Introduction

Ketone bodies are prominent fuel sources for all evolutionary domains of life. The body can use ketones as a source of energy in the absence of a carbohydrate source. Ketones make up 5% to 20% of the human body's total energy expenditure. The liver converts fatty acids into ketone bodies that travel to other organs via blood. This process is especially important when an individual's blood glucose has decreased, and they must maintain an energy source for organs such as the brain. Ketone metabolism consists of the oxidation and utilization of ketone bodies by mitochondria, especially in organs with high energy demand. This process produces NADH and FADH2 for the electron transport chain and delivers acetyl CoA for gluconeogenesis. Prolonged fasting or vigorous exercise may lead to an excess of ketones and cause ketosis. One of the most feared complications in the setting of ketosis is in diabetic patients. When diabetic patients do not receive enough insulin physiologically or from supplementation, they will inappropriately enter ketosis, leading to diabetic ketoacidosis (DKA).[1][2]

Molecular Level

Ketone is a name for a specific elemental structure in organic chemistry. A ketone consists of a single bond to two CH3 or R groups with a double bond to an oxygen molecule. Acetone, 3-B-hydroxybutyrate (3HB), and acetoacetate all contain a ketone group and are therefore very soluble in the body tissues. The solubility of these ketones allows for transport through the body to various tissues.

The important rate-limiting steps in ketone metabolism include hormone-sensitive lipase (HSL), acetyl CoA carboxylase, succinyl CoA-oxoacid transferase (SCOT), and HMG CoA synthase. HSL and HMG CoA synthase are inhibited by insulin and stimulated by glucagon. Acetyl CoA carboxylase is stimulated by insulin and inhibited by glucagon. All three of these enzymes have the same effect of slowing down ketone production in the presence of insulin and increasing it in the presence of glucagon. Finally, increased levels of acetoacetate in the mitochondria of target organs inhibit SCOT and therefore inhibit ketone metabolism.[2][3][4]

Mechanism

The process of ketogenesis begins with fatty acyl CoA molecules. These molecules arise from the lipolysis of long-chain fatty acids via hormone-sensitive lipase. Triglycerols and amino acids may also be sources for Acetyl CoA; however, these sources usually add up to less than 10% of the total. The regulation of hormone-sensitive lipase (HSL) is via negative feedback from increases in insulin and glucose concentration. Positive feedback from glucagon and beta-adrenergic catecholamines increase HSL activity to provide more fatty acyl CoA molecules. HSL regulation takes place via phosphorylation by protein kinase A (PKA). PKA is activated by cyclic AMP (cAMP), which is directly downstream from the cell surface receptor affected by hormones. Fatty acids pass through the cell membrane and circulate in the blood. Certain tissues of the body, such as skeletal muscle, myocardium, and liver can use fatty acids as an energy source, which contrasts with the fact that the brain cannot utilize fatty acids for energy and must use ketone bodies as a means of energy transport from fat stores.

Fatty acids in the blood are converted to ketone bodies when insulin is low, and the fatty acid concentration is high. Fatty acyl CoA is transported into the liver mitochondria by the carnitine shuttle system. This system involves two transmembrane proteins to move fatty acyl CoA molecules across the mitochondrial membrane. The first protein is carnitine palmityl transferase I (CPT I), this protein on the cytosolic side of the mitochondrial membrane transfers the fatty acyl CoA across the outer membrane. During this process, a carnitine molecule is attached to the fatty acyl CoA molecule to make an acylcarnitine. The acylcarnitine is carried through the mitochondrial matrix by a transporter protein called carnitine/acylcarnitine translocase. At the inner mitochondrial membrane, the acylcarnitine molecule is converted back to acyl CoA and carnitine by CPT 2.

Ketone synthesis in the liver produces acetoacetate and beta-hydroxybutyrate from two acetyl CoA molecules. This process begins in the mitochondria of the liver after transporting the fatty acyl CoA molecule into the inner mitochondrial membrane by the carnitine shuttle. The fatty acyl CoA molecules undergo beta-oxidation to become acetyl CoA molecules. Acetyl CoA molecules are either converted to malonyl CoA by acetyl CoA carboxylase or acetoacetyl CoA by 3-ketothiolase. Malonyl CoA serves as negative feedback to the liver CPT-1. Acetoacetyl CoA is further converted to 3-hydroxy-3-methylglutaryl CoA (HMG CoA) by HMG CoA synthase. HMG CoA synthase is essential to this process, as it is the rate-limiting step for the synthesis of ketone bodies. HMG CoA synthase regulation is influenced positively by glucagon and negatively regulated by insulin. HMG CoA is finally converted to acetoacetate by HMG CoA lyase. At this point, acetoacetate may be converted to 3-B-hydroxybutyrate (3HB) by 3HB dehydrogenase. Acetoacetate and 3HB are organic acids that diffuse freely across cell membranes into the blood and other organs of the body.

