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Prof. Dr. Tarik Al-Khayat.

Fructose metabolism :1. The major source of fructose is sucrose, which when

cleared, release equimolar amounts of fructose &glucose.

2. Entry of glucose into the cell is insulin independent .3. Fructose fructokinase fructose -1- P.

fructose -1- P aldolase B Dihydroxyacetone phosphate + glyceraldehyde .

fructokinase and aldolase Bare found in the liver, kidney,and small intestinal mucosa.

4. A deficiency of fructokinase causes a benign condition,but of aldolase B causes a hereditary fructoseintolerance in which severe hypoglycemia and liverdamage can lead to death if the amount of fructose (andtherefore sucrose) in the diet are not severely limited.

5. Mannose, an important component of glycoproteins, isphosphorylated by hexokinase to mannose -6- P whichreversibly isomerized to fructose -6- P byphosphomannose isomerase.

6. Glucose can be reduced to sorbitol (glucitol) by aldosereductase in many tissues including lens, retina, Schwanncells, liver, kidney, ovaries, sperm and seminal vesicle.

Metabolism Of Mono- & Disaccharides

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A second enzyme (sorbital dehydrogenase )is present in thecells of the liver, ovaries and seminal vesicles which oxidizesorbitol to produce fructose. Hyperglycemia results inaccumulation of sorbitol in those cells lacking sorbitoldehydrogenase. The resulting osmotic events cause cellswelling and contribute to the cataract formation.

Galactose metabolism :1. The major dietary source of galactose is lactose. The

entry of galactose into cells is not insulin-dependent.2. Galactose is first phosphorylated by galactokinase, which

produces galactose -1- P.this compound is converted toUDP – galactose by galactose -1- P uridyl transferasewith the nucleolide supplied by UDP – glucose.

3. A deficiency of uridyl transferase causes classicgalactosemia. Galactose -1- P accumulates causingphosphate sequestration, and excess galactose isconverted to galactitol by aldose reductase. This causesliver damage, severe retardation and cataracts.Treatment requires removal of galactose ( and thereforelactose ) from the diet.

4. In order for UDP-galactose to enter the main stream ofglucose metabolism, it must first be converted to UDP-glucose, by UDP-hexose 4-epimerase. This enzyme can

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also be used to produce UDP-glucose when the former isrequired to the synthesis of structural carbohydrates.

1. galactose galactokinase galactose-1-P.

ATP ADP

2. galactose-1-P+UDP-glucose galactose-1-P uridyl transferase UDP-galactose +glucose-1-P.

3. UDP-galactose epimerase UDP-glucose.4. UDP-glucose glycogen G-1-P G-6-P Glycolysis.

Lactose Metabolism :1. Lactose is a disaccharide that consists of a molecule of

galactose and glucose. Milk and other dairy products arethe dietary sources of lactose.

2. Lactose is synthesized by lactose synthase from UDP-galactose and glucose in the lactating mammary gland.the enzyme has two subunits. Protein A ( which isgalactosyl transferase found in most cells where itsynthesizes N-acetyllactoseamine ). and protein B ( α-lactalbumin, which found only in lactating mammaryglands, and whose synthesis is stimulated by the peptidehormone, prolactin ). When both subunits are present, thetransferase produces lactose.

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Questions :Q1/ Explain how does fructose enter glycolysis ?Q2/ Define : hereditary fructose intolerance ?Q3/ How does mannose enter glycolytic pathway ?Q4/ Explain how does mannose enter glycolysis ?Q5/ How is fructose formed from glucose ?Q6/ What is the biochemical basis of cataract formation ?Q7/ How do galactose enter the glycolytic pathway ?Q8/ What is the cause of galactosemia ? What are the clinical signsof this disease ?Q9/ Explain in detail how is lactose synthesized ?

.أخرب تقله: وقال عليه السالم؛القلب مصحف البصر: ) عليه السالم(اإلمام عليقال

-------------------------------------------------------------------Printed by Mortadha A. Hadi.

17/12/2011 1:34:58 AM