In this article we will discuss about Ketosis:- 1. Meaning of Ketosis 2. Ketogenesis in the Liver 3. Regulation of Ketogenesis 4. Metabolism of Ketone Bodies 5. Effects 6. Prevention 7. Test for Ketone Bodies in the Urine.
Contents:
- Meaning of Ketosis
- Ketogenesis in the Liver
- Regulation of Ketogenesis
- Metabolism of Ketone Bodies
- Effects of Ketosis
- Prevention of Ketosis
- Test for Ketone Bodies in the Urine
1. Meaning of Ketosis:
The fatty acids undergo excessive oxidation in the liver under certain metabolic conditions producing large quantities of keto acids—acetoacetic acid and β-hydroxybutyric acid, which pass into the blood by diffusion.
Acetoacetic acid then undergoes spontaneous decarboxylation to produce acetone. These three substances—acetoacetate, β- hydroxybutyrate and acetone—are collectively known as the ketone bodies (acetone bodies or ketones).
Normally, the blood of mammals contains ketone bodies not exceeding 1 mg/100 ml. The concentration is little higher than this in ruminants. Daily excretion of ketone bodies of normal person is less than 1 mg.
Higher than normal quantities in the blood or urine constitute ketonemia (hyper-ketonemia) or ketonuria, respectively. The condition in which there is a high concentration of ketone bodies in tissues and blood is called ketosis.
Acetoacetic acid and β-hydroxybutyric acid are moderately strong acids and are buffered in blood or tissues. Their excretion in large quantities admits some loss of buffer cation (in spite of ammonia production by the kidney) which depletes the alkali reserve causing ketoacidosis.
The process of formation of ketone bodies is termed ketogenesis and the process of breakdown of ketone bodies taking place in peripheral tissues is called ketolysis.
2. Ketogenesis in the Liver:
Ketosis generally occurs in severe diabetes mellitus, prolonged starvation, glycogen storage diseases, toxemia of pregnancy, infective hepatic disease and continued fever.
Experimentally, it occurs in the oral administration of fatty acids, high fat diet, low carbohydrate diet, pancreatectomy, administration of growth hormone or ACTH. Under these conditions, there is diminished carbohydrate utilization and increased fat mobilization.
In ruminants, the rumen converts butyric acid formed from fermentation to β-hydroxybutyrate which enters the blood stream. The ruminant lactating mammary gland also produces ketone bodies. But these ketone bodies do not cause ketosis in these species.
Ketone bodies are formed in the liver but utilized in the extra hepatic tissue. Enzymes responsible for ketone body formation are associated mainly in the mitochondria. Acetyl-CoA (C2 units) formed in β-oxidation of fatty acids is the basic unit for the formation of ketone bodies. Two molecules of acetyl-CoA condense to form acetoacetyl-CoA by a reversal of thiolase reaction.
Two pathways have been proposed for the formation of acetoacetate from acetoacetyl-CoA:
a. First Pathway (Minor Pathway):
The first pathway is by simple deaeylation catalyzed by the enzyme acetoacetyl-CoA deacylase which is shown in Fig. 18.19.
b. The Second Pathway (Major Route):
It involves the condensation of acetoacetyl-CoA with another molecule of acetyl-CoA to form β-hydroxy-β-methyl glutaryl-CoA by HMG-CoA synthetase. HMG-CoA is splitted into acetoacetic acid and acetyl-CoA by HMG-CoA lyase present in mitochondria. From acetoacetic acid, acetone and β-hydroxybutyrate are formed which are shown in Fig. 18.20.
3. Regulation of Ketogenesis:
a. Ketosis does not occur in vivo unless the concentration of circulating free fatty acids increases by the lipolysis of triacylglycerol in adipose tissue. Free fatty acids are the precursors of ketone bodies in the liver. The factors regulating mobilization of free fatty acids from adipose tissue are important in controlling ketogenesis.
b. The activity of carnitine palmitoyl transferase I in the outer mitochondrial membrane regulates the entry of long chain acyl groups into mitochondria prior to β-oxidation. Fatty acid oxidation is depressed in fed state since the enzyme activity is low; but high in starvation when fatty acid oxidation increases.
In fed state, there is the increase in concentration of malonyl-CoA which inhibits this enzyme and thereby switch off β-oxidation. In fed state, free fatty acids enter the liver cells in low concentrations and all are esterified to acylglycerols and transported out of the liver in VLDL.
