Insulin stimulates the dephosphorylation of phosphothreonine from fat-pad ATP-citrate lyase.

ATP-citrate lyase is phosphorylated in vivo at three amino acid residues on two peptide sequences (peptides a and b). Insulin action is known to increase the phosphorylation of peptide a. To study the effect of insulin on peptide b phosphorylation ATP-citrate lyase was radiolabeled in vivo by incubating fat pads with 32Pi. Following "cold chase", insulin action decreased the calculated specific radioactivity of peptide b to less than 30% of control whereas the specific radioactivity of peptide a increased 5-6 fold. The insulin induced decrease in peptide b phosphorylation was mainly due to a decrease in phosphothreonine phosphorylation. Isoproterenol treatment increased peptide a phosphorylation 4-6 fold but did not decrease peptide b phosphorylation. Specific radioactivity of ATP did not change significantly with hormone treatment. These results suggest that insulin action increases the dephosphorylation of peptide b by increasing the activity of a putative phosphothreonine phosphatase.