Prostaglandin H synthase-catalyzed metabolism and DNA binding of 2-naphthylamine.
The oxidation of the bladder carcinogen 2-naphthylamine (2-NA) by prostaglandin H synthase (PHS) in vitro was examined. Oxygen uptake studies of 2-NA oxidation in the presence of glutathione, as well as extensive product analysis data, are consistent with a one-electron mechanism of 2-NA oxidation by PHS. The formation of 2-nitrosonaphthalene is not observed under any condition. Metabolism studies with a purified PHS preparation confirm that 2-NA oxidation is dependent upon the peroxidase activity of the enzyme complex, and that a variety of organic hydroperoxides may support the reaction. Horseradish peroxidase oxidizes 2-NA to the same products but, depending on pH, in very different proportions from those obtained with PHS. Oxidation of 2-NA by a one-electron chemical oxidant results in a product profile similar to that obtained in the enzymatic systems. The above data are consistent with a one-electron mechanism of 2-NA oxidation by PHS. The metabolism data provide evidence for the formation of two types of potentially reactive electrophiles: 2-imino-1-naphthoquinone and a free radical species. We further examine the time course of covalent binding of [3H]2-NA products to DNA in vitro, and compare this with the reaction of authentic [3H]2-amino-1-naphthol (2-A-1-N) product(s) with DNA and protein. A significant amount of the PHS-catalyzed binding of 2-NA to DNA is derived from a short-lived intermediate; furthermore, the time course of binding is very rapid. Conversely, the binding to DNA of 2-A-1-N (presumably in the form of 2-imino-1-naphthoquinone) occurs to a lower extent and is not time dependent under the conditions studied. 2-A-1-N binds to protein, however, at a rapid rate and to three orders of magnitude greater extent than to DNA. The PHS-catalyzed binding of 2-NA to DNA was studied under several conditions; binding was shown conclusively to result from the peroxidase activity of the PHS complex. In addition, greater levels of binding were observed at pH 5.0 than at pH 7.6, and when catalyzed by horseradish peroxidase/H2O2 rather than PHS. These are conditions under which 2-A-1-N formation is negligible or nonexistent. These results demonstrate that in the PHS system, a reactive product(s) in addition to 2-A-1-N is generated which binds to DNA, and that this product is probably a free radical.(ABSTRACT TRUNCATED AT 400 WORDS)[1]References
- Prostaglandin H synthase-catalyzed metabolism and DNA binding of 2-naphthylamine. Boyd, J.A., Eling, T.E. Cancer Res. (1987) [Pubmed]
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