Ethyl glucuronide (EtG) is formed by conjugation with glucuronic acid catalyzed by the enzyme UDP-glucuronosyltransferase.
My question: Is it possible for EtG to form spontaneously in a urine sample that contains a small amount of ethanol?
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No, because the equilibrium lies on the side of the reactants. It works enzymatically inside the cell because glucuronic acid first reacts with UTP, turning it into a mixed anhydride (phosphate of UDP bound to carboxylic acid of glucuronic acid) plus inorganic phosphate. Then, the enzyme swaps UDP for ethanol or other molecules. Both of these reactions are spontaneous (equilibrium on the side of the products).
Attaching glucuronic acid makes molecules more water-soluble, helping to excrete them in the urine 1.