Page 321 - Color Atlas of Biochemistry
P. 321
312 Tissues and organs
Lipid metabolism B. Biosynthesis of ketone bodies
At high concentrations of acetyl-CoA in the
The liver is the most important site for the
formation of fatty acids, fats (triacylglycerols), liver mitochondria, two molecules condense
to form acetoacetyl CoA [1]. The transfer of
ketone bodies, and cholesterol. Most of these
products are released into the blood. In con- another acetyl group [2] gives rise to
3-hydroxy-3-methylglutaryl-CoA (HMG CoA),
trast, the triacylglycerols synthesized in adi-
pose tissue are also stored there. which after release of acetyl CoA [3] yields
free acetoacetate (Lynen cycle). Acetoacetate
can be converted to 3-hydroxybutyrate by
A. Lipid metabolism reduction [4], or can pass into acetone by
nonenzymatic decarboxylation [5]. These
Lipid metabolism in the liver is closely linked three compounds are together referred to as
to the carbohydrate and amino acid metabo- “ketone bodies,” although in fact 3-hydroxy-
lism. When there is a good supply of nutrients butyrate is not actually a ketone. As reaction
in the resorptive (wellfed) state (see p. 308), +
the liver converts glucose via acetyl CoA into [3] releases an H ion, metabolic acidosis can
occur as a result of increased ketone body
fatty acids.The livercan also take up fatty
acids from chylomicrons, which are supplied synthesis (see p. 288).
Theketonebodies are released by theliver
by the intestine, or from fatty acid–albumin into the blood, in which they are easily solu-
complexes (see p. 162). Fatty acids from both
ble. Blood levels of ketone bodies therefore
sources are converted into fats and phospho- rise during periods of hunger. Together with
lipids. Together with apoproteins, they are
packed into very-low-density lipoproteins free fatty acids, 3-hydroxybutyrate and ace-
toacetate are then the most important energy
(VLDLs; see p. 278) and then released into suppliers in many tissues (including heart
the blood by exocytosis. The VLDLs supply
extrahepatic tissue, particularly adipose tis- muscle). Acetone cannot be metabolized and
is exhaled via the lungs or excreted with
sue and muscle.
In the postresorptive state (see p. 292)— urine.
particularly during fasting and starva- To channel ketone bodies into the energy
tion—the lipid metabolism is readjusted and metabolism, acetoacetate is converted with
the organism falls back on its own reserves. In the help of succinyl CoA into succinic acid
and acetoacetyl CoA, whichis brokendown
theseconditions, adiposetissuereleases fatty
acids. They are taken up by the liver and are by β-oxidation into acetyl CoA (not shown;
see p.180).
mainly converted into ketone bodies (B). If the production of ketone bodies exceeds
Cholesterol can be derived from two sour-
ces—food or endogenous synthesis from ace- the demand for them outside the liver, there
is an increase in the concentration of ketone
tyl-CoA. A substantial percentage of endo-
genous cholesterol synthesis takes place in bodies in the plasma (ketonemia)and they are
also eventually excreted in the urine (ketonu-
the liver. Some cholesterol is required for ria). Both phenomena are observed after pro-
the synthesis of bile acids (see p. 314). In ad-
dition, it serves as a building block for cell longed starvation and in inadequately treated
diabetes mellitus. Severe ketonuria with ke-
membranes (see p. 216), or can be esterified
with fatty acids and stored in lipid droplets. toacidosis can cause electrolyte shifts and
loss of consciousness, and is therefore life-
The rest is released together into the blood in threatening (ketoacidotic coma).
the form of lipoprotein complexes (VLDLs)
and supplies other tissues. The liver also con-
tributes to the cholesterol metabolism by tak-
ing up from the blood and breaking down
lipoproteins that contain cholesterol and cho-
lesterol esters (HDLs, IDLs, LDLs; see p. 278).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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