STUDIES ON THE MERCAPTURIC ACID SYNTHESIS IN ANIMALS

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Biotransformation of 4-dimethylaminophenol in man

1) The biotransformation of 4-dimethylaminophenol (DMAP) was studied on six healthy male volunteers. After i.v. injection of 3.2 mg DMAP-HCl/kg the metabolites were analyzed in the 24 h urine with an isotope dilution technique. 2) For the isotope dilution technique the 14C-labeled metabolites were synthetized from 14C-DMAP. 14C-DMAP-glucuronide was produced in isolated hepatocytes and purified. 3) Two thirds of the DMAP applied were identified in the 24 h urine: 40% DMAP-glucuronide, 12% DMAP-sulfate, and 15% DMAP-thioethers. About 65% of the thioether fraction consisted of S,S,S-(2-dimethylamino-5-hydroxy-1,3,4-phenylene)tris-cysteine and 3% of the corresponding N-acetyl derivative. 4) As compared to dogs and rats humans produced more glucuronide at the expense of sulfate whereas thioether formation was equal in man and dog.