Fructose, Galactose, Sorbitol, and Ethanol Metabolism
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32 Terms
What enzyme phosphorylates fructose?
Fructokinase
What does fructokinase produce?
Fructose-1-phosphate (F-1-P)
What enzyme cleaves F-1-P?
Aldolase B
What are the products of F-1-P cleavage?
DHAP and glyceraldehyde
Which glycolytic steps are bypassed by fructose metabolism?
Hexokinase and PFK-1
What transporter brings fructose into cells?
GLUT5
What is essential fructosuria?
Fructokinase deficiency; benign condition with fructose in urine
What is hereditary fructose intolerance?
Aldolase B deficiency; causes phosphate trapping and severe hypoglycemia
What is the consequence of excess fructose intake?
Citrate accumulation → fatty acid synthesis → hyperlipidemia
What enzyme converts glucose to sorbitol?
Aldose reductase
What enzyme converts sorbitol to fructose?
Sorbitol dehydrogenase
Where is sorbitol dehydrogenase low or absent?
Lens, retina, kidney, nerve cells
Why is sorbitol accumulation harmful?
Strong osmotic effect → cell swelling
What conditions are associated with sorbitol accumulation?
Cataracts, peripheral neuropathy, nephropathy, retinopathy
Why does sorbitol accumulate in diabetes?
High intracellular glucose enters insulin-independent tissues and is converted to sorbitol
What enzyme phosphorylates galactose?
Galactokinase
What does galactokinase produce?
Galactose-1-phosphate (Gal-1-P)
What enzyme converts Gal-1-P to glucose-1-phosphate?
Galactose-1-phosphate uridyltransferase
What is classic galactosemia?
Uridyltransferase deficiency; causes Gal-1-P and galactitol accumulation
What are symptoms of classic galactosemia?
Cataracts, hepatomegaly, failure to thrive, neutrophil dysfunction
What is galactokinase deficiency?
Benign condition; causes galactitol accumulation and cataracts
What is lactose intolerance?
Deficiency of lactase; causes bloating, gas, diarrhea
What is the fate of galactose in metabolism?
Converted to glucose-1-phosphate → glycolysis or glycogenesis
What enzyme converts ethanol to acetaldehyde?
Alcohol dehydrogenase
What enzyme converts acetaldehyde to acetate?
Aldehyde dehydrogenase
What cofactors are used in ethanol metabolism?
2 NAD⁺ per ethanol molecule
What toxic intermediate is produced from ethanol?
Acetaldehyde
What is disulfiram’s mechanism?
Inhibits aldehyde dehydrogenase → acetaldehyde buildup
How does ethanol affect gluconeogenesis?
Consumes NAD⁺ → inhibits gluconeogenesis → lactic acidosis
How does ethanol intersect with thiamine deficiency?
Thiamine is needed for pyruvate → acetyl-CoA; deficiency worsens lactic acidosis
What toxic metabolites are produced from methanol and ethylene glycol? (less important than ethanol)
Formic acid and oxalic acid
Why is ethanol used as an antidote for methanol or ethylene glycol poisoning?
Alcohol dehydrogenase has higher affinity for ethanol
