Utilization of ketone bodies and ketosis- lecture 3

Utilization of ketone bodies

The ketone bodies are water-soluble and are transported across the inner mitochondrial membrane as well as across the blood-brain barrier and cell membranes. Thus, they can be used as a fuel source by a variety of tissues including the CNS. They are preferred substrates for aerobic muscle and heart, thus sparing glucose when they are available.

Tissues that can use fatty acids can generally use ketone bodies in addition to other energy sources. The exceptions are the liver and the brain. The liver synthesizes ketone bodies but has little β-ketoacyl-CoA transferase, and therefore little ability to convert acetoacetate into acetyl-CoA. The brain does not normally use fatty acids, which do not cross the blood-brain barrier; under ordinary circumstances, the brain uses glucose as its sole energy source.

The metabolic rate of the brain is essentially constant. While other tissues reduce their metabolic requirements during starvation, the brain is unable to do so. After a few days of fasting,  the brain undergoes metabolic changes to adapt to the decreased availability of glucose. One major change is increased amounts of the enzymes necessary to metabolize ketone bodies. Extrahepatic tissues utilize ketone bodies via a series of cytosolic reactions that are essentially a reversal of ketone body synthesis, and the ketones must be reconverted to acetyl CoA in the mitochondria:

Steps (figure 1)

1) Beta-hydroxybutyrate is first oxidized to acetoacetate with the production of one NADH (1). It is important to appreciate that under conditions where tissues are utilizing ketones for energy production their NAD⁺/NADH ratios are going to be relatively high, thus driving the β-hydroxybutyrate dehydrogenase catalyzed reaction in the direction of acetoacetate synthesis. 

2) Coenzyme A must be added to the acetoacetate. The thioester bond is a high energy bond, so ATP equivalents must be used. In this case, the energy comes from transesterification of the CoASH from succinyl CoA to acetoacetate by Coenzyme A transferase (2), also called Succinyl co A: Acetoacetate co A transferase, also known as Thiophorase.

The succinyl CoA comes from the TCA cycle. This reaction bypasses the succinyl-CoA synthetase step of the TCA cycle. Hence there is no GTP formation at these steps although it does not alter the amount of carbon in the cycle.

The liver has acetoacetate available to supply to other organs because it lacks this particular CoA transferase, and that is the reason that “Ketone bodies are synthesized in the liver but utilized in the peripheral tissues.” The latter enzyme is present at high levels in most tissues except the liver. Importantly, a very low level of enzyme expression in the liver allows the liver to produce ketone bodies but not to utilize them. This ensures that extrahepatic tissues have access to ketone bodies as a fuel source during prolonged fasting and starvation. Also, a lack of this enzyme in the liver prevents the futile cycle of synthesis and breakdown of acetoacetate.

3) The Acetoacetyl CoA is now cleaved to two acetyl CoA’s with Thiolase (3).

This implies that the TCA cycle must be running to allow ketone body utilization, a necessarily true fact because the  TCA  cycle is necessary to allow generation of energy from acetyl-CoA.

D(-)-3-Hydroxybutyrate (beta-hydroxybutyrate) is oxidized to produce acetoacetate as well as NADH for use in oxidative phosphorylation.

Figure-1- showing the steps of the utilization of ketone bodies

If the blood level is raised, oxidation of ketone bodies increases until, at a concentration of approximately 12 mmol/L, they saturate the oxidative machinery. When this occurs, a large proportion of the oxygen consumption may be accounted for by the oxidation of ketone bodies.

Ketonemia

In most cases, ketonemia is due to increased production of ketone bodies by the liver rather than to a deficiency in their utilization by extrahepatic tissues. While extrahepatic tissues readily oxidize acetoacetate and D(-)-3-hydroxybutyrate, acetone is difficult to oxidize in vivo and to a large extent, is volatilized in the lungs.

Causes of ketosis

Excessive formation of ketone bodies occurs in the following conditions:

• Uncontrolled diabetes mellitus
• Starvation
• Chronic alcoholism
• Von- Gierke’s disease
• Heavy exercise
• Low carbohydrate diet- For weight loss
• Glycogen storage disease type 6(Due to phosphorylase kinase deficiency)
• Pyruvate carboxylase deficiency
• Prolonged ether anesthesia
• Toxemia of pregnancy
• Certain conditions of alkalosis 

Non-pathologic forms of ketosis are found under conditions of :

• High-fat feeding and after severe exercise in the post-absorptive state.

Ketoacidosis

Both β-hydroxybutyrate and acetoacetate are organic acids. These compounds are released in the protonated form, which means that their release tends to lower the pH of the blood. In normal individuals, other mechanisms compensate for the increased proton release. Individuals with untreated Type I diabetes mellitus often release ketone bodies in such large quantities that the normal pH-buffering mechanisms are overloaded; the reduced pH, in combination with a number of other metabolic abnormalities associated with lack of insulin results in diabetic ketoacidosis, an acute life-threatening disorder of Type I diabetes. In most cases, the increase in ketone body concentration in blood is due to increased synthesis in the liver; in severe ketoacidosis, cells begin to lose the ability to use ketone bodies also.

What is the effect of insulin, glucagon, or epinephrine upon lipolysis in adipose tissue? How can a decrease in the insulin/glucagon ratio explain the increased production of ketone bodies during a fast?

The primary regulator of ketone body synthesis is fatty acid availability. When hormonal conditions (e.g., high glucagon, low insulin) cause fatty acid concentration in the plasma to be high, malonyl CoA concentration in the liver cytoplasm is low (because acetyl CoA carboxylase is in the less active phosphorylated state). Fatty acyl CoA can enter the mitochondria at a high rate (because there is no inhibition of CAT I), and beta-oxidation proceeds at a high rate (figure2).

Figure 2- showing the effect of malonyl co A  on transportation of fatty acids (through carnitine shuttle) and the rate of ketogenesis

The ensuing high mitochondrial concentration of acetyl CoA results in active ketone body synthesis. Therefore, during conditions of glucose deprivation such as prolonged fasting/starvation or in diabetes mellitus, the rate of ketogenesis increases to accommodate the influx of acetyl co A and to provide fuel to the brain. The process of ketogenesis during conditions of glucose deprivation has been shown in figure 3.

Figure-3-showing the biochemical basis of ketosis during conditions of glucose deprivation.