Fat is stored in the body in adipose tissue. Storage of fat is not regulated, but release of fat from storage is controlled hormonally.

The principal enzyme in mobilizing stored fat, triacylglycerol lipase, is activated by phosphorylation in a cAMP-mediated kinase cascade that is stimulated by binding of glucagon or epinephrine or -corticotropin to a plasma membrane receptor (Figure 18.11).

Once activated, triacylglycerol lipase catalyzes the release of fatty acids from carbons #1 or #3 of the glycerol moiety of the fat. Diacylglycerol lipase and monoacylglycerol lipase catalyze the release of the other fatty acids.

The glycerol and fatty acid products exit the adipocyte by passive diffusion and move to the blood plasma where they are bound by albumin. Each albumin can bind to up to 10 molecules of free fatty acid. Fatty acids move from albumin to tissues largely by passive diffusion. Fatty acid uptake by cells is therefore driven by concentration. Glycerol is taken up by the liver for use in gluconeogenesis.