N-(Phosphonacetyl)-L-aspartate inhibition of the enzyme complex of pyrimidine biosynthesis

The inhibition of aspartate carbamoyltransferase (ACTase) from rat Novikoff tumor by N-(phosphonacetyl)-L-aspartate (PALA) was studied in a substrate mixture permitting endogenous synthesis of carbamoyl phosphate. Among the components required for carbamoyl phosphate synthetase activity, ATP, Mg(C2H3O2)2 and KCl interfered with inhibition by PALA (with added carbamoyl phosphate). The inhibition was also decreased when the concentration of partially purified enzyme was increased. In the system dependent on carbamoyl phosphate synthetase, the 50% inhibitory concentration of PALA was lower than that in the same mixture plus 0.2 mM carbamoyl phosphate, but higher than in the usual simple assay mixture with 0.2 mM carbamoyl phosphate.