Acetyl Coenzyme A Carboxylase III. FURTHER STUDIES OK THE RELATIOK OF CATALYTIC ACTIVITY TO POLYnIERIC STATE*

Catalytic activity of the filamentous polymeric form of avian liver acetyl-CoA carboxylase, (spO+ = 45 to 47 S) undergoes first-order decay in assay reaction mixture (f; 10 min at Z”), which is rapidly reversed ( <lO s) by addition of tricarboxylic acid activator, i.e. citrate or isocitrate. As previously shown by sucrose density gradient centrifugation, extended exposure to assay reaction mixture leads to the formation of a catalytically inactive 13 to 15 S protomeric species of the enzyme. If avidin, the biotin-binding protein from egg white, is added following this decay, reactivation by citrate is blocked completely. However, when citrate is present in the assay reaction mixture, thereby maintaining the enzyme in the polymeric state, inactivation by avidin is prevented. The presence of avidin in the assay reaction mixture does not alter the ft for activity decay, although reactivation is no longer possible upon addition of citrate. Only that fraction of carboxylase activity remaining can be protected by citrate from further erosion of activity and inactivation by avidin. Thus, the characteristics and kinetics of the decay of catalytic activity are compatible with a transition from an avidin-insensitive, active polymeric species to an avidin-sensitive, catalytically inactive form. Consistent with a polymer (intrinsic viscosity, 83) to protomer (intrinsic viscosity, 11.5) transition, the decay of enzyme activity is tightly coupled to an apparent first order (ft = 10 min) decline in viscosity, fg (activity) = fg (viscosity) . Citrate addition results in the rapid restoration of carboxylase activity and an abrupt rise in viscosity, equivalent to the percentage of reactivation. Tight coupling of reversible loss of enzyme activity to depolymerization is further supported by the observation that Mg2f ATP and HCO1-, which carboxylate the enzyme, promote a rapid loss of catalytic activity (f; = 4 min), the formation of the avidinsensitive species, as well as, a rapid decrease in viscosity. The presence of carboxyl acceptor (acetyl-CoA), which retards activity decay (f+ = 71 min), and susceptibility to avi-