Polymer-protomer transition of acetyl-CoA carboxylase occurs in vivo and varies with nutritional conditions.

The protomeric form of purified acetyl coenzyme A carboxylase is inactivated by the binding of avidin to the biotinyl prosthetic group; the catalytically active filamentous form of the enzyme is resistant to avidin. This differential sensitivity to avidin was used to examine the influence of nutritional state on the proportion of polymeric and protomeric carboxylase occurring in avian liver. Hepatic carboxylase was 80% avidin-resistant (polymeric) in the fed chick. Food deprivation for 2 and 6 h reduced the avidin resistance to 54% and 30%, respectively. Similarly, within 1 h after fat intubation, the fraction of polymeric carboxylase had significantly decreased. Accompanying the change in carboxylase transformation was a comparable reduction in 3H2O incorporation into liver fatty acid. These data indicate that the protomer-polymer transition defined for purified acetyl-CoA carboxylase also occurs with the enzyme in vivo and that a lower polymer/protomer ratio is associated with reduced rates of fatty acid synthesis.