Index
Lipid peroxidation and the generation of oxygen radicals may play a role in vascular disease in diabetes.
Ketone bodies or ketone bodies are present in small amounts in the blood of healthy individuals during prolonged fasting exercise and play a key role in glucose utilization and reduction of proteolysis.
Unlike most other tissues, the brain cannot use fatty acids for energy when blood glucose levels are compromised.
In this case, the ketone bodies provide the brain with an alternative energy source, which equates to almost 2/3 of the brain’s energy needs during periods of prolonged fasting and starvation.
Abnormally large amounts of ketone bodies are found in the blood of individuals experiencing diabetic ketoacidosis, alcoholic ketoacidosis , salicylate poisoning, and other rare conditions.
Ketone bodies stimulate insulin release in vitro, generate oxygen radicals, and produce lipid peroxidation.
Ketone body metabolism
The body’s metabolism of ketone includes synthesis of ketone bodies (ketogenesis) and breakdown (ketolysis).
When the body goes from eating to the fasting state, the liver goes from an organ of carbohydrate utilization and fatty acid synthesis to one of fatty acid oxidation and ketone production of the body.
This metabolic change is amplified in uncontrolled diabetes.
In these states, energy derived from fat (ketone bodies) generated in the liver enters the bloodstream and is used by other organs, such as the brain , heart, renal cortex, and skeletal muscle.
Ketone bodies are particularly important to the brain that has no other substantial energy source not derived from glucose.
The two main ketone bodies are acetoacetate (AcAc) and 3-hydroxybutyrate (3HB), also called β-hydroxybutyrate, with acetone being the third and least abundant.
Ketone bodies are always present in the blood and their levels increase during prolonged fasting and exercise.
After an overnight fast, ketone bodies provide between 2% and 6% of the body’s energy requirements, while meeting 30-40% of energy needs after a 3-day fast.
When they accumulate in the blood, they spill into the urine. The presence of elevated ketone bodies in the blood is called ketosis, and the presence of ketone bodies in the urine is called ketonuria.
The body can also rid itself of acetone through the lungs, which gives the breath a fruity smell. Diabetes is the most common pathological cause of elevated blood ketones.
In diabetic ketoacidosis, high levels of ketone bodies are produced in response to low levels of insulin and high levels of counter-regulatory hormones.
Cetogenesis
Ketogenesis is the process by which fatty acids are transformed into acetoacetate (AcAc) and 3-hydroxybutyrate (3HB). This process takes place in the liver in specialized organelles called mitochondria.
Under normal aerobic conditions, glucose and fatty acids are metabolized to acetyl CoA by glycolysis and β-oxidation, respectively.
Acetyl CoA is then metabolized to two CO2 molecules by the tricarboxylic acid cycle (TCA cycle) which comprises eight sequential enzymatic reactions.
The energy released for each turn of the cycle is stored as high-energy phosphate in a GTP molecule.
It is also stored as high-energy electrons in three molecules of NADH + H + and one molecule of the reduced cofactor, coenzyme Q (QH2) via FADH2.
The three NADHs and one FADH2 produced by each turn of the cycle are re-oxidized and generate ATP in a process called oxidative phosphorylation.
The theoretical yield is 3 ATP per NADH molecule and 2 ATP per FADH2 molecule, for a total of 11 ATP and 1 GTP per cycle.
Inefficiencies in oxidative phosphorylation reduce the actual yield to ~ 2.5 per NADH and 1.5 per FADH2 (10 ATP equivalents) per cycle.
Energy production through the TCA cycle and oxidative phosphorylation takes place in the mitochondria, the same organelles where ketogenesis occurs.
The availability of oxaloacetate (OAA) is critical for the oxidation of acetyl CoA.
If intracellular glucose levels become too low in the liver, oxaloacetate is depleted due to its preferential use in the gluconeogenesis process.
Therefore OAA is not available for condensation with acetyl CoA for oxidative metabolism of the latter through the TCA cycle.
In contrast, in the liver, acetyl CoA is diverted to the formation of ketone bodies (ketogenesis).
The liver also lacks one of the key enzymes required for the utilization of the ketone body – acetoacetyl succinyl CoA transferase.
The unavailability of OAA and the lack of the above transferase explain why ketone bodies are synthesized in the liver but used in peripheral tissues.
Fatty acid oxidation in liver mitochondria generates acetyl CoA. Under conditions of low glucose availability, acetyl CoA cannot be oxidized by the TCA cycle.
This happens because in the liver (its condensation with acetyl CoA to form citrate), the oxaloacetate required for the first step is not available. It is then redirected to the glucose process (gluconeogenesis).
Consequently, acetyl CoA is converted to ketone bodies that are used by non-liver tissues for energy production (ATP).
Note that the first and last enzyme in both processes are reversible and operate in both processes. In healthy adults, the liver is capable of producing up to 185 g of ketone bodies per day.
The above process includes the steps below:
- β-oxidation of fatty acids to acetyl CoA.
- Formation of acetoacetyl CoA from two molecules of acetyl CoA.
- Conversion of acetoacetyl CoA to 3-hydroxy-3-methylglutaryl CoA (HMG CoA).
