Guanosine triphosphate and colchicine affect the activity and the polymeric state of acetyl-CoA carboxylase

Acetyl-CoA carboxylase was purified 300-fold from rat liver, in the absence of added citrate, by precipitation from an 18,000g supernatant in the presence of Triton X-100 at 105,000g and 20 °C, followed by chromatography on phosphocellulose. Acetyl-CoA carboxylase activity in this preparation was activated by preincubation with GTP (0.1–2.0 mm) and with citrate (20 mm). Colchicine (10^?6–10^?3m) inhibited enzyme activity and counteracted the effects of GTP and citrate. Sucrose density gradient centrifugation demonstrated that GTP and citrate preincubation promoted the formation of the polymeric, active enzyme, while colchicine engendered disassembly. Preincubation of the purified acetyl-CoA carboxylase at 4 °C caused inactivation and disassembly, which was countered by preincubation at 37 °C in the presence of GTP or citrate. These results suggest that GTP, like citrate, activates acetyl-CoA carboxylase by enhancing the conversion of the protomeric form of the enzyme to its more active, polymeric state.