Investigation of the role of lipoxygenase in bioactivation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in human lung

Abstract

4-Methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) is a potent tobacco-specific carcinogen believed to play a role in human lung cancer. Bioactivation of NNK involves alpha-carbon hydroxylation that could be catalyzed by cytochrome P450, hemoglobin, and lipoxygenases (LOX). In the present study, the role of LOX in NNK bioactivation was investigated. Formation of keto acid, the endpoint metabolite of alpha-methylene NNK hydroxylation, was observed in human lung cytosols incubated with 4.2 microM [5-(3)H]NNK (N = 6). Following concanavalin A affinity chromatography to enrich human lung lipoxygenase (HLLO), the fraction containing cytosolic components less LOX (fraction 1) retained the ability to bioactivate NNK. Although enriched HLLO exhibited the characteristic dioxygenase and hydroperoxidase activities, it did not bioactivate NNK. The LOX inhibitor nordihydroguaiaretic acid inhibited dioxygenase activity of HLLO by 83 +/- 19% (P < 0.05, N = 6), but did not inhibit keto acid formation in the crude cytosols (N = 6, P > 0.05). Failure of soybean LOX to catalyze NNK bioactivation supported the results observed in human lung cytosols, and failure of chemically generated alkylperoxyl radicals to bioactivate NNK further suggested that the dioxygenase activity of LOX is not likely to be involved in NNK bioactivation. Horseradish peroxidase and myeloperoxidase catalyzed NNK bioactivation were also nondetectable. Our results demonstrate that, although human lung cytosols can bioactivate NNK to form keto acid, LOX is not involved. We have attributed the ability of crude human lung cytosols to bioactivate NNK to hemoglobin. The inhibitory effect of 1-aminobenzotriazole and arachidonic acid on keto acid formation in the crude cytosols and in fraction 1, respectively (P < 0.05, N = 6), is consistent with hemoglobin-catalyzed NNK bioactivation.