The regulation of carbamyl phosphate synthetase (ammonia) in rat liver mitochondria. Effects of acetylglutamate concentration and ATP translocation.

1. Mitoplasts, unlike mitochondria, are readily permeable to acetylglutamate; by incubating mitoplasts with different external [acetylglutamate] a wide range of concentrations of this cofactor in the matrix can be obtained, which can be correlated directly with the velocity of carbamyl phosphate synthetase (ammonia). 2. The relationship between carbamyl phosphate synthetase (ammonia) activity and matrix [acetylglutamate] is hyperbolic. One-half of the maximal velocities observed using mitoplasts are obtained when matrix [acetylglutamate] is about 0.2 n”; the velocity is twice as high at 1.3 n” matrix acetylglutamate. The velocity of carbamyl phosphate synthesis by mitochondria is about one-half of the maximum observed using mitoplasts, and the matrix [acetylglutamate] is 0.4 m ~ . 3. On the basis of these and other published data, we propose that carbamyl phosphate synthetase (ammonia) of mitochondria of animals fed a normal diet is approximately half-saturated with acetylglutamate, and that the most important involvement of acetylglutamate in urea synthesis is in bringing about the inactivation of carbamyl phosphate synthetase (ammonia) when the supply of ammonia decreases. 4. In coupled mitochondria, ADP inhibits carbamyl phosphate synthesis; the inhibition is inversely proportional to the external ATP/ADP when this ratio is between 1 and 14. In uncoupled mitochondria, using exogenous ATP, carbamyl phosphate synthesis is maximal even at external ATP/ADP of 1. The effect of ADP translocation on the activity of carbamyl phosphate synthetase (ammonia) may provide a mechanism for the coordination of the mitochondrial and the cytoplasmic portions of the urea cycle.