PHOSPHORYLATION OF RAT MUSCLE ACETYL-COA CARBOXYLASE BY AMP-ACTIVATEDPROTEIN-KINASE AND PROTEIN-KINASE-A

This study was designed to compare functional effects of phosphorylati on of muscle acetyl-CoA carboxylase (ACC) by adenosine 3',5'-cyclic mo nophosphate-dependent protein kinase (PKA) and by AMP-activated protei n kinase (AMPK). Muscle ACC (272 kDa) was phosphorylated and then subj ected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis fol lowed by autoradiography. Functional effects of phosphorylation were d etermined by measuring ACC activity at different concentrations of eac h of the substrates and of citrate, an activator of the enzyme. The ma ximal velocity (V-max) and the Michaelis constants (K-m) for ATP, acet yl-CoA, and bicarbonate were unaffected by phosphorylation by PKA. Pho sphorylation by AMPK increased the K-m for ATP and acetyl-CoA. Sequent ial phosphorylation by PKA and AMPK, first without label and second wi th label, appeared to reduce the extent of label incorporation, regard less of the order. The activation constant (K-a) for citrate activatio n was increased to the same extent by AMPK phosphorylation, regardless of previous or subsequent phosphorylation by PKA. Thus muscle ACC can be phosphorylated by PKA but with no apparent functional effects on t he enzyme. AMPK appears to be the more important regulator of muscle A CC.