Rat mammary gland ATP‐citrate lyase is phosphorylated by cyclic AMP‐dependent protein kinase

Fatty acids and sterols are synthesized in the cytoplasm from acetyl-CoA, but acetyl-CoA is produced in the mitochondrion by the pyruvate dehydrogenase reaction. The carbon atoms of acetyl-CoA leave the mitochondrion as citrate, which is then reconverted to acetyl-CoA in the cytoplasm by the enzyme ATP citrate lyase [ 11. ATP citrate lyase is therefore a key lipogenic enzyme in non-ruminants, and this conclusion is supported by the finding that dietary manipulation changes its activity in parallel with other lipogenic enzymes, such as acetyl-CoA carboxylase and fatty acid synthetase [2]. The activity of hepatic ATP-citrate lyase is depressed on starvation, and is elevated by refeeding a fat-free diet, and there is evidence that these changes are regulated by glucagon and insulin [3]. While these long-term effects on activity over a period of days probably represent alterations in the amount of the ATP-citrate lyase protein, the possibility that a mechanism exists for the shortterm regulation of its activity cannot yet be excluded. Linn and Srere [4] reported that ATP-citrate lyase purified from rat liver contained 0.5 mol acid-stable phosphate/subunit, which raised the possibility that it is regulated by reversible phosphorylation. ATPcitrate lyase has now been shown to become labelled in vivo in rat liver after injection of 32Pi [4], and the incorporation of 32P-radioactivity into the enzyme in rat hepatocytes is stimulated by both insulin and glucagon [ 5,6]. Here we report that ATP-citrate lyase purified to homogeneity from lactating rat mammary gland can be phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase at a rate sufficient to account for the effect of glucagon on the phosphorylation of the enzyme in hepatocytes.