Citrulline synthesis: Regulation by alterations in the total mitochondrial adenine nucleotide content

The total adenine nucleotide content of rat liver mitochondria was varied in vitro over a wide range in order to investigate a possible relationship between net changes in the total matrix ATP + ADP + AMP content and the overall rate of citrulline synthesis. Isolated mitochondria were specifically depleted of matrix adenine nucleotides by incubating with inorganic pyrophosphate (G. K. Asimakis and J. R. Aprille, 1980, Arch. Biochem. Biophys.203, 307–316); alternatively, matrix adenine nucleotides were increased by incubating mitochondria with 1 mm ATP at 30 °C. No exogenous ATP or ADP was included in the subsequent incubations for the determination of citrulline synthesis. Rates varied from 0.1 to 1.6 μmol citrulline/mg protein/h as a linear function of total adenine nucleotide content in the range 2–15 nmol (ATP + ADP + AMP)/mg protein. Further increases in the matrix ATP + ADP + AMP content caused no further increase in citrulline synthesis rates. Changes in the total adenine nucleotide content were reflected in proportional changes in both the ATP and ADP content of the matrix. The $\text{ATP}\text{ADP}$ ratio did not change significantly. Therefore, the variations in citrulline synthesis were most simply explained as the effect of different concentrations of ATP on the activity of carbamoyl-phosphate synthetase. It was concluded that net changes in the total adenine nucleotide content can contribute to the control of citrulline synthesis. These findings are significant in the context of recent evidence which shows that the matrix adenine nucleotide pool size is under hormonal control.