The process of breaking down of lipids (fats) into fatty acids and glycerol
Lipolysis is the process of breaking down lipids. It entails hydrolysis whereby a triglyceride, for instance, is broken down into free fatty acids and glycerol. The process occurs mainly in the adipose tissues. It is used to mobilize the stored energy for use by cells.
Certain hormones (e.g. epinephrine, norepinephrine, testosterone, and cortisol) induce lipolysis. They activate certain receptors on the surface of adipocytes. This, in turn, activates the enzyme adenylate cyclase. This enzyme boosts cAMP levels, causing the activation of protein kinase A, which activates lipases. Lipase is an enzyme that speeds up the breakdown of lipids.
In particular, the epinephrine from the bloodstream binds to the beta-adrenergic receptors of the adipocyte. This causes the adipocyte to produce more cAMPs. cAMP, in turn, binds and activates protein kinase A. Activated protein kinase A activates in turn the lipases. The lipases in the adipocyte catalyze the breakdown of triglyceride into glycerol and fatty acid. Adipose triglyceride lipase, for instance, catalyzes the hydrolysis of triacylglycerol into diacylglycerol. The diacylglycerol is also hydrolyzed into monoacylglycerol through the aid of hormone-sensitive lipase. The hydrolysis of monoacyglycerol, in turn, is catalyzed by monoacylglycerol lipase. The resulting glycerol and the free fatty acids are then released out of the adipocyte into the bloodstream. The free fatty acids are then absorbed by cells, such as muscle cells. Free fatty acids in the bloodstream that are not immediately taken up bind to albumin. The glycerol is taken up by the liver for gluconeogenesis.
In the presence of high glucose level (such as from dietary carbohydrates), the level of insulin (released by pancreas) is increased. The resulting increase in the amount of insulin in the bloodstream leads to the slowing down of lipolysis. Insulin induces cascade of reactions that lead to the reduction of cAMP level. While insulin slows down lipolysis, it induces lipogenesis. This ensures that lipolysis and lipogenesis do not occur at the same time.
Word origin: Ancient Greek lípos (“animal fat”) + Ancient Greek lúsis (“a loosening”)