NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

Ketone Bodies: Synthesis, Utilization and Therapies

IntroductionKetone bodies are important as a source of energy for the brain and biosynthetic process when there is

lacks of glucose. Recent studies proved that cultured astrocytes are ketogenic cells, despite general

assumption that the ketone bodies are supplied by the liver to the brain (Guzmán & Blázquez, 2004).

Ketone bodies are the by-products of the partial oxidation of fatty acids in the liver and it is assumed

that when the availability of CHO is decreased for a short period of time in a significant degree, the

maximization of the oxidation of the fat is stimulated, forcing the ketone bodies to become the source

of fuel to the body (Adam-Perrot, Clifton & Brouns, 2006). Basically, ketone bodies are produced by the

liver to be used as an energy source when glucose is not readily available. Acetoacetate and 3-β-

hydroxybutyrate are the two main ketone bodies, while acetone is the least abundant ketone body.

They can be in higher concentration during prolonged exercise and fasting (Laffel, 1999). Even though

elevated ketone bodies can cause illnesses like diabetes, they also can be used as therapies for

neurological disease like epilepsy and Alzheimer’s.

The Synthesis of Ketone BodiesAcetyl-CoA is formed in the liver during the fatty acids oxidation and it either being converted to ketone

bodies or entered citric acid cycle. There is an increase in the formation of ketone bodies like

acetoacetate, acetone, and D-β-hydroxybutyrate. Acetone which is produced in lesser amount than the

other ketone bodies is exhaled, while acetoacetate and D-β-hydroxybutyrate are transported to tissues

except the extraheptic tissues by the blood (Lehninger, Nelson & Cox, 2005). The ketone bodies are then

converted to acetyl-CoA and oxidized in the citric acid cycle so that the energy needed by tissues. Even

though the brain preferably uses glucose as its source of fuel, it can still utilizes acetoacetate or D-β-

hydroxybutyrate when there is lacks of glucose, like during starvation. The export of ketone bodies to

extraheptic tissues from the liver enable the oxidation of fatty acids to be continued when there is no

oxidation of acetyl-CoA in the citric acid cycle (Lehninger, Nelson & Cox, 2005).

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NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

There is an increase in the formation of ketone bodies during starvation due to a number of factors.

Prolonged low levels of insulin cause an increase in the release of fatty acid from adipose, resulting in

increases in the amount of the enzymes needed to synthesize and utilize ketone bodies (Brandt, 2002).

Furthermore, increases demand of gluconeogenesis in the liver causes the oxaloacetate to deplete,

forcing the decreases of the capacity of the TCA cycle, which in turn causes an increase in the amount of

acetyl-CoA that acts as the substrate for ketone bodies production (Brandt, 2002).

Figure 1.

Thiolase, which is the same enzyme used in the cleavage step in β-oxidation, is the first enzyme used in

the ketone body synthesis pathway. Thiolase catalyzes the condensation of two-acetyl-CoA molecules,

resulting in the production of acetoacetyl-CoA (Brandt, 2002). A third acetyl-CoA molecule is added by

HMG-CoA synthase resulting in the formation of β-hydroxy-β-methylglutaryl-CoA (HMG-CoA). The

product is used for the synthesis of ketone bodies in mitochondria and it is also important to

biosynthetic pathway. Acetyl-CoA is then released from HMG-CoA by HMG-CoA lyase, forming

acetoacetate (Brandt, 2002).

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NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

Β-hydroxybutyrate dehydrogenase is the final enzyme in the synthesis of ketone bodies. It reduces

acetoacetate to β-hydroxybutyrate. Β-hydroxybutyrate dehydrogenase stores energy equivalent to an

NADH in the ketone body to be exported to the tissue and it also produces a more stable molecule.

Acetoacetate can spontaneously decarboxylate, producing an acetone, as it is a β – ketoacid. Acetone is

mostly excreted through the lungs and it is a volatile waste product (Brandt, 2002).

Acetoacetate and β-hydroxybutyrate are then released into circulation, with the ration depending on

the concentration of NADH available in the liver mitochondria. A higher concentration of β-

hydroxybutyrate will be released by the liver if the amount of NADH is high. Acetoacetate and β-

hydroxybutyrate are released in the protonated form, resulting in the lower blood pH as they are

released in the protonated form (Brandt, 2002). Patients suffering from untreated Type I diabetes

mellitus tend to release higher concentration of ketone bodies, causing the overloading of the normal

pH-buffering mechanisms. Consequently, the pH level decreases, lack of insulin causes metabolic

abnormalities associated with ketoacidosis, which is an acute disorder of Type I diabetes. In some

ketoacidosis cases, cells unable to use ketone bodies (Brandt, 2002).

The reason for multienzyme pathway is still unknown but the third acetyl-CoA used functions as a

catalyst. The cell requires HMG-CoA synthase for other reasons; HMG-COA lyase is needed. Having

HMG-CoA synthase ad HMG-CoA lyase, which are two mitochondrial enzymes, are important in

controlling the pathway involving ketone body synthesis (Brandt, 2002).

