Glucagon activates mitochondrial 3‐hydroxy‐3‐methylglutaryl‐CoA synthase in vivo by decreasing the extent of succinylation of the enzyme

Abstract

1. 3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase (EC 4.1.3.5) in extracts of rat liver mitochondria can be inactivated by succinyl-CoA and activated by incubation in a medium designed to cause desuccinylation ('desuccinylation medium'). 2. The enzyme is less active in extracts of whole liver from control rats than from rats treated with glucagon or mannoheptulose. Incubation in desuccinylation medium raises the activity in extracts from control rats to the same value as treated rats, suggesting that the extent of succinylation in vivo is greater in controls than in hormone-treated animals. 3. This result is also obtained in liver homogenates and in isolated mitochondria. 4. Increasing the succinyl-CoA content of mitochondria to the same high level lowers the enzyme activity to the same value in mitochondria isolated from control or treated rats. In each case subsequent incubation of the lysates in desuccinylation medium raises the enzyme activity by the same extent. 5. Measurement of the incorporation of radiolabel from 2-o~o[S-'~C]glutarate into protein is consistent with the proposal that all these changes in activity in isolated mitochondria may be explained by changes in the extent of succinylation of the enzyme. 6. From these data and our earlier work we conclude that, in vivo, mitochondrial HMG-CoA synthase in fed rats is normally substantially succinylated (about 40%) and inactivated, and that glucagon increases the activity of HMG-CoA synthase by lowering the concentration of succinyl-CoA and thus decreasing the extent of succinylation of the enzyme (to less than 30%). This may be an important control mechanism in ketogenesis.The control of ketogenesis in liver is not fully understood [I -31 but it is clear that the production of ketone bodies can be stimulated by treatment of animals with glucagon [4-91. Although there is good evidence that some control of ketogenic flux is exerted at carnitine palmitoyltransferase I [3, 9 -121, there is also strong evidence that under certain conditions control shifts to an unidentified intramitochondrial control site [13-201. On the basis of experiments in vitro with mitochondrial HMG-CoA synthase purified from ox liver, Lowe and Tubbs [21] proposed that glucagon might increase ketogenic flux through the HMG-CoA pathway by lowering the succinylCoA concentration and therefore the succinylation state of HMG-CoA synthase.In support of this hypothesis we have observed increased HMG-CoA synthase activity and decreased intramitochondrial succinyl-CoA levels in extracts prepared from rapidly frozen livers of rats after short-term treatment with glucagon or mannoheptulose. These effects persisted in isolated mitochondria, validating their use as a model system in which to examine the effects of hormones on HMG-CoA synthase. We also showed that succinyl-CoA inhibited HMG-CoA synthase activity in isolated mitochondria and that the differences in the activity of the enzyme in mitochondria isolated from glucagon-treated or mannoheptulose-treated rats and shamCorrespondence ...

Ketogenic HMGCS2 Is a c-Myc Target Gene Expressed in Differentiated Cells of Human Colonic Epithelium and Down-Regulated in Colon Cancer

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Gene expression of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase in a poorly ketogenic mammal: effect of starvation during the neonatal period of the piglet

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