Fructose Metabolism Overview

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Study Notes

These tissues contain the enzyme fructokinase, converting fructose into fructose-1-P.
Aldolase B enzyme then hydrolyzes fructose-1-P into dihydroxyacetone phosphate and glyceraldehyde.
Glyceraldehyde is converted into glyceraldehyde-3-P by triokinase.
Glyceraldehyde-3-P can enter glycolysis to produce energy or combine with dihydroxyacetone phosphate to create glucose.

Glucose is converted into fructose through the sorbitol pathway:
- Glucose is converted to sorbitol by aldolase reductase using NADPH+H+.
- Sorbitol is then converted to fructose by sorbitol dehydrogenase using NAD+.

Galactose is a simple sugar found in dairy milk, specifically lactose.
Lactose breaks down into glucose and galactose.
Galactose is a component of lactose, glycolipids, glycoproteins, and proteoglycans.

Galactose is converted to glucose in the liver through a series of steps involving enzymes:
- Galactokinase converts galactose to galactose-1-P.
- Uridyl transferase converts galactose-1-P to UDP-galactose, using UDP-glucose.
- UDP-galactose 4-epimerase converts UDP-galactose to UDP-glucose.
- UDP-glucose is then converted to glucose-1-P and ultimately to glucose-6-P.

Dietary Carbohydrates: Glucose, galactose, and fructose. Galactose and fructose are converted to glucose in the liver.
Gluconeogenic Substances: Amino acids, lactate, glycerol.
Liver Glycogen: Through glycogenolysis.

Defined as blood glucose levels below 60 mg/dl.
Symptoms: Faintness, dizziness, lethargy, potentially progressing to coma and even death from brain damage if untreated.

Stimulative Hypoglycemia:
- Insulin Overdoses or Poisons: Such as chloroform.
- Inborn Errors of Metabolism: Galactosemia and hereditary fructose intolerance.
- Post Gastrectomy: Rapid glucose absorption leads to excessive insulin secretion.
- Excess Alcohol Intake.
- Essential Reactive Hypoglycemia: Exaggerated insulin response to ingested carbohydrates.
- Idiopathic Hypoglycemia.
Fasting Hypoglycemia: Failure to maintain normal blood glucose during fasting.
- Starvation and Malnutrition: Exhaustion of liver glycogen.
- Inborn Errors of Metabolism: Dicarboxylic aciduria, maple syrup urine disease, and tyrosinemia can cause neonatal or childhood hypoglycemia.
- Pancreatic Disease: Insulinoma, hyperinsulinism of childhood, pancreatitis, and pancreatic tumors can lead to enhanced glucose utilization due to insulin overproduction.
- Defective Glucose Production:
  - Endocrine Disease: Adrenocortical insufficiency, hypothyroidism, and growth hormone deficiency.
  - Liver Disease: Severe liver cirrhosis, hepatic tumors.
  - Renal Disease: Defective gluconeogenesis.

Diabetes Mellitus: Hyperglycemia due to insulin deficiency.
Renal Glucosuria: Low renal threshold, where glucose leaks into urine even at low blood glucose concentrations.
- Inherited defects in the kidney.
- Nephritis and nephrosis.
- Drugs like phlorizin (inhibit glucose reabsorption in renal tubules).
- Pregnancy: Impaired glucose tolerance and hyperglycemia due to low renal threshold and increased glomerular filtration rate.
- Fanconi's Syndrome: Defective renal tubular reabsorption of glucose, amino acids, and phosphate.