Synthesis and Degradation of Malic Enzyme in Chick Liver

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Abstract The role of synthesis and of degradation in regulation of malic enzyme concentration in the liver of growing chicks has been studied. Malic enzyme was assayed by enzymic activity and by immunological analysis with immunoglobulin containing specific anti-malic enzyme. Quantitative immunoprecipitin curves and equivalence point determinations indicated that there was a constant relationship between immunologically precipitable protein and units of enzyme activity in livers of neonatal and growing chicks during a time period in which total enzyme activity increased 60-fold. Hence, changes in malic enzyme activity were due to changes in enzyme content of the liver rather than to activation or inhibition of preformed enzyme. The relative rate of malic enzyme synthesis was determined by measuring incorporation of counts into malic enzyme protein after pulse labeling total protein with l-[4,5-3H2]leucine. Malic enzyme protein was purified by quantitative precipitation of the enzyme by antibody followed by disc gel electrophoresis in the presence of sodium dodecyl sulfate. Radioactivity in the band corresponding to malic enzyme was estimated after electrophoresis of the immunoprecipitate. Relative to synthesis of total protein, the rate of synthesis of malic enzyme increased more than 50-fold when neonatal chicks were fed. Starvation of week-old chicks for 2 days caused a 60 to 70% decrease in synthesis of malic enzyme. Degradation of malic enzyme was very slow in unfed, neonatal chicks (tfrac12; = 350 hours). Feeding, which increased the synthesis and concentration of malic enzyme also caused an increase in the rate of degradation. In 8- and 11-day-old chicks degradation of malic enzyme was found to be first order with a tfrac12; of 55 hours. In fasting chicks a t½ of 28 hours was observed, but there was also an increase in the rate of degradation of total liver protein suggesting that part of the increased rate of degradation of malic enzyme was due to a general increase in protein breakdown. The temporal relationship between synthesis of malic enzyme and synthesis of fatty acids was studied in unfed neonatal chicks given a single glucose meal. Fatty acid synthesis from [1-14C]acetate was increased 7-fold at 1 ½ hours and 20-fold at 3 hours after the glucose meal. Neither the activity nor the synthesis of malic enzyme were increased at 3 hours but both were significantly increased at 6 hours (100% and 160%, respectively). These results suggest that the increased synthesis of malic enzyme may have been initiated by an increased flux through the pathway for fatty acid synthesis.