DNA adducts formed by ring-oxidation of the carcinogen 2-naphthylamine with prostaglandin H synthase in vitro and in the dog urothelium in vivo.

Abstract

The presence of relatively high levels of prostaglandin H synthase (PHS) in the dog urinary bladder and its ability to mediate the activation of carcinogenic arylamines to DNA-bound products in vitro suggests the involvement of this enzyme in arylamine-induced bladder carcinogenesis. Since the PHS-dependent metabolism of 2-naphthylamine (2-NA) had been shown to yield both ring- and N-oxidation products in vitro, we compared the reactivity of 3H-labeled N-hydroxy-2-naphthylamine (N-OH-2-NA), 2-nitrosonaphthalene, and 2-amino-1-naphthol (2-AN) toward DNA and protein. In the PHS-incubation system, all three derivatives bound at high levels to protein, but only N-OH-2-NA and 2-AN bound appreciably to DNA. Though ring-oxidation has usually been considered a detoxification pathway, the covalent binding of [3H]2-AN to DNA was found to occur readily under aerobic conditions and was enhanced at acidic pH. At pH 5 in air, the reactivity of [3H]2-AN with nucleic acids and protein was in the order: serum albumin greater than tRNA greater than poly G greater than poly C greater than DNA greater than poly A greater than rRNA greater than poly U. Enzymatic hydrolysis of DNA reacted with [3H]2-AN and subsequent analysis by h.p.l.c. indicated the presence of several carcinogen-nucleoside adducts. The major product was characterized as N4-(deoxyguanosin-N2-yl)-2-amino-1,4-naphthoquinoneimine; and two minor products were tentatively identified as N4-(deoxyadenosin-N6-yl)-2-amino-1,4-naphthoquinoneimine and a deoxyguanosin-N2-yl adduct of a naphthoquinoneimine dimer. These adducts accounted for approximately 60% of the total DNA binding obtained by incubation of [3H]2-NA with PHS in vitro and for approximately 20% of the [3H]2-NA bound to dog urothelial DNA in vivo. The remaining adducts were identical to those previously reported as products of the reaction of N-OH-2-NA with DNA. These results suggest that a minor proportion of the DNA adducts found in vivo may be formed by PHS-activation of 2-NA in the target tissue. Furthermore, the reactivity of 2-AN with cellular nucleophiles, presumably through formation of 2-imino-1-naphthoquinone or a protonated 4-naphthocarbenium ion, indicates that ring-oxidation products of arylamines and of other carcinogenic aryl compounds should be evaluated as proximate carcinogenic metabolites.