Ketogenesis & Ketolysis Ketosis (ketoacidosis) Metabolism of Cholesterol

Ketogenesis It is the formation of ketone bodies in the liver mitochondria. Ketone bodies are: CH3-CO-CH2-COOH Acetoacetic acid CH3-CHOH-CH2-COOH β-hydroxybutyric acid CH3-CO-CH3 Acetone (non-metabolized product)

Ketone bodies are formed from acetyl CoA resulting from β oxidation of FA in excess of optimal function of Kreb's cycle. Under normal fed state: the hepatic production of acetoacetate and β hydroxybutyrate is minimal and the concentration of these compounds in the blood is very low (does not exceed 1 mg% or <0.2 mM). Most acetyl CoA fatty acid or pyruvate oxidation enter the citric acid cycle only if fat and carbohydrates degrdation are balanced.

Steps synthesis of Ketone bodies: Two molecules of acetyl CoA react with each other in the presence of thiolase enzyme to form acetoacetyl CoA.

(3 or β hydroxyl- 3or β methyl glutaryl CoA) Condensation of acetoacetyl CoA with acetyl CoA to form HMG CoA (3 or β hydroxyl- 3or β methyl glutaryl CoA) catalyzed by HMG CoA synthetase,

HMG-CoA lyase enzyme catalyzes the cleavage of HMG-CoA to acetoacetate and acetyl CoA.

Acetoacetate produces β-hydroxybutyrate in a reaction catalyzed by β-hydroxybutyrate dehydrogenase in the present NADH.

Both acetoacetate and β-hydroxybutyrate can be transported across the mitochondrial membrane and the plasma membrane of the liver cells, enter to the blood stream to be used as a fuel by other cells of the body.

In the blood stream, small amounts of acetoacetate are spontaneously (non- enzymatically) decarboxyated to acetone.

Acetone is volatile and can not be detected in the blood. The odor of acetone may be detected in the breath and also in the urine of a person who has high level of ketone bodies in the blood. e.g. in severe diabetic ketoacidosis, while under normal conditions, acetone formation is negligible.

Regulation of Ketone body synthesis: HMG COA synthase is the regulatory enzyme Induced by increased fatty acids in the blood. It is inhibited by high level of CoASH, thus when fatty acids flows to the liver, CoASH used for its activation and for thiolase. Thus, CoASH levels are reduced and HMG CoA synthase is active and vice versa.

Importance of Ketogenesis Ketogenesis becomes of great significant during starvation when carbohydrate store are depleted and oxidation of fats becomes a major source of energy to the body. The brain normally uses glucose as the only fuel. After the diet has been changed to lower blood glucose for 3 days, the brain gets 25% of its energy from ketone bodies. After about 40 days, this goes up to 70%, but can not utilize FA.

Ketolysis Ketolysis is the complete oxidation of ketone bodies to C02 and water. Site: Mitochondria of extrahepatic tissues due to high activity of the enzymes acetoacetate thiokinase and thiophorase, but not in the liver due to deficiency of these enzymes

During glucose is in short supply (starvation) or in insulin deficiency, the mitochondria of Cardiac (70% of its energy) ,skeletal muscles and kidney can use free fatty acids as a source of energy. However, during prolonged starvation when supply of glucose is limited, the brain may utilize ketone bodies as the major fuel.

Mechanism: β-hydroxy butyrate is dehydrogenated forming acetoacetate, the reaction is catalyzed by β-hdyroxybutyrate dehydrogenase.

Activation of acetoacetate to acetoacetyl CoA occurs by one of two pathways:  One mechanism involves succinyl CoA and the enzyme succinyl CoA acetoacetate CoA transferase (CoA transferase). Other mechanism involves the activation of acetoacetate with ATP in presence of CoA SH catalyzed by thiokinase (Acetoacetyl CoA synthetase).

Acetoacetyl CoA is split to acetyl CoA by thioalse and oxidized via citric acid cycle to C02 and H20.

Importance of ketolysis: Utilization of acetone by tissues is very slow, it may be converted to propandiol which becomes oxidized to pyruvate, or splits to acetate and formate. Importance of ketolysis: Ketolysis completes the oxidation of FA which started in the liver. It is a major source of energy to extrahepatic tissues during starvation.