The concentration of free fatty acids increases with the onset of starvation, acetyl-CoA carboxylase is inhibited directly by acyl-CoA, and malonyl-CoA concentration is decreased releasing the inhibition of carnitine palmitoyl transferase- 1 and as a result, more acyl-CoA is β- oxidized.
There is the decrease in the [Insulin]/[Glucagon] ratio. Thus, β-oxidation from free fatty acids is controlled by the carnitine palmitoyl transferase-1 gateway into the mitochondria.
c. When the concentration of serum free fatty acids is raised, more free fatty acid is converted to ketone bodies and less is oxidized via the citric acid cycle to CO2.
The complete oxidation of one mol of palmitate produces 129 mol of ATP via β-oxidation and CO2 production in the citric acid cycle, whereas only 33 mol of ATP is produced when acetoacetate is the end product and only 21 mol when 3-hydroxybutyrate is the end product.
4. Metabolism of Ketone Bodies:
Acetoacetic acid and β-hydroxybutyric acids are carried from liver to extra hepatic tissues mainly kidney and muscle where they are oxidized for energy production after conversion to acetyl-CoA. The enzyme responsible for the activation of acetoacetate to acetoacetyl-CoA is absent from liver for which liver cannot utilize these acids.
Two reactions take place in extra hepatic tissues for the activation of acetoacetate to acetoacetyl-CoA.
In the first reaction, succinyl-CoA reacts with acetoacetic acid in presence of the enzyme acetoacetate-succinyl-CoA transferase (Thiophorase) to form acetoacetyl-CoA and succinate.
In the second reaction, acetoacetate is activated by ATP in presence of CoA catalyzed by acetoacetyl-CoA synthetase to form acetoacetyl-CoA.
Alternatively, β-hydroxybutyric acid is converted directly by β-hydroxy butyrate dehydrogenase in extra hepatic tissues to form acetoacetic acid which is then converted to acetoacetyl-CoA by any of these above two reactions. The acetoacetyl-CoA formed by these reactions is split to acetyl-CoA by thiolase and oxidized in the citric acid cycle. The reactions are given in Fig. 18.22.
Acetoacetate and β-hydroxybutyrate are readily oxidized by extra hepatic tissues but acetone is oxidized with difficulty and its rate of utilization is also very slow. Several pathways have been proposed for the utilization of acetone.
a. First Pathway:
Acetone is converted to acetoacetate by reversal of decarboxylation.
b. Second Pathway:
Acetone is converted to propanediol which can form 1 carbon (formate) unit and 2 carbon (acetate) units. Most of the evidence suggests that ketonemia is due to increased production of ketone bodies by the liver rather than to a deficiency in their utilization by extra hepatic tissues.
In moderate ketonemia, the loss of ketone bodies through the urine is only a few per cent of the total ketone body production and utilization. The formation, utilization and excretion of ketone bodies are shown in Fig. 18.23.
5. Effects of Ketosis:
a. Both acetoacetate and p-hydroxybutyrate are moderately strong acids. They neutralize bicarbonates resulting depletion of alkali of the body and produce metabolic acidosis. In case of severe ketosis, death may ensure from acidosis.
b. The excretion of ketone body in the urine involves the loss of Na+ in particular, leading to total electrolyte and Na+ deficiency.
c. The severe diabetic patient excretes large quantities of both ketone bodies and glucose in the urine with a large quantity of water and thus developing dehydration. In diabetic acidosis, there is severe alteration in cation-anion balance in the plasma.
6. Prevention of Ketosis:
a. In case of diabetic ketosis, carbohydrate diet, intramuscular injection of insulin and anti-ketogenic substance (aspartic acid) which may provide oxaloacetate by transamination should be administered.
b. In case of prolonged starvation ketosis, carbohydrate diet and anti-ketogenic substance (aspartic acid) which may provide oxaloacetate by transamination should be given.
c. The electrolytes and the fluids of the body must be restored by intravenous injection of isotonic solution of sodium salts such as NaCl, NaHCO3 or sodium lactate. Potassium salts are desired to be added.
7. Test for Ketone Bodies in the Urine:
Rothera’s test:
a. 5 ml of the urine is saturated with solid ammonium sulphate by shaking it vigorously.
b. 2 drops of freshly prepared 5 per cent solution of sodium nitroprusside and 1 ml of ammonium hydroxide are added.
c. Allowed to stand for a while.
A permanganate colour which appears just above the layer of the un-dissolved ammonium sulphate indicates the presence of ketone bodies. In normal individuals, the ketone bodies are excreted less in quantities in the urine. This negligible amount does not respond to Rothera’s test.
Hence, the ketone bodies in the normal urine are not detected by this test:
