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338 Tissues and organs
Muscle metabolism II B. Protein and amino acid metabolism
The skeletal muscle is the most important site
A. Cori and alanine cycle for degradation of the branched-chain amino
acids (Val, Leu, Ile; see p. 414), but other amino
Whitemusclefibers(see p. 336) mainlyob-
tain ATP from anaerobic glycolysis—i. e., they acids are also broken down in the muscles.
Alanine and glutamine are resynthesized
convert glucose into lactate. The lactate aris- from the components and released into the
ing in muscle and, in smaller quantities, its blood. They transport the nitrogen that arises
precursor pyruvate are released into the during amino acid breakdown to the liver
blood and transported to the liver, where lac-
tate and pyruvate are resynthesized into glu- (alanine cycle; see above) and to the kidneys
(see p. 328).
cose again via gluconeogenesis, with ATP being During periods of hunger, muscle proteins
consumed in the process (see p. 154). The serve as an energy reserve for the body. They
glucose newly formed by the liver returns are broken down into amino acids, which are
via the blood to the muscles, where it can be
used as an energy source again. This circula- transported to the liver. In the liver, the car-
bon skeletons of the amino acids are con-
tion system is called the Cori cycle,after the
researchers who first discovered it. There is verted into intermediates in the tricarboxylic
also a very similar cycle for erythrocytes, acid cycle or into acetoacetyl-CoA (see p. 175).
These amphibolic metabolites are then avail-
which do not have mitochondria and there-
fore produce ATP by anaerobic glycolysis (see able to the energy metabolism and for gluco-
neogenesis. After prolonged starvation, the
p. 284). brain switches to using ketone bodies in order
The muscles themselves are not capable of to save muscle protein (see p. 356).
gluconeogenesis. Nor would this be useful, as The synthesis and degradation of muscle
gluconeogenesis requires much more ATP
than is supplied by glycolysis. As O 2 deficien- proteins are regulated by hormones. Cortisol
cies do not arise in the liver even during in- leads to muscle degradation, while testo-
tensive musclework, thereis always suf - sterone stimulates protein formation. Syn-
thetic anabolics with a testosterone-like ef-
cient energy there available for gluconeogen- fect have repeatedly been used for doping
esis.
There is also a corresponding circulation purposes or for intensive muscle-building.
system for the amino acid alanine. The alanine
cycle in the liver not only provides alanine as Further information
a precursor for gluconeogenesis, but also
transports to the liver the amino nitrogen Smooth muscle differs from skeletal muscle in
arising in muscles during protein degrada- various ways. Smooth muscles—which are
tion. In the liver, it is incorporated into urea found, for example, in blood vessel walls
for excretion. and in the walls of the intestines—do not
Most of the amino acids that arise in contain any muscle fibers. In smooth-muscle
muscle during proteolysis are converted into cells, which are usually spindle-shaped, the
glutamate and 2-oxo acids by transamination contractile proteins are arranged in a less reg-
(not shown; cf. p.180). Again by transamina- ular patternthaninstriated muscle. Contrac-
tion, glutamate andpyruvategiveriseto ala- tion in this type of muscle is usually not
nine, which after glutamine is the second im- stimulated by nerve impulses, but occurs in
portant form of transport for amino nitrogen alargely spontaneousway. Ca 2+ (in the form
2+
in the blood. In the liver, alanine and 2-oxo- of Ca -calmodulin; see p. 386) also activates
glutarate are resynthesized into pyruvate and contraction in smooth muscle; in this case,
glutamate(seep. 178). Glutamatesupplies however, it does not affect troponin, but acti-
the urea cycle (see p. 182), while pyruvate is vates a protein kinase that phosphorylates the
available for gluconeogenesis. light chains in myosin and thereby increases
myosin’s ATPase activity. Hormones such as
epinephrine and angiotensin II (see p. 330)
are able to influence vascular tonicity in this
way, for example.
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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