The molecular weights of two forms of carbamoyl phosphate synthase from rat liver.

1. N-Acetylglutamate-dependent carbamoyl phosphate synthase from rat liver was centrifuged in sucrose density gradients. The concentration-dependence of s was consistent with a chemical equilibrium existing between the 11S and 7.5S forms of the enzyme. 2. Under conditions favouring the 11S form, the properties of the enzyme in ultra-short-column equilibrium experiments suggest a molecular weight of 316000+/-42000 for the 11S form. 3. Under conditions favouring the 7.5S form, high-speed equilibrium-sedimentation measurements gave a value of 160000+/-10000 as the molecular weight of the 7.5S form of the enzyme.