Regulation of Acetyl - coenzyme A Carboxylase 11 . EFFECT OF FASTING AND REFEEDING

Acetyl-coA carboxylase isolated from freezeclamped livers of fed rats has relatively low phosphate content (6.0 mol of PJmol of subunit) and high specific activity (3.6 unitslmg in the absence of citrate). The enzyme from rats fasted for 12, 18, 24, and 48 h exhibited decreasing specific activities of 2.75, 1.86, 1.7, and 0.9 unitslmg, respectively. Citrate activated all preparations of carboxylase, with most activation observed with the least active preparation. There was no significant change in the sensitivity of the enzyme to citrate since half-maximal activation was observed at 0.2 mM for carboxylase from fed as well as fasted rats. With the decrease in activity as a function of fasting, there was a concomitant increase in the phosphate content of carboxylase, with values of 5.3, 6.6, 6.7, and 7.6 mol of Pdmol of subunit obtained for preparations from rats fasted for 12, 18,24, and 48 h, respectively. Refeeding the fasted rats resulted in increased specific activity of carboxylase (3.4 unitslmg) and decreased phosphate content (6.1 mol of PJmol of subunit). Moreover, dephosphorylation by [acetyl-coA carboxylase]-phosphatase 2 activated the carboxylase from 48-h fasted rats to a value of 2.9 unitslmg, assayed in the absence of citrate, indicating that the low activity of carboxylase from fasted rats was due to its increased phosphate content. Superose 6 chromatography showed that the enzyme exists in two polymeric forms, a highly active polymer of 340 subunits and less active octamer. The former predominates in livers of fed rats, whereas the latter predominates in livers of fasted rats. The octamer could be converted to the highly active polymer by dephosphorylation. These observations indicate that fasting/refeedhg results in phosphorylation/dephosphorylation of acetyl-coA carboxylase with concomitant depolymerizationlpolymerization of the protein and ultimately decreasing or increasing its specific activity.