The covalent binding of acetaminophen to protein. Evidence for cysteine residues as major sites of arylation in vitro.

Covalent binding of the reactive metabolite of acetaminophen has been investigated in hepatic microsomal preparations from phenobarbital-pretreated mice. Low molecular weight thiols (cysteine and glutathione) were found to inhibit this binding, whereas several other amino acids which were tested did not. Bovine serum albumin (BSA), which contains a single free sulfhydryl group per molecule and which thus represents a macromolecular thiol compound, inhibited covalent binding of the reactive acetaminophen metabolite to microsomal protein in a concentration-dependent manner. The acetaminophen metabolite also became irreversibly bound to BSA in these experiments, although this binding was reduced by approx. 47% when the thiol function of BSA was selectively blocked prior to incubation. Covalent binding of the acetaminophen metabolite to bovine alpha s1-casein, a soluble protein which does not contain any cysteine residues, was found to occur to an extent of 37% of that which became bound to native BSA. These results were taken to indicate that protein thiol groups are major sites of covalent binding of the reactive metabolite of acetaminophen in vitro. The covalent binding characteristics of synthetic N-acetyl-p-benzoquinoneimine (NAPQI), the putative electrophilic intermediate produced during oxidative metabolism of acetaminophen, paralleled closely those of the reactive species generated metabolically. These findings support the contention that NAPQI is indeed the reactive arylating metabolite of acetaminophen which binds irreversibly to protein.