Effects of organic acids on the synthesis of citrulline by intact rat liver mitochondria.

Citrulline synthesis, mostly regulated at the carbamoyl-phosphate synthase I (EC 6.3.4.16) step by the intramitochondrial concentration of ATP and/or N-acetylglutamate is tested with four organic acids: propionate, alpha-ketobutyrate, dipropyl-acetate and 4-pentenoate. In the presence of 10 mM succinate, as the oxidizable substrate, citrullinogenesis was only inhibited by propionate and 4-pentenoate. With 10 mM L-glutamate, a significant inhibition was observed with the four acids. After the addition of ATP and N-acetylglutamate to uncoupled mitochondria, no inhibition could be demonstrated with dipropylacetate and 4-pentenoate. However, a slight inhibition remained with propionate and alpha-ketobutyrate. When mitochondria were incubated with 10 mM L-glutamate, ATP decreased with propionate, dipropylacetate and 4-pentenoate. Under the same conditions, N-acetylglutamate synthesis was strongly inhibited by each organic acid. The decrease of N-acetylglutamate synthesis was related to the constant diminution of intramitochondrial acetyl-coenzyme A (CoA) and to the increase of propionyl-CoA with propionate and alpha-ketobutyrate. Acetyl-CoA and propionyl-CoA are respectively substrate and competitive inhibitor of the N-acetylglutamate synthase (EC 2.3.1.1). Each acid displayed its optimum inhibition at concentrations between 1 and 2 mM. At these acid concentrations, mitochondria had the lowest acetyl-CoA content and the highest propionyl-CoA content.