Ketosis and Ketoacidosis

Ketosis is a metabolic state in which the bodies energy supply comes from ketone bodies in the blood in contrast to a state of glycolysis in which there is adequate glucose breakdown. In most cases, ketosis results from a high metabolism of fatty acids which are converted to ketone bodies. Ketone bodies are formed from ketogenesis when liver glycogen stores are depleted. Most cells in the body utilize both ketone bodies and glucose for energy, and while in ketosis the body works to maintain normal metobolism so it ramps up gluconeogenesis. Gluconeogenesis is glucose synthesis used to go through glycolysis.

Most of the time ketosis is a short interval of time, although long-term ketosis may be a result of fasting or a dietary insufficiency of carbohydrates. In glycolysis, high levels of insulin are released which promotes storage of fat and delayed release of fat from adipose tissue. In ketosis, fat reserves are readily available and are consumed. For this reason, ketosis has become one of the more recent diet fads as a way to burn fat quickly and lose weight.

Ketoacidosis

Although similar, ketosis is not ketoacidosis. Ketoacidosis is a physiological life-threatening situation due to insulin deficiency. Ketone bodies are acidic, and acid-base homeostasis in the blood is normally maintained through bicarbonate buffering, respiratory compensation, and renal compensation. Prolonged excess of ketone bodies can overwhelm the normal compensatory mechanisms and cause a state of acidosis when the blood pH falls below 7.35.

There are multiple precipitating factors that leads to ketoacidosis, which is most prevalent in patients with type 1 diabetes. Ketoacidosis in the case of a patient with type 1 diabetes is deemed diabetic ketoacidosis (DKA). In established type 1 diabetes, patients often forget to take insulin, with non-compliance being the bigger issue. This does not rule out other causes of ketoacidosis, as those are still prevalent. Acute major illnesses such as a myocardial infarction, cerebrovascular accident, sepsis, or pancreatitis. Certain drugs that affect carbohydrate metabolism such as glucocorticoids, diuretics, or anti-psychotic agents can cause ketoacidosis. General malnutrition associated with physiological problems can also lead to ketoacidosis. Such disorders lead to psychological starvation, which leads to ketone production and if prolonged ketoacidosis.

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Clinical Presentation

The clinical presentation of DKA is a two headed monster. The earliest symptoms of marked hyperglycemia is polyuria, polydipsia, and unexplained weight loss. As the duration of hyperglycemia continues neurological symptoms, including lethargy, focal signs and obtundation develop. Further progression can lead to a coma. The other head is the extent of the metabolic acidosis due to the excess ketone bodies. As the acidemia worsens accompanied with it is abdominal pain which can sometimes be severe. The electrolyte imbalance and metabolic acidosis causes delayed gastric emptying and an ileus (obstruction of the bowel). Vomiting and nausea are common.

Diagnostic Evaluation

The initial laboratory evaluation of patients with suspected DKA should include a serum glucose to establish whether or not the patient is hyperglycemic or not. Its helpful to measure the serum electrolytes and calculate the anion gap, BUN, plasma creatinine, and a plasma osmolality. This gives a broad picture of the metabolic state of the patient. Urinalysis is commonly performed along with urine ketones measured by dipstick method. Serum ketones are also measured to assess whether or not the patient is undergoing ketogenesis. An arterial or venous blood gas can be helpful to determine whether the serum bicarbonate is substantially reduced, which presumptively leads to metabolic acidosis. This also aids in determining hypoxia if it is present.

Findings

Hyperglycemia and hyperosmolality are the two primary laboratory findings in patients with DKA. Patients with DKA have a high anion gap metabolic acidosis. Serum glucose often times exceeds 350-500 mg/dL. Three ketone bodies are produced and accumulate in DKA; acetoacetic acid, beta-hydroxybutyric, and acetone. Acetoacetic acid is the only true ketoacid. A serum ketone measurement gives levels of beta-hydroxybutyric, while a urine dipstick measures the presence of acetoacetic acid using the nitroprusside method.

Other findings that may or may not be present are leukocytosis, and lipidemia. The majority of patients with hyperglycemic emergencies present with leukocytosis, which is proportional to the degree of ketonemia. Patients with DKA also present with marked hyperlipidemia. Lipolysis, primarily caused from insulin deficiency, and to a lesser extent elevated levels of lipolytic hormones including catecholamines, GH, ACTH, and glucagon. Lipolysis releases glycerol and free fatty acids into circulation which causes insulin resistance and serves as the substrate for ketoacid generation in the hepatocyte mitochondria.

To recap, ketosis is a dietary manipulation that if done right can lead to results. Ketoacidosis is a life-threatening metabolic state that requires immediate medical care.

This discussion will be continued with the next article focusing on ketoacid generation.

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Glycolysis

Glycolysis is the first phase of a series of reactions for the catabolism of carbohydrates. Catabolism is the breakdown of larger molecules into its respective smaller constituents. Glycolysis is the first part of cellular respiration that generates pyruvate to be used in either anaerobic respiration in the absence of oxygen or in the TCA cycle in aerobic respiration which yields useable energy for cells. This will be a general outline of the steps in glycolysis.

The whole process can be broken down into an energy investment phase where ATP is being used and an energy payoff phase where ATP is being generated. Fructose-1,6-biphosphate is where the energy investment phase ends. That is where the last ATP has to be used for energy to drive glycolysis.

A simple equation can be remembered as a summary of glycolysis.

