Slide Set 11: Lactose Biosynthesis Flashcards
What are the carbohydrates found in milk?
o Lactose o Monosaccharides o Oligosaccharides o Sugar phosphate o Nucleotide sugars o Free glucose and galactose o Free myo-inositol
What is the essential molecule for lactose biosynthesis?
glucose
Blood glucose levels of ruminants vs monogastric
blood glucose levels of ruminants is 1/2 of monogastric animals
Where does the blood glucose come from in ruminants?
45-60% of blood glucose in ruminants comes from
gluconeogenesis
Where does lactose synthesis take place?
Golgi
What are the steps in lactose synthesis?
- One glucose to UDP glucose then UDP galactose
- One glucose to the Golgi
- Active transport of UDP galactose to the Golgi
- Lactose is impermeable (i.e. cannot diffuse from
the Golgi membrane - UDP is hydrolyzed to UMP and inorganic P
- Lactose synthesis is one was reaction
- Lactose synthesis is catalyzed by lactose synthase
What are the two subunits of lactose synthase?
Galactosyl transferase
A-lactalbumin
Where is a-lactalbumin expressed?
only in the mammary gland and it is under hormonal control
What are the main features of galactosyl transferase?
• The active enzymatic subunits of lactase synthase
• Involves in glycoprotein synthesis (glycosylation of
protein)
• Its function is modified by the presence of a-
lactalbumin
What are the main features of a-lactalbumin?
- 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
How is lactose transported out of the golgi body?
Lactose cannot diffuse out the Golgi so secretory vesicles
How is lactose biosynthesis inhibited?
- High concentration of lactose does not inhibit its own synthesis
- Rate of synthesis depends on the a-lactalbumin:GT (500:1)
The rate of lactose biosynthesis depend on ______
the a-lactalbumin:GT (500:1)
T/F
High concentration of lactose inhibits its own synthesis
FALSE
High concentration of lactose does not inhibit its own synthesis
What is lactose intolerance?
Intestinal disorder results from the lack of lactase.
What are the characteristics of lactose intolerance?
o The inability to digest lactose
o Increased microbial fermentation of lactose
o Osmotic entry of fluids into the gut Diarrhea
What are the methods of diagnosing lactose intolerance?
- 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. - 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 - 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
Which diagnosis methods are suitable for infants and children?
- lactose intolerance test
- hydrogen breath test
What is natural decline in lactase activity post-weaning called?
Primary lactase non-persistency
What is a rare genetic condition where lactase activity is decreased or absent at birth?
Congenital lactase deficiency
What is the temporary condition due to damage of the intestinal lining called?
Secondary lactase deficiency
What is galactosemia?
- 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
How is lactose intolerance treated?
- 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