- Conversion of 3-hydroxy-3-methylglutaryl CoA to acetoacetate (AcAc).
- AcAc reduction to 3-β-hydroxybutyrate (3HB).
- Spontaneous decarboxylation of acetoacetate to acetone.
The conversion of 2 acetyl CoA molecules into acetoacetyl CoA and free CoA is catalyzed by the reversible enzyme acetoacetyl CoA thiolase. Which is formed from acetoacetyl CoA by mitochondrial HMG CoA synthase.
This step is stimulated by starvation, low insulin levels, and eating a high-fat diet. HMG CoA is also produced from ketogenic amino acids such as leucine, lysine, and tryptophan through a separate enzymatic process during amino acid catabolism.
HMG CoA is then cleaved to release acetoacetate in a step mediated by 3-hydroxy-3-methylglutaryl CoA lyase (HMG CoA lyase).
The reduction of acetoacetate (AcAc) to 3-hydroxybutyrate (3HB) is catalyzed by 3-hydroxybutyrate dehydrogenase, a phosphatidylcholine-dependent enzyme.
During this step, NADH is oxidized to NAD +. The maximum ratio of 3HB to AcAc in the blood depends on the redox potential (ie, the NADH / NAD + ratio) within the hepatic mitochondria.
Acetoacetate and 3-hydroxybutyrate are short-chain (4-carbon organic acids) that can diffuse freely through cell membranes.
Therefore, ketone bodies can serve as an energy source for the brain (which does not use fatty acids) and the other peripheral organs mentioned above.
Ketone bodies are filtered and reabsorbed in the kidney and these organic acids are completely dissociated.
The large hydrogen ion charge generated during its pathological production quickly overwhelms the normal buffering capacity and leads to metabolic acidosis with increased anion.
Ketolysis
Ketolysis is the process by which ketone bodies produced in the liver are converted (in non-liver tissues) to acetyl CoA.
Then after complete oxidation through the tricarboxylic acid cycle and oxidative phosphorylation, it provides energy for various intracellular metabolic activities.
Ketolysis occurs in the mitochondria of many extrahepatic organs.
The central nervous system is particularly dependent on the administration of ketone bodies produced in the liver for the ketolysis process.
Since ketogenesis occurs very slowly, if at all, in the central nervous system .
Free fatty acids are released into the circulation by lipolysis and are broken down into multiple copies of acetyl CoA by β-oxidation.
Under conditions of low glucose availability, ketogenesis occurs in the liver producing the three ketone bodies, 3-hydroxybutyrate, acetoacetate, and acetone.
Acetone is formed by non-enzymatic decarboxylation of acetoacetate and cannot be used as an energy source.
Acetoacetate and 3-hydroxybutyrate pass from the liver into the general circulation and are absorbed by non-hepatic tissues where they can be used as fuel.
3-Hydroxybutyrate is oxidized to acetoacetate by 3-hydroxy butyrate dehydrogenase and then converted to acetoacetyl CoA by acetoacetyl succinyl CoA transferase (II).
Acetoacetyl CoA is then cleaved by acetoacetyl CoA thiolase (III) into two acetyl CoA molecules that are metabolized to CO 2 and H 2O by the TCA cycle and oxidative phosphorylation generating many ATP molecules.
Free fatty acids are released into the circulation by lipolysis and are broken down into multiple copies of acetyl CoA by β-oxidation.
Under conditions of low glucose availability, ketogenesis occurs in the liver producing the three ketone bodies, 3-hydroxybutyrate, acetoacetate, and acetone.
The production of the first two is catalyzed by four enzymes: acetoacetyl CoA thiolase (indicated by 1), HMG CoA synthase (2), HMG CoA lyase (3) and 3-hydroxy butyrate dehydrogenase (4).
Acetone is formed by non-enzymatic decarboxylation of acetoacetate and cannot be used as an energy source.
Acetoacetate and 3-hydroxybutyrate pass from the liver into the general circulation and are absorbed by non-hepatic tissues where they can be used as fuel.
3-Hydroxybutyrate is oxidized to acetoacetate by 3-hydroxy butyrate dehydrogenase and then converted to acetoacetyl CoA by acetoacetyl succinyl CoA transferase (II).
Acetoacetyl CoA is then cleaved by acetoacetyl CoA thiolase (III) into two acetyl CoA molecules which are metabolized to CO2 and H2O through the TCA cycle and oxidative phosphorylation generates many ATP molecules.
Ketolysis involves three steps, two of which are reversible reactions carried out by two (3-hydroxy butyrate dehydrogenase and acetoacetyl CoA thiolase). These four enzymes are involved in ketogenesis.
The first step in ketolysis is the oxidation of 3-hydroxybutyrate to acetoacetate by the reversible enzyme 3-hydroxy butyrate dehydrogenase.
Followed by the reconstitution of acetoacetyl CoA from acetoacetate by the enzyme acetoacetyl succinyl CoA transferase (also called succinyl CoA: 3-oxoacid CoA transferase).
This enzyme uses succinyl CoA, an intermediate of the tricarboxylic acid cycle, as the CoA donor.
