This document summarizes the metabolism of fructose and galactose. It explains that fructose is metabolized differently in the liver versus muscle, and outlines the key steps in each tissue. It also discusses hereditary defects that can occur in fructose metabolism, including essential fructosuria and hereditary fructose intolerance. The document then summarizes galactose metabolism, noting it is converted to glucose-1-phosphate in the liver through a series of enzymatic steps before entering glycolysis. Finally, it discusses galactosemias which can result from inherited defects in galactose metabolism.
Under normal dietary intake the majority of the ingested fructose is metabolized by the enterocytes of the small intestine primarily to glucose which is then delivered to the systemic circulation. In addition to glucose, the carbon atoms from dietary fructose are converted, by intestinal enterocytes, into several other metabolites including glycerate, glutamate, glutamine, alanine, ornithine, and citrulline.
However, diets containing large amounts of sucrose, high fructose corn syrup, or fructose alone, overwhelm the ability of the small intestine to metabolize it all and under these conditions a significant amount of fructose is then metabolized by the liver and to a lesser extent by other organs such as skeletal muscle.
Under normal dietary intake the majority of the ingested fructose is metabolized by the enterocytes of the small intestine primarily to glucose which is then delivered to the systemic circulation. In addition to glucose, the carbon atoms from dietary fructose are converted, by intestinal enterocytes, into several other metabolites including glycerate, glutamate, glutamine, alanine, ornithine, and citrulline.
However, diets containing large amounts of sucrose, high fructose corn syrup, or fructose alone, overwhelm the ability of the small intestine to metabolize it all and under these conditions a significant amount of fructose is then metabolized by the liver and to a lesser extent by other organs such as skeletal muscle.
Carbohydrates are the sugars, starches and fibers found in fruits, grains, vegetables and milk products. Though often maligned in trendy diets, carbohydrates — one of the basic food groups — are important to a healthy diet.
Carbohydrates are the sugars, starches and fibers found in fruits, grains, vegetables and milk products. Though often maligned in trendy diets, carbohydrates — one of the basic food groups — are important to a healthy diet.
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3. Fructose metabolism
Diets containing large amounts of sucrose (a
disaccharide of glucose and fructose) can utilize
the fructose as a major source of energy.
Fructose is found in foods containing sucrose
(fruits), high-fructose corn syrups, and honey.
The pathway to utilization of fructose differs in
muscle and liver.
4. In liver, dietary fructose is converted to Fructose-1-P by
fructokinase (also in kidney and intestine).
Then, by the action of Fructose-1-P adolase (aldolase B),
Fructose-1-P is converted to DHAP and glyceraldehyde.
Glyceraldehyde is conveted to glyceraldehyde-3-P by
triose kinase which together with DHAP may undergo:
1. Combine together and converted into glucose (main
pathway)
2. They may be oxidized in glycolysis
Fructose metabolism- Liver
5. The utilization of fructose
by fructokinase then
aldolase bypass the steps of
glucokinase and PFK-1
which are activated by
insulin
This explains why
fructose disappears
from blood more
rapidly than glucose in
diabetic subjects
Continue…
6. Continue…
The affinity of aldolase B
for fructose-1-P is much
poorer than that of fructose-
1,6-biphosphate, thus
fructose-1-P accumulates in
fructokinase-expressing
tissues.
Thus, aldolase B is the rate-
limiting enzyme for fructose
metabolism (not glucose).
7. Fructose metabolism- Muscle
Muscle which contains only hexokinase can
phosphorylate fructose to F6P which is a direct
glycolytic intermediate
However, hexokinase has a very low affinity to fructose
compared to glucose, So it is not a significant pathway
for fructose metabolism. Unless it is present in very high
concentration in blood.
9. In the testes (seminal vesicles), fructose is converted into
glucose through sorbitol formation by aldolase reductase and
sorbitol dehydrogenase.
Deficiency of fructose correlates with male infertility.
Continue…
10. Hereditary defects of fructose
metabolism
1. Essential fructosuria
• Cause: due to deficiency of fructokinase enzyme
• Effect: not serious condition. The excess accumulated
fructose is lost in urine
2. Fructose 1,6 biphosphatase deficiency
It leads to accumulation of fructose 1,6 biphosphate which
inhibits phosphorylase enzyme (glycogenolysis) and fasting
hypoglycemia
11. 3. Hereditary fructose intolerance
• Cause: due to deficiency of aldolase B. This leads to
accumulation of fructose-1-P
• Effect: the accumulation of fructose-1-P leads to :
Damage of liver and kidney tissues.
Inhibition of glycogen phosphorylase leading to
inhibition of glycogenolysis and fasting hypoglycemia
Continue…
12. Galactose Metabolism
The major source of galactose is lactose (a disaccharide of
glucose and galactose) obtained from milk and milk
products.
Galactose enters glycolysis by its conversion to glucose-1-
phosphate (G1P). This occurs through a series of steps.
Site: liver
13. Galactose metabolism
First the galactose is phosphorylated by
galactokinase to yield galactose-1-p.
Epimerization of galactose-1-phosphate
to G-1-P requires the transfer of UDP
from uridine diphosphoglucose (UDP-
glucose) catalyzed by galactose-1-
phosphate uridyl transferase.
This generates UDP-galactose and G-1-P.
14. Galactose metabolism
The UDP-galactose is
epimerized to UDP-glucose by
UDP-galactose-4 epimerase.
The UDP portion is exchanged
for phosphate generating glucose-
1-phosphate which then is
converted to G6P by
phosphoglucose mutase.
Glucose can be converted to
galactose, thus performed
galactose is not essential in the
diet.
15. Galactosemias
• Definition: it is increase blood galactose concentration due to
inability of the body to metabolize galactose
• Causes: inherited defects in galactokinase, uridyl-transferase
(the most common) or 4-epimerase.
• Effect:
1. Cataract (opacity of eye lens): Galactose is reduced in
the eye by aldose reductase to form galactitol which
accumulates causing cataract
2. Liver failure
3. Mental retardation
4. Galactosuria (excretion of galactose in the urine)