Metabolism of Hexoses and Blood Glucose

Metabolism of Other Hexoses and Blood Glucose

• This presentation covers the metabolism of hexoses, specifically fructose and galactose, and blood glucose regulation.
• Key molecules discussed include glucose, fructose, galactose, and various enzymes involved in their metabolism.

Fructose Metabolism

• Sources of fructose: Sucrose (table sugar), honey, fruits, and vegetables.
• Importance of fructose: Contributes 15% of the body's energy, the only source for sperm energy.
• Fructose metabolism in liver, kidney, and intestine:
  • Tissues contain fructokinase, converting fructose to fructose-1-P.
  • Aldolase B then hydrolyzes fructose-1-P into dihydroxyacetone phosphate and glyceraldehyde.
• Fructose metabolism pathways are shown schematically.

Fructose Metabolism in Muscles and Adipose Tissues

• These tissues lack fructokinase but contain hexokinase.
• Hexokinase phosphorylates fructose into fructose-6-phosphate.
• This fructose-6-phosphate then enters glycolysis, leading to energy production.

Glucose Conversion to Fructose

• In specialised tissues (testes, lens, peripheral nerves, renal glomeruli), glucose is converted into fructose via the enzyme aldolase reductase.
• The conversion involves a series of enzymatic reactions.
• This conversion is also observed under diabetic conditions, where high glucose levels are associated with sorbitol formation leading to cataracts through increased osmotic pressure.

Galactose Metabolism

• Sources of galactose: Dietary lactose (milk sugar).
• Importance of galactose: Galactose is incorporated into various structures in the body, including lactose, glycolipids, glycoproteins, and proteoglycans.
• Galactose metabolism pathway is detailed, showing galactose transformation within the liver, involving galactokinase, UDP-glucose, and UDP-galactose 4-epimerases steps to eventually integrate into glucose
• A separate pathway shows galactose conversion to glucose in the mammary gland

Blood Glucose

• Normal blood glucose range: Fasting blood glucose (FBG) is 70-110 mg/dL; 2-hour postprandial (PP) glucose is up to 140 mg/dL.
• Sources of blood glucose:
  • Dietary carbohydrates (glucose, galactose, and fructose).
  • Gluconeogenesis (e.g., amino acids, lactate, glycerol).
  • Liver glycogenolysis.

Variations in Blood Glucose: Hypoglycemia

• Definition: A condition characterized by decreased plasma glucose below 60 mg/dL.
• Symptoms: Faintness, dizziness, lethargy, potentially progressing to coma and death if untreated.
• Causes: Two types:
  • Stimulatory hypoglycemia: Occurs due to external factors such as drugs (insulin overdose), inborn errors of metabolism (Galactosemia, Hereditary fructose intolerance), post-gastrectomy, excessive alcohol intake, and reactive hypoglycemia.
  • Fasting hypoglycemia: Results from problems producing or releasing glucose during a fast—this can result from starvation, malnutrition, Inborn Errors of Metabolism, pancreatic disease, or diseases affecting the liver or endocrine system.

Variations in Blood Glucose: Glucosuria

• Definition: Glucose in the urine, detectable by standard methods.
• Occurrence: Glucose concentrations in the blood surpassing the renal threshold for glucose (180 mg/dL).
• Causes:
  • Diabetes mellitus (high blood glucose due to insulin deficiency).
  • Renal glucosuria (kidney dysfunction).