Ketone bodies are organic compounds that are produced in the liver from fatty acids when glucose is not available or in limited supply. The three main types of ketone bodies are acetoacetate, ẞ-hydroxybutyrate, and acetone.
Biosynthesis of Ketone bodies:
Site of synthesis– Mitochondria of hepatocytes
Substrate– Acetyl CoA (produced from β-Oxidation)
Regulation- long period of starvation or unmanageable diabetes mellitus (sometimes in pregnancy)
Products- Ketone bodies
1. β-hydroxy butyrate
2. Acetoacetate
3. Acetone
Ketone bodies are predominantly produced in the liver. The initial phase of ketogenesis consists of the β-oxidation of fatty acid to acetyl-CoA. In the synthesis, two molecules of acetyl CoA condense together to form acetoacetyl CoA, a reaction catalyzed by thiolase. Another molecule of acetyl CoA reacts with the acetoacetyl CoA to form 3-Hydroxy-3-methyl glutaryl CoA (HMG-CoA).
This step is the rate limiting step and the reaction is catalyzed by HMG-CoA synthase enzyme. Note that this compound is also an intermediate in the synthesis of cholesterol in the liver cell cytosol, but the mitochondrial HMG-CoA utilizes ketone body synthesis.
In the bloodstream, both acetoacetate and β-hydroxybutyrate are released.
Utilization of ketone bodies in the brain, muscle, and other tissues is simple. β-Hydroxybutyrate is re-dehydrogenated to acetoacetate, which then takes coenzyme A from succinyl-CoA to form acetoacetyl-CoA (occurs in extrahepatic cells).
Thiolase then splits acetoacetyl-CoA into two acetyl-CoA molecules, which subsequently enter the TCA cycle. The HMG-CoA formed in the hepatocyte’s mitochondria by the action of the enzyme HMG-CoA lyase is converted into acetoacetate.
A very high level of acetoacetate in blood is spontaneously decarboxylated to acetone. Acetoacetate can be converted to β-hydroxybutyrate by a dehydrogenase enzyme. It is a reversible reaction.
| Step | Reaction | Enzyme |
| 1 | β-oxidation of fatty acids to produce acetyl-CoA | β-oxidation |
| 2 | Condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA | Thiolase |
| 3 | Addition of a third molecule of acetyl-CoA to acetoacetyl-CoA to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA) | HMG-CoA synthase Rate Limiting and regulated enzyme |
| 4 | Cleavage of HMG-CoA to form acetoacetate and acetyl-CoA | HMG-CoA lyase |
| 5 | Reduction of acetoacetate to β-hydroxybutyrate | β-hydroxybutyrate dehydrogenase |
| 6 | Spontaneous decarboxylation of acetoacetate to form acetone | – |
Figure: Synthesis of ketone bodies.
Note: The odor of acetone (fruity odor) can be detected in the breath of a person who has a high level of acetoacetate, like diabetic patients. During starvation and severe diabetes mellitus peripheral tissues completely depend on ketone bodies for energy metabolism. Even tissues like the heart and brain depend mainly on ketone bodies during such conditions to meet their energy demand.
Utilization of Ketone Bodies:
Ketone bodies are produced in the Liver, but they are utilized in extrahepatic tissues because hepatocyte does not contain the enzyme required for activation of ketone bodies i.e. Succinyl CoA:acetoacetyl CoA transferase or thiophorase.
Acetoacetate is activated by two processes for its utilization.
- Acetoacetate + ATP + CoA → Acetoacetyl CoA + AMP. The enzyme is Synthetase
2. Acetoacetate + Succinyl CoA → Acetoacetyl CoA + Succinate. The enzyme is Thiophorase (Absent in Liver)
3. Aceto acetyl CoA is broken down to two molecules of Acetyl CoA, which enters the TCA cycle to produce energy.
Note: Acetoacetate and β-hydroxybutyrate are the normal substrates for respiration and important sources of energy. Renal cortex and heart muscle use acetoacetate in preference to glucose. Brain switches over to utilization of ketone bodies for energy during starvation and in uncontrolled diabetes.
Acetone is exhaled out. It does not produce energy. Normal level of ketone bodies in blood is 1mg/dl. In ketonemia, the level increases. Excretion of ketone bodies increases in urine, called ketonuria. If the patient suffers from both the signs, it is called ketosis.
