Ketone body metabolism

1. KETONE BODIES METABOLISM BY Dr KHALED SALEH ALGARIRI IMS – MSU January 2015

2. OBJECTIVES What are Ketone Bodies ? How they are produced and Utilized ? How ketone body metabolism is regulated ? When and why excess amount of ketone bodies are produced ? What is Ketosis and Ketoacidosis ? Biochemical basis for diabetic and starvation ketosis and ketoacidosis.

3. INTRODUCTION When there is a adequate balance between . Lipid and carbohydrate metabolism, most of the acetyl CoA produced from the B-oxidation pathway is further processed through the citric acid cycle. The first step of the citric acid cycle involves the reaction between oxaloacetate and acetyl CoA . Sufficient oxaloacetate must be present for the acetyl CoA to react with .

4. Certain body conditions upset the lipid-carbohydrate balance required for acetyl CoA generated by fatty acids to be processed by the citric acid cycle. These condition include dietary intake high in fat and low in carbohydrates Diabetic condition where the body cannot adequately processed glucose even though it is present Prolonged fasting condition, including starvation, where glycogen supplies are exhausted . Under these conditions, the problem of adequate oxaloacetate supplies arises, which is compounded by the body’s using oxaloacetate that is present to produce glucose through gluconeogenesis.

5. Ketone body is one of the three substances ( acetoacetate, B- hydroxybutyrate and acetone ) produced from acetyl CoA when an excess of acetyl CoA from fatty acid dergadation accumulates because of triacylglycerol- carbohydrate metabolic inbalances . The structural formulas for the three ketone bodies, two of which are C4 molecules and the other a C3 molecule .

6. Triglyceride Fatty Acids Glycerol Albumin adipose tissue Fatty Acids liver Fatty Acids Fatty Acids albumin Acetyl Co A b -oxidation Energy for the Brain and Extrahepatic tissues TCA Cycle Hormone Sensitive Lipase Glucagon Epinephrine Glucocorticoids + 1 3 2 Fatty Acids mitochondria Ketone Bodies

7. KETOGENESIS Ketogenesis takes place in liver using Acetyl co A as a substrate or a precursor molecule. Enzymes responsible for ketone body formation are associated mainly with the mitochondria step 1 : first condensation : Ketogenesis begins as two acetyl CoA molecules combine to produce acetoacetate CoA, a reversal af the step of the B- oxidation. Step 2 : second condensation: Acetoacetyl CoA then reacts with a third acetyl CoA and water to produced 3-hydroxy-3-methlglutary CoA and CoA-SH

8. Step 3 : Chain cleavage . HMG-CoA is then cleaved to acetyl CoA and Acetoacetate. Step 4 : Reduction : Acetoacetate is reduced to B- hydroxybutyrate . The reducing agent is NADH.

9. Acetyl-CoA Acetyl-CoA Acetoacetyl -CoA HMG-CoA Acetoacetate Acetone β - Hydroxybutyrate CoA-SH Acetyl-CoA CoA-SH H 2 O Acetyl-CoA NADH+H + NAD CO 2 Thiolase HMG-CoA synthase HMG-CoA lyase β - Hydroxybutyrate dehydrogenase KETOGENESIS

10. The ratio of [3 Hydroxybutyrate ] / [Acetoacetate] in blood varies from 1:1 to 10:1 Acetone is volatile – Expelled out through Lungs Acetoacetate and 3 Hydroxybutyrate are excreted through urine Liver is not able to utilize ketone bodies due to the absence of the enzyme required to activate acetoacetate Extrahepatic tissues contain the enzyme required to activate acetoacetate (They are able to utilize ketone bodies) KETOGENESIS

11. β - Hydroxybutyrate Acetoacetate Acetoacetatyl CoA Acetyl-CoA 2 Succinyl CoA Succinate CoA transferase CoA-SH Thiolase KETOLYSIS NADH NAD + The liver has acetoacetate available to supply to other organs because it lacks the particular CoA transferase and that is the reason that “Ketone bodies are synthesized in the liver but utilized in the peripheral tissues”.   β- Hydroxybutyrate dehydrogenase

12. The liver extracts about 30% of the free fatty acids passing through it The factors regulating mobilization of free fatty acids from adipose tissue are important in controlling oxidation of fatty acids Increased Mobilization of fatty acid Increased Ketogenesis REGULATION OF KETOGENESIS Regulated at three crucial steps 1) Lipolysis in Adipose tissue

13. Ketogenesis 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).

14. 2) Fate of fatty acid - free fatty acids are either  oxidized  to CO 2  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).

