Relationship between the rate of citrulline synthesis and bulk changes in the intramitochondrial ATP/ADP [adenosine triphosphate/adenosine diphosphate] ratio in rat-liver mitochondria

1 The relationship between intramitochondrial and extramitochondrial ATP‐utilizing systems and the intramitochondrial ATP/ADP ratio was studied in isolated rat‐liver mitochondria. Citrulline synthesis was used as an intramitochondrial ATP‐utilizing system, and glucose‐6‐phosphate synthesis as an extramitochondrial ATP‐utilizing system. The intramitochondrial ATP/ADP ratio was manipulated in three ways: with succinate and different concentrations of malonate and/or hexokinase; with 2‐oxoglutarate (plus oligomycin) and different concentrations of hexokinase; and with added ATP in uncoupled mitochondria (oligomycin present). 2 Under all conditions used, citrulline synthesis was strictly correlated with the bulk intramitochondrial ATP/ADP ratio. 3 The curve relating citrulline synthesis and intramitochondrial ATP/ADP was shifted towards lower ATP/ADP ratios when the activity of carbamoyl‐phosphate synthetase was enhanced by increasing the mitochondrial content of N‐acetylglutamate. 4 It is concluded that under the experimental conditions used the intramitochondrial adenine nucleotides behave as a homogeneous pool.