Mechanism of the hormone-sensitive lipase activation

Hormone-sensitive lipase in adipocytes
is an
intracellular lipase
that through
hydrolysis of triacylglycerols
mobilizes the fat energy reserves
stored in adipose tissue.
The activity of this lipase is controlled by hormones:.
Glucagon
(at low blood glucose) and
adrenaline/noradrenaline
(in stress) cause an increase in lipase activity by its phosphorylation.
Both free fatty acids and glycerol are released into the blood.
Fatty acids
are taken up promptly from the blood plasma by tissues that require
nutrients (fatty acids are transported bound to albumin).
Glycerol
cannot be utilized in adipocytes (they are lacking in glycerol kinase) and
serves as the substrate for gluconeogenesis in the liver:
Fatty acids serve as an
energy source
for most of the cells
(not for the nervous system and for red blood cells).
The tissues gain fatty acids
- either from lipoprotein particles after the triacylglycerols have been hydrolysed
by lipoprotein lipase,
- or as fatty acids mobilized by the action of hormones on the fat stores in
adipose tissue and supplied bound onto albumin.
The utilization of fatty acids in the cells requires
three stages of processing
1
Activation by linking to coenzyme A
,
2
transport of acyl CoA into the mitochondrial matrix
by conjugating it to carnitine,
3
?-oxidation of
acyl CoA
in the mitochondrial matrix to
acetyl CoA that enters the citrate cycle.

Insulin
exhibits opposite effect
: it support dephosphorylation of the
lipase in adipocytes and initiates the
synthesis of triacylglycerols.