Control of Ketone Body SynthesisThere are many factors affecting the production of ketone body. The first is the production of the high

level of circulation fatty acids, which can be promoted by low-carbohydrate intakes that causes the

reduction of the circulating insulin level. The high level of circulating fatty acids is then used for oxidation

and production of ketone bodies (Adam-Perrot, Clifton & Brouns, 2006). Current studies shows that low-

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NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

carbohydrate diets not only decrease the levels of fasting glucose, insulin and circulating triglyceride, but

also increase weight loss and improves blood pressure (Adam-Perrot, Clifton & Brouns, 2006). Two

things can happen to the fatty acids: conversion to triacylglycerol, or to ketone body. However, the fatty

acids are being converted to ketone body when glycolytic and glucogenic substrates are limiting the liver

as the glycerol needed for triacylglycerol synthesis is derived from glycolysis (Brandt, 2002).

The second factor affecting the production of ketone body is the liver must have excess energy because

high concentration of circulationg fatty acids supplies the energy so that the liver can divert excess

acetyl-CoA to synthesize ketone body (Brandt, 2002). Production of mitochondrial HMG-CoA synthase

and HMG-CoA lyase, which are stimulated with the prolonged of low levels of insulin, favor ketone body

production if there are high concentration of acetyl-CoA and decreases of glucogenic substrates (Brandt,

2002).

Ketone Body Utilization

The rate of uptake of ketone bodies at physiological concentration in a rate that had fasted for 48 hours

is the same as the glucose. It is possible that the oxidation of the ketone bodies occurs as glucose

oxidation in the fasted and fed rat are inhibited. Glucose oxidation in the fasted rats seems to be

completely abolished. There is a negligible uptake from perfusate containing ketones at concentration

close to those in the in vivo plasma as ketone bodies are not fuel for the intestine of the fed rats

(Hanson & Parsons, 1978). The lack of a difference between fasted and fed rats in acetoacetate

utilization and D-β-hydroxybutyrate shows that the metabolism of ketone bodies are influenced by the

concentration of ketone bodies in the plasma. Unlike in the intestine, rates of uptake of acetoacetate

and D-β-hydroxybutyrate is different, with the uptake of the acetoacetate is higher, in skeletal muscle.

This is because muscle has a lower activity of D-β-hydroxybutyrate than intestine. The intestinal smooth

muscle and the intestinal epithelial cells also utilize ketone bodies as both contain the enzyme needed

to metabolized ketone bodies (Hanson & Parsons, 1978).

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NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

Ketone bodies are used in the tissues than can utilize fatty acids, except the liver and the brain. This is

because the liver uses ketone bodies but has lack of ability to produce acetyl-CoA from acetoacetate as

there is little β-ketoacyl-CoA transferase (Brandt, 2002). As for the brain, fatty acids cannot cross the

blood-brain barrier; so, for its main energy source, the brain normally utilizes glucose. Normally, the

brain has a constant metabolic rate even during starvation where the brain is unable to reduce its

metabolic requirement unlike other tissues. The brain experiences metabolic changes after days of

fasting so that it can adapt to the decreased in the availability of glucose. In order to metabolize ketone

bodies, one major change are required which is to increase the amounts of the enzyme needed (Brandt,

2002).

To utilize ketone bodies, β-ketoacyl-CoA transferase is needed to convert acetoacetate to acetoacetyl-

CoA. This enzyme do not present in the biosynthetic pathway of the ketone body. Without this enzyme

in the liver, synthesis and breakdown of acetoacetate is not futile (Brandt, 2002).

Figure 2.

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NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

Succinyl-CoA donates its CoA to β-ketoacyl-CoA transferase, resulting in the formation of succinate and

acetoacetyl-CoA. Even though it does not alter the count of carbon in the cycle, this reaction bypasses

the succinyl-CoA synthetase step of the TCA cycle. This indicates that to allow ketone body utilization,

the TCA cycle must be running as the cycle is needed to allow acetyl-CoA to generate energy. Thiolase

and β-hydroxybutyrate dehydrogenase, which are the other enzyme of the ketone body utilization

pathway, are identical to the enzyme used for ketone body synthesis (Brandt, 2002).

Potential for Therapeutic UsesKetosis, which is an elevation of D-β-hydroxybutyrate and acetoacetate, is the main causes to the

survival of human are starvation by supplying non-glucose substrate to the hypertrophied brain,

consequently saving the muscle from destruction from the glucose synthesis. In addition, D-β-

hydroxybutyrate also provides an efficient source of energy for brain per unit oxygen. It has proven to

lower the cell death in a model of Alzheimer’s and Parkinson’s disease. These show the possibility that

ketosis might be beneficial to neurological disorder, whether it is acquired or genetic (Veech, Chance,

Kashiwaya, Lardy & Cahill Jr, 2001).