Ketone Bodies and its regulation:
The fundamental concept of ketone body metabolism is to convert free fatty acids into more water-soluble substrates that are easier to transport and metabolize. When the amount of acetyl CoA produced by β-oxidation rises to a level that is more than that required for entry into the citric acid cycle due to the limited amount of oxaloacetic acid. It undergoes ketogenesis (ketone body synthesis) in the mitochondria of hepatocytes. The three compounds viz., acetoacetate, ẞ-hydroxybutyrate, and acetone are collectively known as ketone bodies. The synthesis of ketone bodies takes place during long periods of starvation or unmanageable diabetes mellitus. During such conditions, the body totally depends on the metabolism of stored triacylglycerols to fulfill its energy demand.
The synthesis of ketone bodies is regulated by several factors, including hormonal and metabolic signals. The primary regulators of ketone body synthesis are insulin and glucagon.
- Insulin: Insulin inhibits the breakdown of fatty acids in adipose tissue and promotes glucose uptake by cells. This results in decreased fatty acid availability for the liver to produce ketone bodies, leading to decreased ketone body synthesis.
- Glucagon: Glucagon promotes the breakdown of glycogen in the liver, leading to increased fatty acid availability for the liver to produce ketone bodies. Glucagon also stimulates the expression of key enzymes involved in ketone body synthesis, such as HMG-CoA synthase.
- Metabolic state: Ketone body synthesis is increased during periods of low glucose availability, such as fasting or starvation. This is because the body needs an alternative energy source to glucose, and ketone bodies can provide energy for the brain and other tissues.
- Free fatty acids: The availability of free fatty acids in the liver is a key determinant of ketone body synthesis. When fatty acid availability is high, ketone body synthesis increases. This is because fatty acids are a precursor for ketone body synthesis.
- HMG-CoA synthase: The rate-limiting step in ketone body synthesis is the conversion of acetyl-CoA to HMG-CoA by HMG-CoA synthase. The activity of this enzyme is regulated by hormonal and metabolic signals, such as glucagon and insulin.
The level of ketone bodies in the blood can vary depending on the underlying disease or condition.
- Diabetic ketoacidosis (DKA): In uncontrolled diabetes, Due to a deficiency of insulin, the body produces a high level of ketone bodies. This can result in the buildup of ketone bodies in the blood, a condition known as diabetic ketoacidosis. In DKA, the concentration of ketone bodies in the blood can reach 10 to 20 mmol/L or higher.
- Alcoholic ketoacidosis: The consumption of alcohol has been found to result in a reduction in glucose production and an elevation in fatty acid oxidation, ultimately leading to the generation of ketone bodies. Alcoholic ketoacidosis is characterized by elevated levels of ketone bodies in the bloodstream, which can exceed 5-10 mmol/L.
- Starvation: During prolonged fasting or starvation, the body produces ketone bodies as an alternative source of energy. The level of ketone bodies in the blood can increase to 0.5-5 mmol/L or higher during fasting.
- High-fat, low-carbohydrate diets: A high-fat, low-carbohydrate diet can also lead to the production of ketone bodies. The level of ketone bodies in the blood can increase to 0.5-3 mmol/L or higher during a ketogenic diet.
- Inborn errors of metabolism: Some inborn errors of metabolism, such as medium-chain acyl-CoA dehydrogenase deficiency (MCADD), can cause an accumulation of fatty acids and ketone bodies in the blood. This can lead to a condition called metabolic acidosis and an increase in the level of ketone bodies in the blood.
Important Key Point: When glucose levels are low, such as during fasting or in diabetes, oxaloacetate is used for gluconeogenesis instead of condensing with acetyl CoA. This diverts acetyl CoA to ketone body formation.
Clinical Notes: At higher than physiological plasma concentrations, acetone acts like a general anesthetic, as do many organic solvents (e.g. chloroform and diethyl ether). This is also the case for some antiepileptic drugs. An important common target for anesthetics and antiepileptic drugs is the GABA receptor, which is one of the two major inhibitory neurotransmitter receptors in the central nervous system. Accordingly, acetone has been proposed to be responsible for the effectiveness of the ketogenic diet in epilepsy
Acetone is volatile and is exhaled, and in uncontrolled diabetes the breath has a characteristic odor sometimes mistaken for ethanol. The overproduction of ketone bodies, called ketosis, results in greatly increased concentrations of ketone bodies in the blood (ketonemia) and urine (ketonuria).
Ketoacidosis is the most prevalent clinical complication related to ketone bodies. Typically, ketosis is a physiologically normal condition. However, if it is not controlled, it can lead to a potentially fatal condition known as ketoacidosis. Diabetic ketoacidosis (DKA) and alcoholic ketoacidosis (AK) are the two most common forms of ketoacidosis.