The third and final step in ketosis is the generation of 2 molecules of acetyl CoA from CoA and acetoacetyl CoA by the reversible enzyme acetoacetyl CoA thiolase.
The acetyl CoA formed is then oxidized in non-liver tissues through the TCA cycle. Acetoacetyl succinyl CoA transferase is the rate determining step in ketolysis.
Its activity is highest in the heart and kidney, followed by the central nervous system and skeletal muscle.
Due to the large mass of skeletal muscle, this tissue accounts for the highest fraction of the body’s total resting ketone utilization.
Acetoacetyl succinyl CoA transferase activity is negatively regulated by high intracellular levels (> 5 mM) of acetoacetate (AcAc).
This phenomenon is responsible for the observed increase in circulating levels of ketone bodies during the first phases (3 days to 2 weeks) of starvation.
Acetoacetyl succinyl CoA transferase activity is also present, but at very low levels, in the liver.
Acetoacetyl CoA thiolase, the enzyme responsible for the last key step in ketolysis in extrahepatic tissues, tends to enhance the production of acetyl CoA from acetoacetyl CoA.
Acetoacetyl CoA thiolase is also present in the liver, the primary locus of ketogenesis.
There it plays a key role as a first step in ketogenesis: the creation of acetoacetyl CoA from two molecules of acetyl CoA.
Acetoacetyl CoA thiolase is a multipurpose enzyme that participates in several other metabolic pathways, including the metabolism of fatty acids and the breakdown of some amino acids.
Regulation of ketogenesis
The rate of ketogenesis depends on the activity of three enzymes.
One is hormone-sensitive lipase (or triglyceride lipase), which is found in peripheral adipocytes.
The other two are acetyl CoA carboxylase and 3-hydroxy-3-methylglutaryl-CoA synthase (HMG CoA synthase), which are found in the liver.
Lipase is sensitive to hormones that catalyze the conversion of triglycerides to diglycerides for further degradation of free fatty acids (lipolysis) that serve as substrates for ketogenesis.
On the other hand, acetyl CoA carboxylase catalyses the conversion of acetyl CoA to malonyl CoA, increasing the hepatic level of the primary substrate of fatty acid biosynthesis.
Malonyl CoA levels vary in the liver directly in accordance with the rate of fatty acid synthesis and inversely with the rate of fatty acid oxidation.
Therefore, malonyl CoA plays a critical role in the regulation of ketogenesis.
Low levels of malonyl CoA stimulate the transport of fatty acids to the mitochondria via the carnitine shuttle for oxidation to ketone bodies.
Malonyl CoA normally inhibits carnitine palmitoyltransferase, the enzyme that transports fatty acyl CoA across the mitochondrial membrane.
The hormone-sensitive lipase and acetyl CoA carboxylase are exquisitely controlled by the level of circulating insulin.
It works by inhibiting ketogenesis, epinephrine, and glucagon (which act to stimulate ketogenesis).
Therefore, in fasting or diabetes, high glucagon levels and low insulin levels favor ketogenesis by promoting lipolysis in the adipocyte and stimulating fatty acid oxidation in the liver.
Insulin inhibits lipolysis and ketogenesis and stimulates lipogenesis by triggering inhibitory dephosphorylation of hormone-sensitive lipase and activating dephosphorylation of acetyl CoA carboxylase.
In adipocytes, dephosphorylation of hormone-sensitive lipase inhibits the breakdown of triglycerides into fatty acids and glycerol, the limiting step in the release of free fatty acids (lipolysis) from the adipocyte.
This reduces the amount of substrate that is available to generate acetyl CoA (through fatty acid oxidation) for ketogenesis.
Furthermore, insulin-mediated dephosphorylation of hepatic acetyl CoA carboxylase inhibitor sites increases malonyl CoA production.
It simultaneously reduces the rate at which fatty acids can enter the liver mitochondria for oxidation and the production of ketone bodies.
Glucagon stimulates ketogenesis by doing the opposite of insulin.
Glucagon triggers the phosphorylation of both hormone-sensitive lipase and acetyl CoA carboxylase by cyclic AMP-dependent protein kinase.
In adipocytes, phosphorylation of hormone-sensitive lipase by cyclic AMP-dependent protein kinase stimulates the release of fatty acids from triglycerides.
Glycerol diffuses freely out of adipose tissue into the circulation to transport it to the liver.
Free fatty acids enter the circulation and travel (bound to albumin) for uptake and metabolism in other tissues such as the heart, skeletal muscle, kidney, and liver.
In hepatocytes, phosphorylation of acetyl CoA carboxylase by cyclic AMP-dependent protein kinase reduces malonyl CoA production.
This, in turn, stimulates the absorption of fatty acids by the mitochondria, and therefore increases the amount of substrate available for ketogenesis.
Liver mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (HMG CoA synthase) is the third key enzyme involved in the control of ketogenesis.
The activity of this enzyme is increased by starvation and a high-fat diet, and decreased by insulin.
These factors modulate HMG CoA synthase activity by altering mRNA production and the post-translational phase of protein synthesis through reversible succinylation of the enzyme itself.
Increased HMG CoA synthase activity leads to the production of ketone bodies.