Energetics production from degradation of ketone bodies in peripheral tissue Acetoacetate is oxidized into 2 acety1 CoA , which enter the citric acid cycle. Activation of acetoacetate consumes 1 ATP , and the total amount of ATP from metabolism of 2 acety1 CoA is 24 – 1= 23 ATP 2. Conversion of β- hydroxybutyrate back into acetoacetate generates 1 NADH , which produces an additional 3ATP total ATP produce = 26ATP) (24 +3) – 1= 26 ATP after entering the electron transport chain .

which is necessary to convert acetoacetate into 2 acety1 CoA. 3. The liver cannot use ketones for fuel because it lacks the enzyme succiny CoA: acetoacetate CoA transferase (thiophorase), which is necessary to convert acetoacetate into 2 acety1 CoA.

Ketosis (ketoacidosis) It is the accumulation of the ketone bodies in the blood (Ketonemia) and their appearance in the urine (ketonuria) together with acetone odour in the breath and acetone can be detected in urine.

Mechanism: Ketosis can occur in any condition characterized by inhibited carbohydrate utilization and at the same time increased fatty acid oxidation. This condition associated with decreased insulin relative to the anti insulin hormones, leading to increased lipolysis and release of FFA from adipose tissue as well as decreased oxidation of glucose by the liver.

This increases the uptake and oxidation of FA by the liver forming excess acetyl COA. The decreased glucose oxidation decreases the availability of oxalacetic (because it will be directed for gluconeogenesis) and so the excessive amounts of active acetate will be directed for ketone bodies formation.

Causes of ketosis: Diabetes mellitus. Starvation. Unbalanced diet i.e. high fat, low carbohydrate diet. Renal glucosuria.

Effects of ketosis: Increased ketone bodies in blood is neutralized by the alkali reserve (blood buffers), this lead to metabolic acidosis. If ketone bodies are far high than the capacity of alkali reserve they will result in acidemia - uncompensated acidosis with a decrease in blood PH which is a serious that results in death if not treated.

Metabolism of Cholesterol Cholesterol is the most important animal sterols which is the precursor of all other steroid in the body e.g. corticosteroids, sex hormones, bile acids and vitamin D. Cholesterol biosynthesis: Cholesterol is derived about equally from the diet or manufactured de novo in cells of humans especially in liver , intestine, and adrenal cortex . Acetyl CoA is the source of all carbon atoms in cholesterol. All tissues containing nucleated cells are capable of synthesizing cholesterol.

The liver is the main source of plasma cholesterol but intestine also participates. The liver is the principle organ which removes cholesterol from blood. The enzymes involved in cholesterol biosynthesis are present in cytosol and microcosms of the cell.

Synthesis of cholesterol: Cholesterol is synthesized from cytosolic acetyl CoA which is transported from mitochondria via the citrate transport system. It starts by the condensation of three molecules of acetyl CoA with the formation of HMG CoA.

3. HMG CoA reduced to mevalonic acid (C6) is a reaction requiring NADPH+H+ and enzyme HMG CoA reductase. Two molecules of NADPH are consumed in the reaction.

Mevalonic is dehydrated and decarboxyalted to isoprenoid units. 5 Molecules of isopentenyl pyrophosphate are converted to squalene (30 C with liberation of phosphate then by cyclization and demethylation (-3 CH3 ) cholesterol (27 carbon).

Control of cholesterol biosynthesis: 1. Control of HMG CoA enzyme: HMG CoA reductase is the key enzyme, which exists in phosphorylated inactive and dephosphorylated active form. HMG CoA reductase is activated by insulin and by feeding carbohydrate. HMG CoA reductase is inhibited by glucagon, therefore its activity decrease during starvation, as starvation directing acetyl CoA to the formation of ketone bodies.

Cholesterol feeding inhibits liver HMG CoA reductase. Bile salts inhibit the intestinal HMG CoA reductase. 2- A second point of control is the cyclization of squaline to lanosterol.

Blood Cholesterol Plasma cholesterol is in a dynamic state, entering the blood complexed with lipoproteins and leaving the blood as tissues remove cholesterol from lipoproteins or degrade them intracellularly. Cholesterol occurs in plasma lipoproteins in 2 forms: free cholesterol (30%) and esterified form with long chain fatty acids (70%). It is the free cholesterols that exchanges between different lipoproteins and plasma membranes of cells.

Total cholesterol in plasma is normally between 140-300 mg/dl, 2/3 of this is esterified with long chain FA (linoleic). Cholesterol esters are continually hydrolysed in liver and resynthesized in plasma. Cholesterol is present in all the lipoproteins but in fasting more than 60% is carried in p lipoproteins (LDL).

Blood lipids Plasma lipids are usually measured after 12 hours fasting. The total plasma lipids ranges from 400-700 mg/dl (mean value 470 mg/dl). The different types of plasma lipids are as follows: