ADRENALINE AND THE REGULATION OF ACETYL-COENZYME-A CARBOXYLASE IN RAT EPIDIDYMAL ADIPOSE-TISSUE - INACTIVATION OF THE ENZYME IS ASSOCIATED WITH PHOSPHORYLATION AND CAN BE REVERSED ON DEPHOSPHORYLATION

Exposure of rat epididymal fat-pads or isolated fat-cells to adrenaline results in a decrease in acetyl-CoA carboxylase activity measured both in initial extracts and in extracts incubated with potassium citrate; in addition the concentration of citrate required to give half-maximal activation may also be increased. Incorporation of 32P(i) into acetyl-CoA carboxylase within intact fat-cells was investigated and evidence is presented that adrenaline increases the extent of phosphorylation of the enzyme. Dephosphorylation of 32P-labelled acetyl-CoA carboxylase was studied in cell extracts. The rate of release of 32P is increased by 5 mM-MgCl2 and further enhanced by 5 mM-MgCl2 plus 10-100 μM-Ca2+, whereas it is inhibited by the presence of bivalent metal ion chelators such as EDTA and citrate. The effects of adrenaline on the kinetic properties of acetyl-CoA carboxylase disappear if pad or cell extracts are treated with Mg2+ and Ca2+ under conditions that also lead to dephosphorylation of the enzyme. The results of this study represent convincing evidence that adrenaline inactivates acetyl-CoA carboxylase in adipose-tissue preparations by increasing the degree of phosphorylation of the enzyme.