Alzheimer’s DiseaseAlzheimer’s disease is related to about 5 different genes which lead to the accumulation of amyloid

proteins. The metabolisms of ketone bodies can by-pass a blockade of the pyruvate dehydrogenase

multienzyme complex and also can recognize the intra and extracellular accumulation of amyloid

peptides. The activity of glycogen synthase kinase 3β which causes the phosphorylation and inhibition of

pyruvate in primary cultures of hippocampal neurons is stimulated by a fragment of the beta chain of

amyloid. In addition, the beta chain at the fragment 1-42 of amyloid, Aβ1-42, causes the inhibition of

acetyl choline synthesis in cultures of septal neurons (Veech, Chance, Kashiwaya, Lardy & Cahill Jr,

2001). Inhibition of pyruvate dehydrogenase lowers the accumulation of citrate, which is the precursor

of acetyl choline, by the actions of choline acetyl transferase and citrate cleavage enzyme. The decrease

in citrate concentration can be overcome by the metabolisms of ketones. Cultured hippocampal neurons

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NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

die when they are being exposed to Aβ1-42; thus, to prevent the death, D-β-hydroxybutyrate is added. In

addition of this finding, the hope for new therapies is available with the finding that the aspartate

protease is responsible for the cleavage of the 1-42 fragments (Veech, Chance, Kashiwaya, Lardy & Cahill

Jr, 2001)

EpilepsyEpilepsy is a disorder that causes recurrent seizures and epilepsy seizures result from synchronous,

excessive, and abnormal electrical firing patterns. Many epileptics’ seizures occur without signs of

organic brain disorder, unlike acquired or symptomatic seizures that cause by brain injury,

neurostructural change or disease. Even though there are extensive research for antiepileptic drugs,

many epileptic seizures still unmanageable and there are possible negative side-effect like nervousness,

constipation, memory difficulties and somnolence, arise from the current antiepileptic drugs (Green,

Todorova & Seyfried, 2003).

Fasting is one of the accredited antiepileptic therapies that are effective for many seizures disorder. This

method was linked with increased blood ketone levels and decreased blood glucose. However, seizures

protection was lost via the intake of glucose or food as the falling of blood ketone levels and the rising of

blood glucose levels occurs. The metabolism of ketone bodies causes the changes in the distribution and

content of neurotransmitters (Green, Todorova & Seyfried, 2003). Glutamic acid decarboxylase activity,

which increases the GABA content in synaptosomes, is stimulated by ketones. Changes in TCA that are

caused by ketone reduce the concentration of brain aspartate, which is an excitatory neurotransmitter

implicated in epilepsy, as it favor the formation of glutamate. The combination of the increases in GABA

and the decreases in excitatory neurotransmitters lower neuronal excitation (Green, Todorova &

Seyfried, 2003).

ConclusionFatty acids produced from triacylglycerol breakdown can be converted to ketone bodies, which can cross

the blood-brain barrier and also can be synthesized from acetyl-CoA. It is synthesized from acetyl-CoA in

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NUR LINA ABDUL AZIZExtra Credit Research Paper – Ketone Bodies: Synthesis, Utilization and Therapies

the mitochondria of the liver using a short pathway with thiolase as the initial substrate (Brandt, 2002).

The important control in the production of ketone bodies is that the thiolase condensation reaction is

favored by the high levels of acetyl-CoA as it can produce acetyl-CoA. Using β-ketoacyl-CoA transferase,

ketone bodies can be converted back to acetyl-CoA. Liver mitochondria do not have this enzyme so the

brain has to increases the synthesis of β-ketoacyl-CoA transferase during starvation. By doing so, it can

increase the ability to produce energy from these compounds. So, ketone bodies can supply energy to

the brain when shortage of glucose occurred (Brandt, 2002).

References

1. Veech, R. L., Chance, B., Kashiwaya, Y., Lardy, H. A. and Cahill, G. F. (2001), Ketone Bodies,

Potential Therapeutic Uses. IUBMB Life, 51: 241–247. doi: 10.1080/152165401753311780

2. Greene, A. E., Todorova, M. T. and Seyfried, T. N. (2003), Perspectives on the metabolic

management of epilepsy through dietary reduction of glucose and elevation of ketone bodies.

Journal of Neurochemistry, 86: 529–537. doi: 10.1046/j.1471-4159.2003.01862.x

3. Brandt, M. (2002) Ketone bodies. Copyright © 2000-2003 Mark Brandt, Ph.D Retrieved from

http://www.rose-hulman.edu/~brandt/Chem330/Ketone_bodies.pdf

4. Guzmán M, Blázquez C. (2004) Ketone body synthesis in the brain: possible neuroprotective

effects. Prostaglandins Leukot Essent Fatty Acids. ; 70(3):287-92. Review. PubMed PMID:

14769487

5. Hanson, P. J., & Parsons, D. S. (1978). Factors affecting the utilization of ketone bodies and other

substrates by rat jejunum: Effects of fasting and of diabetes. The Journal of Physiology, 278(1),

55-67.

6. Adam-Perrot, A., Clifton, P., & Brouns, F. (2006). high level of circulating fatty acids, used for

oxidation and production of ketone bodies. The International Association for the Study of

Obesity, 7, 49-58. Retrieved from https://docs.google.com/viewer?

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