Oorsig/Review
removed via the production of ketone bodies.
directly to the liver but rather are first converted
in the muscle to the transport amino acids
alanine, glutamine and aspartate. They form an
important part of the substances contributing to
gluconeogenesis. The substances used to form
glucose through the gluconeogenic pathway (i.e.
propionate, lactate, glycerol alanine, aspartate
and glutamine) enter the energy pathways in
several positions.
Butyrate will also enter the ß oxidation pathway
and be transformed into triglycerides or ketone
bodies. Most of the butyrate will in fact already
be converted to β hydroxy butyrate by the rumen
epithelium as it is absorbed from the rumen.
Acetate and butyrate are thus ketogenic in nature.
As ruminants cannot rely on a dietary glucose
source of any magnitude, they need to produce
large amounts of newly formed glucose
(gluconeogenesis). This happens mainly in
the liver through the gluconeogenic pathway.
Skeletal muscle acts as an amino acid depot. Most
amino acids from muscle are not transported
The glucose formed through the gluconeogenic
pathway is mainly used to manufacture lactose
in lactating animals. Milk contains about 4.5
% lactose i.e. 50 kg of milk will contain 2.25 kg
of lactose, nearly all of which must come from
glucose provided by gluconeogenesis.
RUMEN
ADIPOSE TISSUE
MUSCLES
Proteins
Triglycerides
VFA's:
Hormone
Alanine
Glutamine
Aspartate
P ropionate
B utyrate
25%
Acetate
10%
65%
sensitive
Lipase
Glucose
Glycerol
Amino Acids
C3
Acetyl CoA (C2)
Oxaloacetate
(C4)
FFA
C6
Excreted as
lipoproteins
CO2
G lucose (C 6)
C5
LIVER
FFA
*
Stored as TAG
C4
CO2
Acetoacetyl
CoA
Aceto-
Acetate
B eta-O H -
Acetone
B utyrate
KETONE BODIES
Insulin
* This reaction does not take place in the liver,
but can take place in heart, muscle, kidney,
a.o. tissues in the presence of sufficient C4
Diagram illustrating the pathways of energy metabolism in lactating dairy cows.
26
BST
Prolactin
Placental lactogen