Kinetic evidence for multiple chemically reactive intermediates in the breakdown of phenacetin N-O-glucuronide.

N-hydroxyphenacetin glucuronide has been previously shown to be an unstable compound (half-life 8.7 h) that breaks down to phenacetin, 2-hydroxyphenacetin glucuronide, acetaminophen, acetamide, and a deethylated metabolite that covalently binds to protein. Evidence was presented that the acetamide, acetaminophen and a compound that binds covalently were formed from a common intermediate which was postulated to be N-acetylimidoquinone. In the presence of phosphate buffer, 3-hydroxyphenacetin phosphate is formed at the expense of acetaminophen, acetamide and covalent binding. Phosphate buffer, however, only partially blocks covalent binding to protein suggesting that two deethylated reactive metabolites are formed that can covalently bind to protein. These metabolites also may be converted to acetaminophen but only one of them leads to acetamide. Since the phosphate conjugate contains the ethyl group apparently a third reactive intermediate, which can react with phosphate but not with protein, serves as a precursor of one of the metabolites.