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

Abstract

ABSTRACT[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-l-hydroxy-24S-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 substituent 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.Hydroxylation of naphthalene is known to occur in vitro by nonenzymatic rearrangement of the 1,2-oxide intermediate (1) as outlined in Fig. 1. Although not directly stated, it is commonly inferred that this in vitro mechanism for formation of naphthols is operative in vivo. Here we present evidence from studies with rats that the 1,2-oxide is not the immediate precursor for the majority of the naphthols formed in vivo. We propose that most of the naphthols and the 1,2-dihydro-1-hydroxy-2-methylthionaphthalene (CH3S-metabolite) reported by Stillwell et al. (2) are formed during enterohepatic circulation of premercapturic acid pathway (pre-MAP, defined in Fig. 1) metabolites of naphthalene [1,2-dihydro-1-hydroxy-2-S-glutathionyl-, -cysteinylglycyl-, -cysteinyl-, and -(N-acetyl)cysteinylnaphthalene] by a process either mediated by the intestinal microflora or dependent upon their presence in the gut.EXPERIMENTAL PROCEDURES Chemicals. Naphthalene was obtained from Eastman Organic Chemicals, Rochester, NY. [1-14C]Naphthalene (5 mCi/mmol; 1 Ci = 37 GBq) was obtained from Amersham, 1-and 2-naphthol from Fisher, and 2-thionaphthol from Fluka. 1,2-Dihydro-1-hydroxy-2-S-cysteinyl[ C]naphthalene (I); 1,2-dihydro-1-hydroxy-2-S-(N-acetyl)cysteinyl[14C]naphthalene (II); and 1,2-dihydro-1,2-dihydroxy-[14C]naphthalene glucuronide (III) were isolated, as previously described (3), from bile (I) or urine (II and III) from rats during the period 0-24 hr after they were dosed orally with [1-'4C]naphthalene (2.0 mg/0.5 ACi in 0.5 ml of ethanol). I and II were assumed to have t...

Studies on the chromatographic fractionation of metabolites of benzo[a]pyrene in faeces and urine from germfree and conventional rats

“…Therefore, the acidic group may be the remaining part of an cx-hydroxy-dihydro structure. This would fit well with results obtained in studies on simpler aromatic compounds (Stillwell et al, 1982, Lertratanangkoon, 1982, Bakke et al, 1985.…”

Naphthalene?an environmental and occupational toxicant

“…Bieniek (1994) determined an excretion rate of 570 lg/h, with a half-life of approximately 4 h in occupationally exposed workers. Animal studies indicate that the majority of administered naphthalene is eliminated as metabolites in the urine: 77-93% in urine, 6-7% in faeces following oral administration (Bakke et al 1985), and 70-87% in urine, 6-14% in expired air, 2-4% in faeces following dermal administration (Turkall et al 1994). Bakke et al (1985) recovered 7-20% of an administered dose as 1-naphthol and 2-naphthol in rats' urine.…”

Simultaneous analysis of naphthols, phenanthrols, and 1-hydroxypyrene in urine as biomarkers of polycyclic aromatic hydrocarbon exposure: intraindividual variance in the urinary metabolite excretion profiles caused by intervention with β-naphthoflavone induction in the rat

“…A problem of this kind is virtually determined by the metabolism of the parent PAHs, and hence is not relevant to the urinary 1-OHP test. The initial biotransformation of naphthalene (Bakke et al 1985;Bock et al 1976;Jerina et al 1970;Kilanowicz et al 1999) and phenanthrene (Grimmer et al 1997;Nordqvist et al 1981) yields phenols as side pathway products, whereas dihydrodiols are the major products formed by the enzymatic pathways mediated by cytochrome P450 monooxygenase (epoxidation) and epoxide hydrolase (subsequent hydration). Dehydration of dihydrodiols into corresponding phenols occurs under acidic conditions.…”

“…This compound appeared-in the light of earlier studies (Jacob et al 1982a(Jacob et al , 1982b-as an appropriate model inducer for studying possible intraindividual differences in the metabolism of phenanthrene and pyrene. Full clearance of the first PAH dose given 6 days before the second dose was certain according to the data available on the toxicokinetics and tissue half-lives of naphthalene, phenanthrene, and pyrene in the rat (Bakke et al 1985;Corner and Young 1954;Kilanowicz et al 1999;Turkall et al 1994;Viau et al 1999). The intervention with bNF caused marked differences in PAH metabolite excretion.…”