Effect of ketone bodies on lipolysis in adipose tissue in vitro.

Norepinephrine-sensitive lipase activity was measured in rat epididymal fat pads by determining release either of free fatty acids or of glycerol. Stimulation of the lipase activity by norepinephrine in vitro could not be duplicated by injecting norepinephrine into the rats before sacrifice. A reliable method for assay of lipase deactivation rate was developed in which the tissue is incubated for 80 min, norepinephrine is added for a further incubation of 10 min, and the decay of lipase activity is measured during the next 10 min in the absence of hormone. Of the ketone bodies tested, -hydroxybutyrate and probably acetoacetate inhibited the activation of lipase by norepinephrine but had no effect on lipase deactivation rate, whereas acetone increased lipase activity stimulated by norepinephrine when tested at the concentration at which acetoacetate gave an inhibition. Substances other than -hydroxybutyrate that produce reduced nucleotides-alpha-glycerophosphate, malate, and ethanol-had no effect on lipase activity as tested in the present system.