Effect of substrates on the cyclic AMP-dependent lipolytic reaction of hormone-sensitive lipase.

Cyclic AMP-dependent activation of hormone-sensitive lipase (HSL) stimulated lipolysis of [3H]triolein emulsified with gum arabic, but not that of endogenous lipid droplets from rat fat cells. The absence of responsiveness of the lipid droplets to activation of HSL was found to be caused by some factor other than their surface area. The activated HSL showed a higher rate of lipolysis than nonactivated HSL on lipid droplets sonicated with gum arabic. Addition of phosphatidylcholine, which is one of the minor components of intact lipid droplets, to triolein or the sonicated lipid droplet emulsion induced loss of responsiveness to activated HSL, and treatment of these substrates containing phosphatidylcholine with phospholipase C restored the responsiveness. These results suggest that loss of responsiveness of the endogenous lipid droplets in fat cells to activated HSL may be due to phosphatidylcholine in the lipid droplets.