DETERMINATION OF D(-)-BETA-HYDROXYBUTYRIC ACID IN TISSUE EXTRACTS

THE EFFECTS OF ADENINE NUCLEOTIDES ON PYRUVATE METABOLISM IN RAT LIVER

1. The effects of adenine nucleotides on pyruvate metabolism by isolated liver cells and isolated mitochondria have been investigated. The amount of pyruvate carboxylated has been estimated by determining the tricarboxylic acid-cycle intermediates, glutamate and aspartate accumulating in the incubation medium. The extent of pyruvate oxidation has been assessed by measuring oxygen uptake and the yield of (14)CO(2) from [1-(14)C]pyruvate and [2-(14)C]pyruvate. 2. When catalytic amounts of adenine nucleotides (1-2mm) were added to suspensions of isolated liver cells incubated with pyruvate an ATP:ADP ratio greater than 6:1 was maintained. Both pyruvate oxidation to acetyl-CoA and the oxidation of acetyl-CoA through the tricarboxylic acid cycle were stimulated but pyruvate carboxylation was not affected. The production of acetyl-CoA exceeded the capacity of the cells for the oxidation of acetyl-CoA and the excess was converted into ketone bodies. 3. If a low ATP:ADP ratio was maintained in isolated cells or mitochondria by incubating them with dinitrophenol or hexokinase, pyruvate carboxylation was grossly inhibited, oxygen uptake depressed and ketone-body formation stimulated. Measurement of oxaloacetate concentrations confirmed that under these conditions oxaloacetate was rate-limiting for the oxidation of acetyl-CoA via the tricarboxylic acid cycle. The inclusion in the incubation medium of fumarate (1.25mm) completely prevented the ketogenic action of dinitrophenol or hexokinase. 4. When ADP (5mm) was added to a suspension of isolated liver cells incubated with pyruvate an actual ADP concentration of about 1mm was attained. This brought about effects on pyruvate metabolism similar to those obtained with dinitrophenol or hexokinase. 5. These results support the concept that the relative concentrations of adenine nucleotides within the liver cell may play a role in governing the rates of pyruvate oxidation and carboxylation. In addition, they provide further evidence that the availability of oxaloacetate in the liver cell can play a key role in determining whether acetyl-CoA arising from pyruvate is oxidized through the tricarboxylic acid cycle or converted into ketone bodies.

Effect of palmitate concentration on the relative contributions of the beta-oxidation pathway and citric acid cycle to total O2 consumption of isolated rat hepatocytes

The relative contributions of beta-oxidation and citric acid cycle activity to total O2 consumption during fatty acid oxidation were examined in isolated hepatocytes. When hepatocytes were incubated with palmitate alone, a rise in fatty acid concentration induced an increase in O2 uptake that reflected a large stimulation of beta-oxidation and an accompanying smaller inhibition of citric acid cycle oxidation. In the presence of lactate, successive increments in palmitate concentration over the range from 0 to 1.0 mM stimulated glucose synthesis and brought about a concomitant incremental stimulation of both beta-oxidation and citric acid cycle flux. However, above 1.5 mM palmitate, additional increments in fatty acid concentration depressed gluconeogenesis and citric acid cycle activity but induced a further stimulation of beta-oxidation. These findings demonstrate that, during fatty acid oxidation, the rate of citric acid cycle turnover is more closely linked to the rate of glucose synthesis than is the rate of beta-oxidation. This may be relevant to observations that the stimulation of hepatic O2 consumption, induced by fatty acid oxidation, is much greater than can be explained in terms of the ATP-demand arising from exposure of hepatocytes to fatty acid.