Slide Set 11: Lactose Biosynthesis

Question 1

What are the carbohydrates found in milk?

Answer 1

o Lactose
o Monosaccharides
o Oligosaccharides
o Sugar phosphate
o Nucleotide sugars
o Free glucose and galactose 
o Free myo-inositol

Question 2

What is the essential molecule for lactose biosynthesis?

Answer 2

glucose

Question 3

Blood glucose levels of ruminants vs monogastric

Answer 3

blood glucose levels of ruminants is 1/2 of monogastric animals

Question 4

Where does the blood glucose come from in ruminants?

Answer 4

45-60% of blood glucose in ruminants comes from

gluconeogenesis

Question 5

Where does lactose synthesis take place?

Answer 5

Golgi

Question 6

What are the steps in lactose synthesis?

Answer 6

1. One glucose to UDP glucose then UDP galactose
2. One glucose to the Golgi
3. Active transport of UDP galactose to the Golgi
4. Lactose is impermeable (i.e. cannot diffuse from
   the Golgi membrane
5. UDP is hydrolyzed to UMP and inorganic P
6. Lactose synthesis is one was reaction
7. Lactose synthesis is catalyzed by lactose synthase

Question 7

What are the two subunits of lactose synthase?

Answer 7

Galactosyl transferase

A-lactalbumin

Question 8

Where is a-lactalbumin expressed?

Answer 8

only in the mammary gland and it is under hormonal control

Question 9

What are the main features of galactosyl transferase?

Answer 9

• The active enzymatic subunits of lactase synthase
• Involves in glycoprotein synthesis (glycosylation of
protein)
• Its function is modified by the presence of a-
lactalbumin

Question 10

What are the main features of a-lactalbumin?

Answer 10

• Milk why protein
• It is not catalytically active by itself, but is necessary for lactose synthesis
• Alters the sugar binding properties of GT so that lactose is produced

Question 11

How is lactose transported out of the golgi body?

Answer 11

Lactose cannot diffuse out the Golgi so secretory vesicles

Question 12

How is lactose biosynthesis inhibited?

Answer 12

• High concentration of lactose does not inhibit its own synthesis
• Rate of synthesis depends on the a-lactalbumin:GT (500:1)

Question 13

The rate of lactose biosynthesis depend on ______

Answer 13

the a-lactalbumin:GT (500:1)

Question 14

T/F

High concentration of lactose inhibits its own synthesis

Answer 14

FALSE

High concentration of lactose does not inhibit its own synthesis

Question 15

What is lactose intolerance?

Answer 15

Intestinal disorder results from the lack of lactase.

Question 16

What are the characteristics of lactose intolerance?

Answer 16

o The inability to digest lactose
o Increased microbial fermentation of lactose
o Osmotic entry of fluids into the gut Diarrhea

Question 17

What are the methods of diagnosing lactose intolerance?

Answer 17

1. Lactose tolerance test
   o after 6 hours you do a blood test and measure glucose peak, if it increases then you are not lactose intolerant, if it stays stable then you are lactose intolerant.
2. Hydrogen breath test
   o Measures the amount of hydrogen in the breath
   o Undigested lactose is fermented in the colon by bacteria and various
   gasses are produced (e.g. hydrogen)
   o Gases are carried through the blood stream to the lungs and exhaled
   • Both lactose tolerant test and hydrogen breath test are unsuitable for young children
3. Stool acidity test
   o Based on the fact that undigested lactose will be fermented in the colon to lactic acid and short chain fatty acids
   o Glucose may also be present as a result of unabsorbed lactose in the colon
   o Suitable test for infants and young children

Question 18

Which diagnosis methods are suitable for infants and children?

Answer 18

• lactose intolerance test

- hydrogen breath test

Question 19

What is natural decline in lactase activity post-weaning called?

Answer 19

Primary lactase non-persistency

Question 20

What is a rare genetic condition where lactase activity is decreased or absent at birth?

Answer 20

Congenital lactase deficiency

Question 21

What is the temporary condition due to damage of the intestinal lining called?

Answer 21

Secondary lactase deficiency

Question 22

What is galactosemia?

Answer 22

• A disease where galactose cannot be utilized
• Feeding milk to infants with the disease results in increased liver and spleen size and increased incidence of cataracts

Question 23

How is lactose intolerance treated?

Answer 23

• Young children with lactase deficiency should not consume any feed containing lactose
• Lactase enzymes and tablets can be used
• Use of lactose reduced milk products