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 sam ples, a new assay was developed. The CL-citrate coupled CAT assay meas ures the amount of acetyl-CoA formed by transferring radiolabeled acet yl-CoA synthesized from [C-14]citrate to chloramphenicol with chloramp henicol acetyltransferase (CAT). Employing this assay, the rate of inc rease in acetyl-CoA synthesis from citrate is linear with respect to a dded CL. Kinetic values for ATP, CoA and citrate are similar to those obtained using the MDH assay. The effect of CL phosphorylation on enzy me activity was determined. CL phosphorylated by cAMP-dependent protei n kinase or by this kinase and glycogen synthase kinase-3 (GSK-3) decr eases 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 dete rmined. Computational analysis of the data obtained without added OA a nd at three concentrations of OA indicate that the apparent K-m for th e substrate is not altered even though the apparent V-max is decreased . The effect of OA on the activity of phosphorylated enzyme was also d etermined. OA decreases the apparent V-max of the phosphorylated enzym e to the same extent as in control CL. This assay is able to measure C L activity in cytosol from 3T3-L1 adipocytes. The activity in cytosol was inhibited by OA at lower concentrations than found for purified en zyme: K-i, 63 mu M 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.