L12 - Metabolism of Fructose and Galactose

Question 1

How does Benedict’s test or Clinitest work?

Answer 1

Conversion of cupric ions to cuprous ions through reduction

Question 2

Which sugars are the main reducing sugars? Which are not?

Answer 2

Galactose, fructose, and glucose are reducing sugars. Sucrose is not.

Question 3

How does ingestion of sorbitol produce fructose?

Answer 3

Aldose reductase to glucose and then sorbitol DHG

Question 4

Which GLUT protein absorbs dietary fructose?

Answer 4

GLUT-5

Question 5

Where is the majority of dietary fructose metabolized?

Answer 5

Liver

Question 6

Is fructose uptake insulin independent or dependent?

Answer 6

Independent

Question 7

Which two fructose metabolic enzymes are abundant in the liver?

Answer 7

Aldolase B and fructokinase

Question 8

Aldolase B is used in both fructose metabolism and glycolysis. Why does it preferentially participate in glycolysis?

Answer 8

Aldolase B has a higher affinity for F1,6BP than it does for F1P.

Question 9

Fructose metabolism forms glyceraldehyde 3-P and DHAP. What are the two steps?

Answer 9

1) Fructose to F1P by fructokinase

2) F1P to DHAP or glyceraldehyde 3P by aldolase B

Question 10

What is the clinical difference between essential fructosuria and hereditary fructose intolerance?

Answer 10

Essential fructosuria is generally asymptomatic, whereas hereditary fructose intolerance has episodes of hypoglycemia

Question 11

What is the cause of benign or essential fructosuria? Why are patients asymptomatic?

Answer 11

Deficiency of fructokinase in the liver. There are no toxic metabolites that accumulate

Question 12

What is the cause of hereditary fructose intolerance?

Answer 12

Aldolase B deficiency in the liver.

Question 13

When does the hypoglycemia occur that is a symptom of hereditary fructose intolerance? Why

Answer 13

1) occurs 4-5 hours AFTER consumption of fructose
2) occurs due to trapping of F1P/inorganic phosphate which leads to ATP deficiency and inhibition of glycogenolysis and gluconeogenesis

Question 14

What three sugars should not be eaten when a patient has hereditary fructose intolerance? What are the consequences of non-compliance?

Answer 14

1) sucrose, fructose, and sorbitol

2) hepatocellular failure leading to jaundice, hepatomegaly

Question 15

Glucose can be converted to fructose through which metabolite? What are the enzymes and reactions?

Answer 15

1) sorbitol

2) Glucose to sorbitol via aldose reductase; sorbitol to fructose via sorbitol DHG

Question 16

Where is the polyol pathway mostly seen in the human body?

Answer 16

Seminal vesicles

Question 17

What is the cause of the microvascular complications of diabetes mellitus? How does this happen?

Answer 17

Sorbitol formation in the tissues from uncontrolled hyperglycemia. Leads to presence of sorbitol which is osmotically active

Question 18

The polyol pathway is also implicated in the pathogenesis of galactosemia, how?

Answer 18

Galactose is converted to galactitol in the liver by aldose reductase. The osmotic activity of galactitol in the lens causes cataracts

Question 19

What is the classic triad of galactosemia?

Answer 19

1) liver damage
2) development delay
3) cataracts

Question 20

What are the three most common conditions where sugars are found in the urine?

Answer 20

1) diabetes mellitus
2) essential fructosuria or hereditary fructose intolerance
3) galactosemia

Question 21

What glycosidic linkage is present in lactose?

Answer 21

beta 1-4

Question 22

Which transporter absorbs galactose in the small intestine?

Answer 22

SGLT-1

Question 23

Where is dietary galactose mainly metabolized?

Answer 23

liver

Question 24

What are three major enzymes involved in galactose metabolism?

Answer 24

1) galactokinase
2) galctose 1-phosphate uridylyl transferase (GALT)
3) epimerase

Question 25

What enzyme deficiency occurs in nonclassical galactosemia?

Answer 25

galactokinase

Question 26

What enzyme deficiency occurs in classic galactosemia?

Answer 26

GALT

Question 27

What are the substrates for GALT and what are the products?

Answer 27

Substrates: UDP-glucose and galactose 1-phosphate
Products: UDP-galactose and glucose 1-phosphate

Question 28

What is the starting material for glycogen synthesis?

Answer 28

UDP-glucose

Question 29

What type of disorder is classical galactosemia?

Answer 29

autosomal recessive

Question 30

What causes the hypoglycemia found in classical galactosemia?

Answer 30

Phosphate trapping of galactose 1-phosphate in the liver causing inhibition of GNG and glycogenolysis

Question 31

What are the toxic metabolites in classical galactosemia?

Answer 31

galactitol and galactose 1-phosphate

Question 32

What is the cause of cataracts in classical galactosemia?

Answer 32

Accumulation of galactose in the lens, results in galactitol formation by aldose reductase causing osmotic imbalance and increase in water content

Question 33

How is galactosemia treated?

Answer 33

Dietary exclusion of lactose and galactose

Question 34

What is the characteristic sign of non-classical galactosemia?

Answer 34

cataracts

Question 35

Where does lactose synthesis take place?

Answer 35

In the lactating mammary gland during lactation

Question 36

Prolactin stimulates the synthesis of which protein?

Answer 36

alpha-lactalbumin (protein B)