Upon arrival to the mitochondria of distant organs, ketone bodies become utilized for energy. The first step involved is an enzyme that converts acetoacetate to acetoacetyl CoA. The enzyme responsible for this conversion is called succinyl CoA-oxoacid transferase (SCOT), and it is the rate-limiting step for the utilization of ketones for energy. High concentrations of acetoacetate feedback negatively on SCOT to decrease ketone conversion. Finally, acetoacetyl CoA is converted to acetyl CoA by methylacetoacetyl CoA thiolase.

Acetyl CoA may be turned into citrate and churned through the citric acid cycle to produce FADH2 and NADH, or it can be converted to oxaloacetate and used in gluconeogenesis.[2][3][4][5][6]

Testing

A traditional method to detect elevated ketones is to smell the patient's breath. Acetone and the other ketone bodies have a distinct fruit-like smell that is detectable by a clinician. A urine dipstick test can be used to measure the presence of ketone bodies qualitatively. A nitroprusside stain detects acetone and acetoacetate in the urine. The urine test utilizes a range of 0 to +4 where zero is undetectable, and +4 stands for a high amount detected. Newer quantitative lab studies have recently become available that measure the 3HB in the serum.[5][7]

Clinical Significance

The most common clinical manifestation that arises, associated with ketone bodies, is ketoacidosis. Ketosis is usually a normal physiological state. However, if unregulated, it may also drive life-threatening syndromes termed ketoacidosis. The most prevalent subdivisions of ketoacidosis are diabetic ketoacidosis (DKA) and alcoholic ketoacidosis.

DKA most commonly occurs as a result of a lack of appropriate response to insulin in the body. This lack of insulin response could be due to noncompliance with treatment, undiagnosed type 1 DM, or sub-therapeutic administration of insulin. A lack of insulin response results in an increase of glucagon and a decrease in the uptake of glucose. Decreased insulin also facilitates the increased activity of hormone-sensitive lipase. Hormone-sensitive lipase converts triglycerides to fatty acyl CoA molecules. Fatty acyl CoA molecules overload the Krebs cycle and shunt over to ketone metabolism. An abundance of ketone bodies causes an anion gap acidosis. Patients also usually present with hyperglycemia, lethargy, abdominal pain, polyuria, polydipsia, vomiting, and mental status changes.

Alcoholic ketoacidosis occurs in chronic alcoholics after an abrupt withdraw or acute intoxication. The ethanol metabolizes to acetic acid, and NAD+ is converted to NADH while acting as a coenzyme. Acetic acid is shunted to ketogenesis if insulin and glucagon are at favorable concentrations, such as in hypoglycemia. Additionally, NADH acts to inhibit gluconeogenesis because NAD+ is necessary for multiple steps in gluconeogenesis. Finally, during the acute withdraw of ethanol, the release of epinephrine further drives ketogenesis and, at that point, ketoacidosis.

Some inherited defects in ketone metabolism exist as well. The most common inherited deficiencies include a systemic primary carnitine deficiency and medium-chain acyl CoA dehydrogenase deficiency.

A patient with a systemic primary carnitine deficiency (CDSP) is unable to transport fatty acyl CoA molecules into the mitochondria in the liver. CDSP has an autosomal recessive inheritance pattern and is on the SLC22A5 gene of chromosome 5. Patients can present as infants with hyperammonemia, or they may present as adults with elevated liver enzymes and hypoketotic, hypoglycemic episodes. No matter the onset, the disease may be diagnosed by genetic testing or by checking the carnitine level of the blood. Treatment of these patients includes supplementing carnitine, reducing fats in the diet, and decreasing fasting durations.

Medium-chain acyl CoA dehydrogenase (MCAD) deficiency occurs in about every 1 in 10,000 White infants. MCAD deficiency screen is a common test included in a standard newborn screening. MCAD is an enzyme that converts 4-12 carbon fatty acyl CoA molecules into acetyl CoA in the mitochondria for utilization. MCAD has an autosomal recessive inheritance pattern, most commonly of the ACADM gene on chromosome 1. These patients may experience seizures, comas, hypoketotic hypoglycemia, and failure to thrive if not diagnosed early. The mainstay of treatment for these patients is early diagnosis and avoidance of fasting.[7][8][9][10]