Abstract
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.
Highlights
Cyclic cyclic adenosine monophosphate (AMP)-dependent activation of hormonesensitive lipase (HSL) stimulated lipolysis of [3H]triolein emulsified with gum arabic, but not that of endogenous lipid droplets from rat fat cells
Heat-treated phospholipase C did not have any enhancing effect. These results suggest that phosphatidylcholine, especially its phosphate group, in the triolein emulsion inhibits the increase in the rate of lipolysis on activation of HSL
Heat-treated phospholipase C had no enhancing effect. These results suggest that phosphatidylcholine, especially its phosphate group, in the sonicated lipid droplets inhibits the increase in rate of lipolysis due to activation of HSL
Summary
Cyclic AMP-dependent activation of hormonesensitive lipase (HSL) stimulated lipolysis of [3H]triolein emulsified with gum arabic, but not that of endogenous lipid droplets from rat fat cells. Lipase activity is markedly influenced by the physical properties of the surface of the substrate [4].Tsujita, Muderkwa, and Brockman [4]prepared films of mixtures of 1,3-dioleoylglycerol and ~-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine at an argon-buffer interface and exposed these lipid films to various lipases, such as pancreatic carboxyester lipase and milk carboxyester lipase They found that the extent of hydrolysis of 1,3-dioleoylglycerolwas less than 5% up to a mole fraction of 0.5 and increased abruptly to 95% at a mole fraction of 0.6.
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