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.