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Questions and Answers
What provides the sources of galactose in the diet?
What provides the sources of galactose in the diet?
Which enzyme is responsible for converting galactose to glucose in the liver?
Which enzyme is responsible for converting galactose to glucose in the liver?
What enzyme is responsible for converting fructose into fructose-1-P in the liver, kidney, and intestine?
What enzyme is responsible for converting fructose into fructose-1-P in the liver, kidney, and intestine?
Which of the following statements about fructose metabolism is correct?
Which of the following statements about fructose metabolism is correct?
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What is the normal fasting blood glucose range?
What is the normal fasting blood glucose range?
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During which process is glucose converted into galactose in the mammary gland?
During which process is glucose converted into galactose in the mammary gland?
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Where does the conversion of excess glucose to sorbitol occur, leading to the formation of fructose?
Where does the conversion of excess glucose to sorbitol occur, leading to the formation of fructose?
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What is a key product of fructose metabolism that enters glycolysis to produce energy?
What is a key product of fructose metabolism that enters glycolysis to produce energy?
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What condition is characterized by plasma glucose levels falling below 60 mg/dl?
What condition is characterized by plasma glucose levels falling below 60 mg/dl?
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What percentage of the total body energy is derived from fructose?
What percentage of the total body energy is derived from fructose?
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Study Notes
Fructose Metabolism
- Fructose is a simple sugar found in honey, fruits, and vegetables.
- It is also produced from sucrose by the enzyme sucrase.
- Fructose makes up about 15% of the body's energy supply.
- The only source of energy for spermatozoa.
Fructose Metabolism in Liver, Kidney, and Intestine
- 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.
Fructose Metabolism in Muscles and Adipose Tissues
- These tissues lack fructokinase but contain hexokinase.
- Hexokinase converts fructose into fructose-6-P.
- Phosphofructokinase-1 then converts fructose-6-P into fructose 1,6 biphosphate.
- Fructose 1,6 biphosphate then enters glycolysis to produce pyruvate and energy.
Fructose Metabolism in Testis, Lens, Peripheral Nerves, and Renal Glomeruli
- 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 Metabolism
- 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.
Conversion of Galactose to Glucose in the Liver
- 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.
Conversion of Glucose to Galactose in Mammary Gland
- This process is the reverse of galactose to glucose conversion.
- Key enzymes involved include:
- Hexokinase converts glucose to glucose-6-P.
- Phosphoglucomutase converts glucose-6-P to glucose-1-P.
- UDP-glucose pyrophosphorylase converts glucose-1-P to UDP-glucose.
- UDP-galactose 4-epimerase converts UDP-glucose to UDP-galactose.
- Lactose synthase combines UDP-galactose with glucose to create lactose.
Blood Glucose
- Normal blood glucose levels in fasting state: 70 - 110 mg/dl.
- Normal blood glucose levels two hours after a meal (2 hrs PP): up to 140 mg/dl.
Sources of Blood Glucose
- 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.
Variations in Blood Glucose
Hypoglycemia
- 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.
Causes of Hypoglycemia
-
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.
Glucosuria
- Defined as the presence of glucose in urine.
- Occurs when blood glucose exceeds the renal threshold for glucose (180 mg/dl).
Causes of Glucosuria
- 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.
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Description
Explore the intricate processes of fructose metabolism, involving its conversion in the liver, kidney, intestine, muscles, and adipose tissues. Understand the role of various enzymes in breaking down fructose to produce energy and its significance in human physiology.