Laboratory Value of Ketone bodies:
| Ketone bodies | Normal level | Abnormal level |
| Acetoacetate | < 9 mg/dl | > 27 mg/dl |
| β-hydroxybutyrate | < 9 mg/dl | > 54 mg/dl |
Measure the Ketone bodies in laboratory:
In the laboratory, enzymatic reactions and spectrophotometry are the most common techniques for measuring ketone bodies.
- Enzymatic reactions are chemical processes catalyzed by enzymes, which are biological molecules that speed up chemical reactions without being consumed in the process. Methods currently available quantify ketone bodies via enzymatic catalysis, which facilitates the conversion of ketone bodies into a measurable product. The enzyme β-hydroxybutyrate dehydrogenase facilitates the conversion of β-hydroxybutyrate to acetoacetate, and the resulting acetoacetate is quantified using a spectrophotometer. Likewise, acetoacetate decarboxylase converts acetoacetate to acetone, which can be measured by gas chromatography or a colorimetric assay.
- Spectrophotometry is a technique used to measure the light absorption of ketone bodies at a specific wavelength. Using the absorbance of β-hydroxybutyrate at 340 nm, the concentration of β-hydroxybutyrate in a sample can be determined. The specimen is combined with a chromogen-containing reagent, which reacts with the ketone bodies to produce a chromatic output. The intensity of the color is proportional to the sample’s ketone body concentration.
- Dipstick test: The dipstick test is a simple and quick means to measure ketone bodies in urine. The dipstick contains a reagent that reacts with acetoacetate and alters color when ketones are present. The concentration of ketones in the urine sample can then be determined by comparing the intensity of the color to a color chart.
Clinical Conditions
1) Diabetic ketoacidosis (DKA) is frequently associated with inadequate insulin response within the body. insulin response leads to elevated levels of glucagon and reduced glucose uptake. The reduction in insulin levels promotes the heightened functioning of hormone-sensitive lipase. The process of converting triglycerides to fatty acyl CoA molecules is facilitated by hormone-sensitive lipase. The excessive presence of fatty acyl CoA molecules results in an overload of the Krebs cycle, leading to a diversion towards ketone metabolism. The presence of an excessive amount of ketone bodies results in the development of anion gap acidosis. Patients typically exhibit symptoms such as hyperglycemia, lethargy, abdominal pain, polyuria, polydipsia, vomiting, and alterations in mental status.
2) Alcoholic ketoacidosis is a medical condition that manifests in individuals with chronic alcoholism following sudden cessation or acute inebriation. Ethanol undergoes metabolism to produce acetic acid, and during this process, NAD+ serves as a coenzyme and is converted to NADH. Under conditions of favorable concentrations of insulin and glucagon, such as in hypoglycemia, acetic acid is directed towards ketogenesis. Furthermore, the inhibitory effect of NADH on gluconeogenesis can be attributed to the essential role of NAD+ in several stages of the gluconeogenic pathway. Acute withdrawal of ethanol, the release of epinephrine serves to propel ketogenesis, ultimately leading to the onset of ketoacidosis.
3) Medium-chain acyl CoA dehydrogenase (MCAD) deficiency is observed, and MCAD deficiency screening is a common examination included in standard neonatal screening. The enzyme MCAD facilitates the conversion of fatty acyl CoA molecules with carbon chains ranging from four to twelve carbons into acetyl CoA, which can be utilized by the mitochondria. The ACADM gene on chromosome 1 is frequently associated with the autosomal recessive inheritance pattern of MCAD. Patients may exhibit seizures, comas, hypoketotic hypoglycemia, and failure to thrive in the absence of an early diagnosis. For these patients, early diagnosis and avoiding fasting are the primary treatment modalities.
4) Energy production: During prolonged fasting, starvation, or when carbohydrates are restricted, the body uses ketone bodies as an alternative source of energy. The brain can also use ketone bodies for energy during periods of low glucose availability, such as during fasting or in individuals with uncontrolled diabetes.
5) Weight loss: A low-carbohydrate, high-fat diet can lead to the production of ketone bodies and promote weight loss. This is because the body burns fat for energy in the absence of glucose.
6) Epilepsy: A ketogenic diet, which is high in fat and low in carbohydrates, has been shown to reduce seizures in some individuals with epilepsy. This is believed to be due to the production of ketone bodies, which can help regulate brain activity.
Note: Endurance athletes may produce ketone bodies during prolonged exercise, as the body uses fat as a fuel source.