Glucose + 2 ADP + 2 phosphate ions + 2 NAD+ —-> 2 Pyruvate + 2 ATP + 2 NADH + 2 H20 + 2 H+.

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In the first step of glycolysis an addition of a high energy phosphate from ATP yields glucose-6-phosphate and ADP. This step is initialized by the enzyme hexokinase. G6P is more reactive than glucose.

In the next step, glucose-6-phosphate is converted to its isomer, fructose-6-phosphate by phosphoglucose isomerase.

In the third step of glycolysis, fructose-6-phosphate is converted to fructose-1,6-biphosphate by phosphofructokinase (PFK) and the addition of ATP. This is the committed step, meaning that fructose-1,6-biphosphate MUST be converted to pyruvate. This is also the end of the energy investment phase of glycolysis.

Fructose-1,6-biphosphate is converted to glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate catalyzed by aldolase. Glyceraldehyde-3 phosphate maintains a reversible reaction with dihydroxyacetone phosphate through triode phosphate isomerase. The resulting reaction generates two molecules of glyceraldehyde-3-phosphate. A key point going forward is that two molecules of each substrate are produced.

Each G3P molecule gains an inorganic phosphate and with the addition of NAD+ to form the energized carrier molecules NADH. The resulting reaction catalyzed by glyeraldehyde-3-phosphate dehydrogenase generates two molecules of 1,3-bisphophoglycerate which yield two high energy phosphates.

Through the addition of two low energy ADP molecules and the enzyme phosphoglycerate kinase, the two molecules of 1,3-bisphophoglycerate are converted to 3-phosphoglycerate and yields two molecules of ATP. This reaction is called the break even reaction because at this point the energy input is equal to the energy output. Two molecules of ATP were expended and at this step there was a generation of two ATP molecules.

In the next step the two molecules of 3-phosphoglyercate are converted to 2-phosphoglycerate through the enzymatic properties of phosphoglycerate mutase.

The molecules of 2-phosphoglycerate are converted to phosphoenolpyruvate catalyzed by enolase. This step yields H20 molecules.

In the final step of glycolysis, the molecules of phosphoenolpyruvate are converted to pyruvate catalyzed by pyruvate kinase. ATP is generated from the addition of ADP and the two high energy phosphates from the molecules of phosphoenolpyruvate.

Upon the completion of glycolysis, the pyruvate molecules can be oxidized to carbon dioxide in cellular respiration to generate 28 molecules of ATP.

The NADH that is produced is turned back into NAD+ to drive further glycolysis. There are two ways to accomplish this. In the presence of oxygen NADH passes it electrons into the electron transport chain, which regenerates NAD+ for use in glycolysis. In the absence of oxygen, cells regenerate NAD+ by undergoing fermentation.

 

Lactate Dehydrogenase

Lactate dehydrogenase (LD, LDH) is an enzyme that is found in all cells in all tissues of the body. It catalyzes the reversible conversion of lactate to pyruvic acid in glycolysis and gluconeogenesis. It is released from various anatomical sites of the body in response to cellular injury and damage. It is used as a common marker for tissue damage and disease such as heart failure. Its down-fall is that it isn’t very specific to which tissue is damaged, but there are subtle hints that can clue a physician in particular directions.

Lactate dehydrogenase is structurally composed of four subunits, but the two common subunits are LDHA known as LDH-M, and LDHB, known as LDH-H. The only difference between the two subunits is that their is an amino acid substitution of alanine with glutamine within the H subunit. This amino acid change slightly changes the biochemical properties of the two subunits slightly in that the H subunit can bind faster, but the catalytic activity of the M subunit does not deteriorate at the same rate as the H subunit, it holds well.

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The two subunit of LDH can form five isomers which are found in various sites within the body;

LDH-1 (4H)- Found in the heart, RBCs, and the brain.

LDH-2 (3H1M)- Found in the RES.

LDH-3 (2H2M)- Found in the lungs.

LDH-4 (1H3M)- Found in the kidneys, placenta, and the pancreas.

LDH-5 (4M)- Found in the liver and striated muscle.

LDH is a protein that is found in small amounts normally in the body and there are various conditions that can cause an elevation. Cancer can raise the LDH levels within the body. Cancer cells rely on increased glycolysis due to their high energy demand. LDH elevation in cancer is often times referred to the Warburg effect which allows malignant cells to convert glucose stores into lactate even in the presence of aerobic respiration. This shifts glucose metabolism from simple energy production to accelerate cell growth and proliferation.

Hemolysis can be measured as LDH is abundant in RBCs and can be measured. Although measures should be taken to correctly receive the sample as incorrect procedures an cause hemolysis and a false-positive elevation in LDH levels among other substrates and electrolytes.

It can also be used as a marker for myocardial infarction. Normally LDH-2 is at a higher level than LDH-1. When someone experiences a myocardial infarction, levels of LDH-1 will be significantly elevated to a level higher than LDH-2. This is known as the LDH flip and is diagnostic in patients who have experienced myocardial infarction. Elevation of LDH peaks 3-4 days after MI, and can remain elevated for up to 10 days. LDH has since been replaced by the troponin test, which is a much more specific and sensitive test in diagnosing MI.

High levels of LDH in the cerebrospinal fluid (CSF) can indicate bacterial meningitis. Elevated LDH levels in viral meningitis is indicative of a poor prognosis.

LDH is an important tool that physicians don’t always utilize to its lack of specificity, but it can still be helpful in a diagnosis. Its important not to ignore any test and any result as it still contributes to the whole picture.

-Caleb