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Ketone bodies are transported from the liver to other tissues, where acetoacetate and β-hydroxybutyrate can be reconverted to acetyl-CoA to produce reducing equivalents (NADH and FADH 2), via the citric acid cycle. Though it is the source of ketone bodies, the liver cannot use them for energy because it lacks the enzyme thiophorase (β ...
Most acetoacetate is reduced to beta-hydroxybutyrate, which serves to additionally ferry reducing electrons to the tissues, especially the brain, where they are stripped back off and used for metabolism. Acetone, which is generated through the decarboxylation of acetoacetate, either spontaneously or through the enzyme acetoacetate decarboxylase.
β-Hydroxybutyrate, BHB, is also synthesized within liver cells; this is accomplished through the metabolism of fatty acids. Through a series of reactions, acetoacetate is first produced; and it is this acetoacetate that is reduced into β-hydroxybutyrate, catalyzed by the β-hydroxybutyrate dehydrogenase enzyme.
β-Hydroxybutyric acid, also known as 3-hydroxybutyric acid or BHB, is an organic compound and a beta hydroxy acid with the chemical formula CH 3 CH(OH)CH 2 CO 2 H; its conjugate base is β-hydroxybutyrate, also known as 3-hydroxybutyrate. β-Hydroxybutyric acid is a chiral compound with two enantiomers: D-β-hydroxybutyric acid and L-β-hydroxybutyric acid.
Although beta-hydroxybutyrate is the predominant circulating ketone, urine test strips only measure acetoacetate. Urinary ketones often correlate poorly with serum levels because of variability in excretion of ketones by the kidney, influence of hydration status, and renal function. [1] [8]
The test does not measure β-hydroxybutyrate, the most abundant ketone in the body; during treatment of ketoacidosis β-hydroxybutyrate is converted to acetoacetate so the test is not useful after treatment begins [11] and may be falsely low at diagnosis. [14] Similar tests are used in dairy cows to test for ketosis. [15]
Acetoacetate is an intermediate in the Krebs cycle which releases energy from sugars and carbohydrates. [22] In medicine, acetone, acetoacetate, and beta-hydroxybutyrate are collectively called ketone bodies, generated from carbohydrates, fatty acids, and amino acids in most vertebrates, including humans.
Acetoacetate and D-β-hydroxybutyrate freely interconvert through the action of D-β-hydroxybutyrate dehydrogenase. [20] Subsequently, one function of acetoacetate decarboxylase may be to regulate the concentrations of the other, two 4-carbon ketone bodies.