Reference book
- Harper’s Illustrated Biochemistry, Thirty-Second Edition
- Textbook of Biochemistry with Clinical Correlations Hardcover – Illustrated, 22 January 2010 by Thomas M. Devlin
- Lehninger Principles of Biochemistry: 6th Edition
- Lippincott’s Illustrated Reviews – Biochemistry South
Multiple Choice Questions
1. Which of the following is not a ketone body?
A) Acetone
B) Acetoacetate
C) β-hydroxybutyrate
D) Pyruvate
Answer: D
2. In which of the following conditions can ketone body levels increase?
A) Starvation
B) Diabetes
C) Alcoholic ketoacidosis
D) All of the above
Answer: D
3. What is the primary source of energy for the brain during fasting or starvation?
A) Glucose
B) Ketone bodies
C) Fatty acids
D) Amino acids
Answer: B
4. Which enzyme is responsible for the conversion of acetoacetate to β-hydroxybutyrate?
A) β-hydroxybutyrate dehydrogenase
B) Acetoacetate decarboxylase
C) Thiolase
D) Succinyl-CoA synthetase
Answer: A
5. What is the normal range for β-hydroxybutyrate levels in the blood?
A) 0.0-0.6 mmol/L
B) 0.5-5.0 mmol/L
C) 0.02-0.27 mmol/L
D) 0.0-0.3 mmol/L
Answer: C
6. Which of the following is a genetic disorder that affects ketone body metabolism?
A) Alcoholic ketoacidosis
B) Diabetic ketoacidosis
C) Medium-chain acyl-CoA dehydrogenase deficiency (MCADD)
D) Metabolic acidosis
Answer: C
7. Which of the following is a condition that can lead to high levels of ketone bodies in the blood?
A) Hyperglycemia
B) Hypoglycemia
C) Hypoxia
D) Hypertension
Answer: B
8. What is the normal range for acetoacetate levels in the blood?
A) 0.0-0.6 mmol/L
B) 0.5-5.0 mmol/L
C) 0.02-0.27 mmol/L
D) 0.0-0.3 mmol/L
Answer: D
9. Which of the following ketone bodies is produced by the liver?
A) Acetone
B) β-hydroxybutyrate
C) Acetoacetate
D) All of the above
Answer: D
10. Which of the following is the primary precursor for ketone body synthesis?
A) Glucose
B) Amino acids
C) Fatty acids
D) All of the above
Answer: C
11. Which of the following conditions might ketone bodies be produced in high levels?
A) Starvation
B) Diabetes mellitus
C) Alcoholism
D) All of the above
Answer: D
12.Which of the following enzymes is responsible for the conversion of acetoacetate to β-hydroxybutyrate?
A) β-hydroxybutyrate dehydrogenase
B) β-ketothiolase
C) Succinyl-CoA transferase
D) None of the above
Answer: A
13. Which of the following is a common symptom of diabetic ketoacidosis (DKA)?
A) Hyperglycemia
B) Hypoglycemia
C) Acidosis
D) None of the above
Answer: C
14. In which of the following conditions might ketone bodies be produced in high levels due to increased fatty acid oxidation?
A) MCADD
B) Type 2 diabetes
C) Cystic fibrosis
D) All of the above
Answer: A
15. Which of the following can be used as an energy source by the brain during prolonged fasting?
A) Glucose
B) Fatty acids
C) Ketone bodies
D) None of the above
Answer: C
16. Which of the following hormones can stimulate ketone body production?
A) Insulin
B) Glucagon
C) Cortisol
D) All of the above
Answer: B
17. In which of the following conditions might ketone bodies be produced in high levels due to impaired carbohydrate metabolism?
A) Alcoholic ketoacidosis
B) Type 1 diabetes
C) Cushing’s syndrome
D) None of the above
Answer: B
18. Which of the following is an example of a high-fat, low-carbohydrate diet that can promote ketone body production?
A) Atkins diet
B) Mediterranean diet
C) Vegan diet
D) None of the above
Answer: A
19. Which of the following ketone bodies is a gas that can be detected in the breath during ketosis?
A) Acetone
B) β-hydroxybutyrate
C) Acetoacetate
D) None of the above
Answer: A
20. In which of the following conditions might ketone bodies be produced in high levels due to alcohol metabolism?
A) Alcoholic ketoacidosis
B) Type 2 diabetes
C) Pancreatitis
D) None of the above
Answer: A
21. Which of the following is a potential complication of ketone body production in high levels?
A) Metabolic acidosis
B) Hypoglycemia
C) Hyperglycemia
D) None of the above
Answer: A
22. In which of the following conditions might ketone body production be decreased due to impaired fatty acid metabolism?
A) MCADD
B) Type 2 diabetes
C) Cystic fibrosis
D) None of the above
Answer: C