MALIC ENZYME-ACTIVITY IN ADULT AND NEWBORN RAT LUNG

Using a sensitive technique measuring (CO2)-C-14 production from radio labeled malate, we examined malic enzyme activity in both adult and ne wborn rat lung tissue and in L2 cells, a cell culture line of type II pneumocytes. Malic enzyme was present in both cytosolic and mitochondr ial fractions. Time course experiments demonstrated a linear rate afte r the initial 10 min, up to 30 min. The optimal pH in the cytosolic fr action was 8.0, whereas maximal mitochondrial malic enzyme activity oc curred at pH 7.0. The mitochondrial fraction exhibited biphasic kineti cs over the 200-fold range of concentrations examined. The high-affini ty Km was 0.16 mmol with V-max of 7.11 nmol/mg protein/min. The low-af finity Km was 6.95 mmol, with V-max of 31.82 nmol/mg protein/min. In t he cytosol there was a single Km of 0.30 mmol and V-max of 5.95 nmol/m g protein/min. In paired experiments examining differences between 1-d -old and adult rat lung, significantly higher total and mitochondrial malic enzyme activity occurred in the newborn as compared with the adu lt. Malic enzyme activity was also present in the L2 cells. The findin g of malic enzyme activity in the lung suggests that cytosolic malic e nzyme may play a role in generating NADPH needed in the lung for fatty acid synthesis. These findings of developmental differences in malic enzyme activity suggest that alternate substrates such as anaplerotic amino acids may be used in the young animal as energy substrates by wa y of the tricarboxylic acid cycle.