Reversible dissociation of a carbamoyl phosphate synthase-aspartate transcarbamoylase-dihydroorotase complex from ovarian eggs of Rana catesbeiana: effect of uridine triphosphate and other modifiers

Abstract

Glutamine-dependent carbanoyl phosphate synthase [ATP6carbamate phosphotransgerase (dephosphorylating), EC 2.7.2.9], aspartate transcarbamoylase (carbamoylphosphate: L-aspartate carbamoyltransferase, EC 2.1.3.2) and dihydroorotase (L-5,6-dihydroorotate amidohydrolase, EC 3.5.2.3), are copurified as a high-molicular-weight complex from extracts of unfertilized eggs of Rana catesbeiana. UTP is required to maintain the integrity of the complex during the last two purification steps. Removal of the nucleotide results in dissociation of the complex. Based on sedimentation behavior in glycerol gradients, the dissociated carbamoyl phosphate synthase has an apparent molecular weight of 260,000 +/- 20,000 and that of dihydroorotase is estimated at 280,000 +/- 20,000. Aspartate transcarbamoylase is broadly distributed over the gradient. The addition of ATP, 5-phosphoribosyl-1-pyrophosphate, Mg++, or inorganic phosphate to the dossociated complex results in the appearance of a peak of aspartate transcarbamoylase activity with an apparent molecular weight of 110,000 +/- 10,000. Icubation of a mixture of the dissociated enzymes with UTP and Mg++ leads to their reassociation into the high-molecular-weight complex.