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Utilization of ketone bodies, Regulation and Clinical significance of ketogenesis
1) Utilization of β-Hydroxy Butyrate
Beta-hydroxybutyrate is first oxidized to acetoacetate with the production of one NADH (Figure-1, step-1). In tissues actively utilizing ketones for energy production, NAD+/NADH ratio is always higher so as to drive the β-hydroxybutyrate dehydrogenase catalyzed reaction in the direction of acetoacetate synthesis.
D (-)-3-Hydroxybutyrate is oxidized to produce acetoacetate as well as NADH for use in oxidative phosphorylation. D (-)-3-Hydroxybutyrate is the main ketone body excreted in urine.
2) Utilization of Acetoacetate
a) 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 a trans esterification of the CoASH from succinyl CoA to acetoacetate by Coenzyme A transferase (Figure-1, step-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 this step 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 “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, very low-level of enzyme expression in the liver allows the liver to produce ketone bodies but not to utilize them. This ensures that extra hepatic tissues have access to ketone bodies as a fuel source during prolonged fasting and starvation, and also, lack of this enzyme in the liver prevents the futile cycle of synthesis and breakdown of acetoacetate.
b) The Acetoacetyl CoA is now cleaved by thiolase to produce two acetyl CoA molecules (figure-1-step-3).
This implies that the TCA cycle must be running to allow ketone body utilization; a fact which is necessarily true, because the TCA cycle is necessary to allow generation of energy from acetyl-Co A.
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.
Figure-1- Utilization of ketone bodies, Acetoacetate can also be converted to Acetoacetyl co A by direct attachment of Co A from Co ASH, same as in activation of fatty acid, but this is a minor pathway .The major pathway proceeds through transfer of CoA from succinyl co A.
Regulation of ketosis
Ketogenesis is regulated at three steps (Figure-2)
1) Lipolysis in Adipose tissue
- Ketosis does not occur unless there is an increase in the level of circulating free fatty acids that arise from lipolysis of triacylglycerol in adipose tissue.
- When glucose levels fall, lipolysis induced by glucagon secretion causes increased hepatic ketogenesis due to increased substrate (free fatty acids) delivery from adipose tissue.
- Conversely, insulin, released in the well-fed state, inhibits ketogenesis via the triggering dephosphorylation and inactivation of adipose tissue hormone sensitive lipase (HSL).
2) Fate of fatty acid-free fatty acids are either oxidized to CO2 or ketone bodies or esterified to triacylglycerol and phospholipids.
- There is regulation of entry of fatty acids into the oxidative pathway by carnitine Acyl transferase-I (CAT-I)
- Malonyl-CoA, the initial intermediate in fatty acid biosynthesis formed by acetyl-CoA carboxylase in the fed state, is a potent inhibitor of CAT-I.
- Under these conditions, free fatty acids enter the liver cell in low concentrations and are nearly all esterified to Acylglycerols and transported out of the liver in very low density lipoproteins (VLDL).
3) Fate of Acetyl co A
- The acetyl-CoA formed in beta-oxidation is oxidized in the citric acid cycle, or it enters the pathway of ketogenesis to form ketone bodies.
- As the level of serum free fatty acids is raised, proportionately more free fatty acids are converted to ketone bodies and less are oxidized via the citric acid cycle to CO2.
- Entry of acetyl CoA into the citric acid cycle depends on the availability of Oxaloacetate for the formation of citrate, but the concentration of Oxaloacetate is lowered if carbohydrate is unavailable or improperly utilized.
Figure-2- Regulation of ketosis takes place at 3 steps. 1) Lipolysis in adipose tissue, 2) Entry of free fatty acid in to the mitochondrion 3) Entry of acetyl co A in to TCA cycle
Ketonemia – increased concentration of ketone bodies in blood
- It is due to increased production of ketone bodies by the liver rather than to a deficiency in their utilization by extra hepatic tissues.
- The production of ketone bodies occurs at a relatively low rate during normal feeding and under conditions of normal physiological status.
- Normal physiological responses to carbohydrate shortages cause the liver to increase the production of ketone bodies from the acetyl-CoA generated from fatty acid oxidation.
- Ketonemia progresses to ketonuria (excessive excretion of ketone bodies in urine), that can be detected by Nitroprusside test.
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, but their excessive production leads to lowering of pH causing ketoacidosis.
a) Diabetic ketoacidosis
DKA results from relative or absolute insulin deficiency combined with counter regulatory hormone excess (glucagon, catecholamines, cortisol, and growth hormone).
The decreased ratio of insulin to glucagon promotes gluconeogenesis, glycogenolysis, and ketone body formation in the liver, as well as increases in substrate delivery from fat and muscle (free fatty acids, amino acids) to the liver
In most cases, the increase in ketone body concentration in blood is due to increased synthesis in liver; in severe ketoacidosis, cells begin to lose ability to use ketone bodies also.
b) Starvation induced ketosis
Prolonged fasting may result
- From an inability to obtain food
- From the desire to lose weight rapidly, or
- In clinical situations in which an individual cannot eat because of trauma, surgery, neoplasms, burns etc.
Biochemical basis of starvation induced ketogenesis
- In the absence of food the plasma levels of glucose, amino acids and triacylglycerols fall, triggering a decline in insulin secretion and an increase in glucagon release. The decreased insulin to glucagon ratio makes this period of nutritional deprivation a catabolic state, characterized by degradation of glycogen, triacylglycerol and protein (Figure-3)
- This sets in to motion an exchange of substrates between liver, adipose tissue, muscle and brain that is guided by two priorities-
(i) the need to maintain glucose level to sustain the energy metabolism of brain ,red blood cells and other glucose requiring cells and
(ii) to supply energy to other tissues by mobilizing fatty acids from adipose tissues and converting them to ketone bodies to supply energy to other cells of the body.
- In early stages of starvation, heart and skeletal muscle consume primarily ketone bodies to preserve glucose for use by the brain.
- After several weeks of starvation, ketone bodies become the major fuel of the brain.
Figure-3- During starvation, glucagon induced adipolysis increases the flow of free fatty acids to liver for oxidation. Excess of Acetyl co A are channeled towards the pathway of ketogenesis. Acetoacetate and beta hydroxy butyrate are poured in blood to be transported to peripheral cells for utilization. Liver cells lack the enzyme for their utilization.
Conditions causing ketosis
- Uncontrolled diabetes mellitus
- 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
- Nonpathologic forms of ketosis are found under conditions of high-fat feeding
- After severe exercise in the post absorptive state.
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