Phosphopyruvate carboxylase of rat lung

Carbon tetrachloride inhibits synthesis of pulmonary surfactant disaturated phosphatidylcholines and ATP production in alveolar type II cells

Other studies have shown that inhalation of carbon tetrachloride (CCl4) decreases the amount of pulmonary surfactant lining the alveolar surface. Therefore, we studied the effects of CCl4 on the synthesis of surfactant phosphatidylcholines (PCs) in rat alveolar type II cells in vitro. The rate of incorporation of choline, palmitate or glycerol into disaturated PC (DSPC) is decreased in a concentration-dependent manner. The CCl4 concentrations which cause maximal inhibition and 50% inhibition are similar for each substrate. The rate of incorporation of choline or glycerol into total PC is diminished to the same extent as their incorporation into DSPC. In addition, the rate of incorporation of glycerol into phosphatidylglycerol is decreased by the same extent as its incorporation into PC. All of these data suggest that there is a common site(s) at which CCl4 inhibits PC synthesis and that the inhibition occurs early in the biosynthetic pathway. However, individual enzymes involved in phospholipid synthesis do not seem to be affected by the solvent. Exposure of alveolar type II cells to CCl4 does cause a rapid and dramatic loss in cellular ATP, a cofactor required by some enzymes involved in PC synthesis. Studies with isolated lung mitochondria suggest that CCl4 inhibits the enzyme complex which catalyzes the synthesis of ATP from ADP. In addition, CCl4 causes a decrease in the amount of 3-O-methylglucose associated with type II cells, suggesting that glucose influx is impaired. This may also contribute to lower cellular ATP levels. The results of this study suggest that inhalation of CCl4 may impair surfactant phospholipid synthesis by decreasing ATP levels in alveolar type II cells.

Citrate formation by rat lung mitochondrial preparations

Rat lung mitochondrial preparations were incubated in the preasence of pyruvate and malate. The principal metabolic products measured were citrate and CO2. Citrate formation from pyruvate was found to be dependent on the presence of malate. Significant citrate was formed in the presence of isocitrate and the rate of citrate formation was increased by the addition of pyruvate. Small amounts of citrate were formed by lung mitochondrial preparations in the presence of 2-oxoglutarate and succinate only after the addition of pyruvate. The level of acetyl-CoA was significantly greater in the presence of pyruvate than in the presence of pyruvate plus malate. The addition of malate to lung mitoochondrial preparations increased 14CO2 production from [2-14C] pyruvate into malate and citrate. A low level of pyruvate-dependent H14CO3-incorporation into acid-stable products was observed, principally citrate and malate, but this rate did not exceed 5% of the rate of net citrate formation in the presence of malate and pyruvate. The capacity of rate lung mitochondria to form oxaloacetate from pyruvate alone in vitro is very limited, and would appear to cast doubt on a major role of pyruvate carboxylase in citrate formation. It is concluded that the rate of citrate formation from pyruvate is limited by the availability of intramitochondrial oxaloacetate and the rate of citrate efflux across the mitochondrial membrane.