Catabolism of premercapturic acid pathway metabolites of naphthalene to naphthols and methylthio-containing metabolites in rats

[¹⁴C]Naphthalene was given orally to rats with cannulated bile ducts and to germ-free rats. Bile and urine from the cannulated rats and urine from the germ-free rats contained no radioactive 1,2-dihydro-1-hydroxy-2-methylthionaphthalene and only trace amounts of radioactive naphthols or naphthol conjugates. Urine of control rats contained 4.6% of the ¹⁴C dose as naphthols and/or naphthol glucuronides. Appreciable quantities of 1- and 2-naphthol (7-20% of dose) and 1,2-dihydro-1-hydroxy-2-methylthionaphthalene (1-35% of dose) were in urine from rats dosed orally or intracecally with 1,2-dihydro-1-hydroxy-2-S-cysteinylnaphthalene and 1,2-dihydro-1-hydroxy-2-S-(N-acetyl)cysteinylnaphthalene. Apparently, in vivo, naphthols and methylthio-containing metabolites of naphthalene are formed during enterohepatic circulation of 1,2-dihydro-1-hydroxy-2-S-cysteinylnaphthalene and 1,2-dihydro-1-hydroxy-2-S-(N-acetyl)cysteinylnaphthalene in a process dependent upon intestinal microflora. A possible pathway for the formation of naphthols is aromatization of the precursor compounds by elimination of the appropriate substitutent group from these metabolites. This discovery of the essential role of the intestinal microflora in the formation of naphthols from naphthalene indicates the existence of a novel pathway for hydroxylation of aromatic systems and challenges the current concept of the in vivo relevance of the in vitro production of naphthols from naphthalene 1,2-oxide.