Studies on a rat-liver cell-sap protein yielding 3-[32P]-phosphohistidine after incubation with [32P]ATP and alkaline hydrolysis. Identification of the protein as ATP citrate lyase.

1. The rat-liver cell-sap material from which 3-[32P]phosphohistidine was previously isolated after incubation with [gamma-32P]ATP and alkaline hydrolysis, was shown to increase about 6-fold on a high-carbohydrate diet. This increase in 32P labelling corresponded to the increase in ATP citrate lyase activity of livers of rats fed on a high-carbohydrate diet, as reported by others. 2. ATP citrate lyase [ATP:citrate oxaloacetate-lyase (CoA-acetylating and ATP-dephopshorylating), EC 4.1.3.8] was purified from rat liver essentially according to the method of Plowman and Cleland (J. Biol. Chem., 242 (1967) 4239). The purified enzyme was incubated for a short time at 0 degree with [gamma-32P]ATP in the presence of 20 mM magnesium acetate. The phosphorylated protein was hydrolysed in alkali and the main part of the radioactivity was identified as 3-[32P]phosphohistidine. The identity of the phosphorylated amino acid was established by Dowex-1 chromatography, paper electrophoresis, paper chromatography and by analysis of the stability to acid. 3. It is concluded from these and previous results from this laboratory that ATP citrate lyase and nucleoside diphosphate kinase (ATP:nucleoside diphosphate phosphotransferase, EC 2.7.4.6) account for most of the normal rat-liver cell-sap protein which is rapidly phosphorylated by ATP.