Effect of Oxaloacetate and Phosphorylation on ATP-Citrate Lyase Activity

ATP-citrate lyase (CL) catalyzes the conversion of citrate and CoA to oxaloacetate (OA) and acetyl-CoA. As the coupled malic dehydrogenase (MDH) assay is not able either to study the effect of oxaloacetate (OA) on CL activity or to measure accurately CL activity in biological samples, a new assay was developed. The CL-citrate coupled CAT assay measures the amount of acetyl-CoA formed by transferring radiolabeled acetyl-CoA synthesized from [¹⁴C]citrate to chloramphenicol with chloramphenicol acetyltransferase (CAT). Employing this assay, the rate of increase in acetyl-CoA synthesis from citrate is linear with respect to added CL. Kinetic values for ATP, CoA and citrate are similar to those obtained using the MDH assay. The effect of CL phosphorylation on enzyme activity was determined. CL phosphorylated by cAMP-dependent protein kinase or by this kinase and glycogen synthase kinase-3 (GSK-3) decreases the apparent V_max without changing the apparent K_m. The effect of OA, a product of the enzyme reaction, on CL activity was also determined. Computational analysis of the data obtained without added OA and at three concentrations of OA indicate that the apparent K_m for the substrate is not altered even though the apparent V_max is decreased. The effect of OA on the activity of phosphorylated enzyme was also determined. OA decreases the apparent V_max of the phosphorylated enzyme to the same extent as in control CL. This assay is able to measure CL activity in cytosol from 3T3-L1 adipocytes. The activity in cytosol was inhibited by OA at lower concentrations than found for purified enzyme: K_i 63 µM for CL activity in cytosol and 1.2 mM for purified CL. These results suggest that CL activity is affected both by OA and by phosphorylation.