Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 3 Carbohydrate Metabolism and Its Diseases
monosaccharides by the action of amylase and maltase.
This activity ceases as the food matter passes into the
stomach, where the enzymatic action is destroyed by the
hydrochloric acid. Within the stomach, acid hydrolysis
may occur, but the stomach empties too rapidly for complete
hydrolysis to take place. Thus, only a small portion
of the ingested carbohydrate is hydrolyzed before passing
into the small intestine. In the small intestine, digestion
of carbohydrate takes place quickly by the carbohydrate
splitting enzymes contained in the pancreatic juice and in
the succus entericus. Starch and glycogen are hydrolyzed
to glucose by amylase and maltase; lactose to glucose and
galactose by lactase; and sucrose to glucose and fructose
by sucrase. The monosaccharide products of enzymatic
hydrolysis of carbohydrates, glucose, fructose, and galactose,
are the principal forms in which absorption occurs in
the monogastric animal.
III . ABSORPTION
The monosaccharides are almost completely absorbed
through the mucosa of the small intestine and appear in the
portal circulation as the free sugars. Absorption into the
mucosal cell occurs by a sodium-dependent active transport
mechanism using a glucose co-transporter. The absorbed
glucose then leaves the mucosal cell by a facilitated diffusion
process in the presence of a glucose transporter, GLUT-2.
Glucose and galactose are absorbed rapidly, whereas fructose
is absorbed at about half the rate of glucose with a
portion being converted to glucose in the process. Other
monosaccharides (e.g., mannose) are absorbed slowly
at a rate consistent with a diffusion process. As a result,
free sugars appear in the portal circulation for transport
to the liver.
Glucose transporters (GLUT) are known to be involved
in many tissues—for example, GLUT-1 is the hepatocyte/
RBC transporter and is widely distributed; GLUT-2 is in
hepatocytes, β cells, and mucosal cells; GLUT-3 is the brain
transporter; GLUT-4 is the only insulin-responsive glucose
transporter; GLUT-5 is in the intestine; GLUT-6 is not available;
and GLUT-7 is within cell organelles ( Winter and
Signorino, 2002 ). Glucose transporters have been identified
and numbered beyond GLUT-8 and up to GLUT-12 ( Piroli
et al ., 2004 ) but remain to be confirmed.
IV . METABOLISM OF ABSORBED
CARBOHYDRATE S
A . General
Liver cells are readily permeable to glucose. This process is
facilitated by glucose transporter (GLUT) proteins within the
plasma membrane, in particular, GLUT-2 is the transporter
in the liver cell plasma membrane ( Thorens et al ., 1988 ).
FIGURE 3-1 Pathways for hexose metabolism. Abbreviations: ATP,
adenosine triphosphate; UTP, uridine triphosphate; UDP-G, uridine
diphosphate glucose; DHAP, dihydroxy acetone phosphate; GA-3-P,
glyceraldehyde-3-phosphate.
Within the liver, there are several pathways by which the
immediate fate of the hexoses is determined. Glucose, fructose,
and galactose first enter the general metabolic scheme
through a series of complex reactions to form glucose
phosphates ( Fig. 3-1 ). The enzyme, galactose-1-P uridyl
transferase, which catalyzes the reaction
galactose-1-P
UDP-glucose →
UDP-galactose glucose-1-P
is blocked or deficient in congenital galactosemia of
humans. The glucose phosphates are then converted to
and stored as glycogen, catabolized to CO 2 , and water
or, as free glucose, returned to the general circulation.
Essentially, intermediate carbohydrate metabolism of animals
evolves about the metabolism of glucose, and the
liver is the organ of prime importance.
B . Storage as Glycogen
Glycogen is the chief storage form of carbohydrate in animals
and is analogous to the storage of starch by plants. It
is found primarily in liver and in muscle, where it occurs at
about 3% to 6% and about 0.5%, respectively ( Table 3-1 ).
Glycogen is composed solely of α -D-glucose units linked
together through carbon atoms 1 and 4 or 1 and 6. Straight
chains of glucose units are formed by the 1–4 links and
these are cross-linked by the 1–6 links. The result is a complex
ramification of chains of glucosyl units with branch