of hepatic malic enzyme degradation was similar for both groups. Immunotitration of liver malic enzyme from hydrogenated cottonseed oil-fed and corn oil-fed mice revealed identical equivalence points, demonstrating that the catalytic efficiency of mouse liver malic enzyme had not been affected by the type of dietary fat administered. When total liver RNA, isolated from the hydrogenated cottonseed oil- and the corn oil-fed animals, was translated in cell-free translation systems (wheat germ extract and reticulocyte lysate) we found that both dietary treatments had resulted in an increase in the activity of malic enzyme messenger RNA. Furthermore, there were no significant differences between the two dietary groups in this regard. These results suggest that hepatic malic enzyme specific activity in high-carbohydrate polyunsaturated fat-fed mice is regulated principally by dietary-induced changes in the rate of enzyme synthesis and not by the activity of messenger RNA coding for the enzyme.