Fatty acid-mediated disaggregation of acetyl-CoA carboxylase in isolated liver cells

In recent years the rapid regulation of acetyl-CoA (AcCoA) carboxylase (EC 6.4.1.2) has become of major interest because of the important role of malonyl-CoA in fatty acid synthesis, ketogenesis, and triglyceride production. AcCoA carboxylase is acutely regulated by two mechanisms: 1) phosphorylation-dephosphorylation and 2) polymer-protomer transition. Until recently polymer-protomer transition of AcCoA carboxylase in vivo has escaped detection. We developed a technique that estimates the intracellular proportion of polymer and protomer forms of AcCoA carboxylase based on the differential sensitivity of polymeric and protomeric AcCoA carboxylase to avidin inactivation. When the enzyme is in its highly aggregated conformation, the biotin prosthetic group of AcCoA carboxylase is protected from avidin binding. Thus the polymeric AcCoA carboxylase is more resistant than the protomeric conformation to avidin inactivation. Utilizing this technique with isolated liver cells we have been able to develop a model for the involvement of free fatty acids and glucagon in regulating polymer-protomer transition of AcCoA carboxylase, and the role of polymer as an intracellular determinant of AcCoA carboxylase activity. Our data suggest that the physiological regulation of AcCoA carboxylase involves the interaction of the phosphorylation mechanism with fatty acid-induced depolymerization. We propose that during periods of food deprivation the elevation in fatty acid-CoA esters promotes depolymerization of AcCoA carboxylase. In addition, glucagon induces phosphorylation of AcCoA carboxylase, which inhibits the enzyme's activity and facilitates acyl-CoA binding and depolymerization. The two separate mechanisms for regulating hepatic AcCoA carboxylase may work in concert to modulate the level of the regulatory metabolite malonyl-CoA.