Fructose and galactose metabolism Flashcards
What is the transporter for fructose into the enterocyte?
GLUT 5 with Na cotransport
What is the transporter for fructose out of the enterocyte?
GLUT 2 and GLUT 5
What is the typical pathway for digestion of fructose?
Fructose 1-phosphate pathway in liver
What is an alternative pathway of glucose digestion?
By hexokinase to fructose-6P
What is the first step of the fructose 1P pathway?
Fructose to fructose 1P by fructokinase using ATP
What is the second step of the fructose 1P pathway?
Fructose 1P to glyceraldehyde and DHAP by fructose 1P aldolase
DHAP enters glycolysis or gluconeogenesis
What happens to the glyceraldehyde produced in the fructose 1P pathway?
Glyceraldehyde to glyceraldehyde 3P by triose kinase using ATP.
This will enter glycolysis or gluconeogenesis
What is the pathway to produce glucose in seminal plasma?
Sorbitol pathway
What is the sorbitol pathway?
Glucose to sorbitol by aldose reductase using NADPH
Sorbitol to fructose by sorbitol dehydrogenase using NAD
What tissues have low/absent sorbitol dehydrogenase?
Lens of eye, nerves, and kidneys
What is the inheritance for hereditary fructose intolerance?
Autosomal recessive
What enzyme is deficient in hereditary fructose intolerance?
Aldolase B
What are the signs of hereditary fructose intolerance?
Recurrent vomiting, abd pain, and hypoglycemia after ingestion of fructose/sucrose
What is the inheritance for essential fructosuria?
Autosomal recessive
Very rare
What is the enzyme deficiency in essential fructosuria?
Hepatic fructokinase
What are the symptoms of essential fructosuria?
Asymptomatic or mistaken for diabetes
What is the transporter for galactose into the enterocyte?
SGLT-1 with Na contransport
What is the transporter for galactose out of the enterocyte?
GLUT-2
What is the first step of galactose metabolism?
Galactose to galactose-1P by galactokinase using ATP
What is the second step of galactose metabolism?
Galactose-1P to Glucose-1P by GALT coupled with UDP-glucose to UDP galactose
What are the two ways glucose 1P can be used?
To glucose 6P by phosphoglucomutase to enter glycolysis or gluconeogenesis
Into UDP-glucose by UDP glucose pyrophosphorylase using UTP and producing PPi
What is the enzyme responsible for epimerization between UDP-glucose and UDP galactose?
UDP-galactose 4’ epimerase (GALE)
How can UDP glucose be used?
Incorporated into glycogen
Synthesis of UDP galactose, making galactose nonessential
Uronic acid pathway
When is lactose synthesized in the body?
During lactation
How is lactose synthesized from UDP-galactose?
UDP-galactose to lactose by lactose synthase using glucose and producing UDP
When is galactitol produced?
When there is an excess of galactose
What happens to galactitol?
It is excreted in the urine
What enzyme is deficient in classical galactosemia?
GALT
What enzyme is deficient in nonclassical galactosemia?
Galactokinase
What are signs of galactosemia?
Failure to thrive
Vomiting and diarrhea following ingestion of milk
Hepatomegaly and impaired liver function
Hypergalactemia
Excessive galactitol in urine
Cataracts
Hypoglycermia
Mental retardation if untreated