15.   Carnitine Acyl T ransferase -I  (CAT-I) [INSULIN] / [GLUCAGON] AcylCoA Triglycerol Fatty Acid AcylCoA AcetylCoA Carboxylase Citrate Oxaloacetate Β -Oxidation Pyruvate Oxaloacetate Glucose Acetyl CoA Malonyl CoA Pyruvate AcetylCoA Citrate CAT - I _ AcetylCoA [INSULIN] / [GLUCAGON]

16. 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 CO 2 . 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. 

17. Ketosis is a disorder of excessive production of ketone bodies The concentration of total ketone bodies in blood of well fed mammals does not exceed 0.2mmol/L Loss via urine is usually less than 1 mg/24h in humans Blood level of ketone bodies increased – Ketonemia Excretion of ketone bodies in urine increased - Ketonuria KETOSIS / KETONEMIA & KETONURIA

18. CLINICAL SIGNIFICANCE of KETOACIDOSIS Ketone bodies ( acetoacetic acid & 3-OH Butyric acid ) are acidic in nature Hydrogen ions are neutralized by bicarbonate (HCO 3 - ) of the blood . Bicarbonate (HCO 3 - ) level of the blood decreases and results metabolic acidosis. When  ketone bodies are released in large  quantities the normal  pH-buffering mechanisms are overloaded ; the reduced pH, in combination with a number of other metabolic  abnormalities  results in KETOACIDOSIS . In severe ketoacidosis, cells begin to lose ability to use ketone bodies also.

19. CLINICAL FEATURES OF KETOSIS Acidosis Smell of acetone in patient's breath . Osmotic diuresis induced by ketonuria may lead to dehydration Sodium loss ( The ketone bodies are excreted in urine as their sodium salt) Dehydration Coma

20. seen when there is excess fatty acid oxidation by the liver KETOACIDOSIS Excess fatty acid oxidation by the liver – when there is excess mobilization of the fatty acids from adipose tissue Excess mobilization of fat from adipose tissue when (Insulin : Glucagon ) Uncontrolled Diabetes Mellitus : Diabetic ketoacidosis Prolonged Starvation: Starvation Ketoacidosis

21. 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. 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 .

22. The decreased insulin to glucagon ratio, makes this period of nutritional deprivation  a catabolic state , characterized by degradation of glycogen, triacylglycerol and protein. 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. STARVATION INDUCED KETOSIS

23. After about 3 days of starving liver forms lot of ketone bodies Brain fulfils 1/3 of its energy needs from Acetoacetate. Heart also uses Ketone bodies After several weeks of starvation ketone bodies become major fuel of brain ( brain derives 60-75% of energy from ketone bodies under conditions of prolonged starvation) Now only 40gm glucose / day is needed by brain compared to 120 gm/day on 1 st day of starvation

24. DIABETIC KETOACIDOSIS Diabetic Ketoacidosis (DKA) is a state of inadequate insulin levels resulting in high blood sugar and accumulation of organic acids and ketones in the blood. It happens predominantly in type 1 diabetes mellitus, But can also occur in type 2 diabetes mellitus under certain circumstances. This may be due to intercurrent illness  (pneumonia, influenza, gastroenteritis, a urinary tract infection),  pregnancy, inadequate insulin administration  (e.g. defective insulin pen device),  myocardial infarction  (heart attack),  stroke  or the use of cocaine.

25. STARVATION KETOACIDOSIS Excess mobilization of fatty acids Hyperglucagonemia alters hepatic metabolism to favour ketone body formation, through activation of the enzyme carnitine palmitoyltransferase I(CPT-I). Excess b -oxidation of fatty acids in the hepatocytes Excess ketone body formation [ INSULIN] / [GLUCAGON] Blood glucose level is decreased

26. DIABETIC KETOACIDOSIS The  decreased ratio of insulin to Glucagon  promotes Gluconeogenesis, glycogenolysis , and Ketone body formation  in the liver, as well as  increases in substrate delivery fr from fat and muscle  (free fatty acids, amino acids) to the liver Hyperglucagonemia alters hepatic metabolism to favor ketone body formation, through activation of the enzyme carnitine palmitoyltransferase -I . Excess b -oxidation of fatty acids in the hepatocytes Excess ketone body formation DKA results from relative or absolute insulin deficiency combined with counter regulatory hormone excess( Glucagon, cortisol, and growth hormone). [Insulin/Glucagon]

27. Diabetic Ketoacidosis may be diagnosed when the combination of  hyperglycemia (high blood sugars), ketones on urinalysis and acidosis are demonstrated.  DIABETIC KETOACIDOSIS

28. MANAGEMENT OF KETOACIDOSIS Treatment is to give insulin and glucose 1- When glucose and insulin are given intravenously, potassium is trapped within the cells 2-Fatal hypokalemia can occur 3-Clinician should always monitor the electrolytes Administration of bicarbonate, and maintenance of electrolyte and fluid balance