Analytical Identification of Novel Furan Metabolites in Human Hepatocytes and Urine: Implications for Liver and Respiratory Disease Research

Abstract

The gas phase of tobacco smoke has been implicated as a major cause of cancer and chronic obstructive pulmonary disease. Furan, a component of the gas phase, is highly toxic and carcinogenic in rodents; however, its toxicity in humans has not been well investigated. Furan’s toxicity results from cytochrome P450 2E1-catalyzed oxidation to cis-2-butene-1,4-dial (BDA). The structure of furan metabolites demonstrates that BDA reacts with glutathione (GSH) to form a reactive GSH conjugate, 2-(S-glutathionyl)succinaldehyde (GSH-BDA), which reacts with protein lysine groups as well as other cellular nucleophiles. In this report, GSH-BDA-glycerolphosphorylethanolamine (GSH-BDA-GPE), GSH-BDA-ethanolamine, and GSH-BDA-glutamic acid are identified as human hepatocellular metabolites of furan. To determine if the downstream mercapturic acid metabolites are present in human urine, stable isotopically labeled forms of the expected metabolites were used as internal standards in an established liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay. This approach indicated that N-acetyl-S-(1-(2-hydroxyethyl)-1H-pyrrol-3-yl)cysteine sulfoxide (NAC-BDA-ethanolamine sulfoxide) was detected in human urine. A sensitive LC-MS/MS assay for this metabolite was developed and applied to urine samples from a variety of smoke-exposed individuals as well as nonsmokers. Its levels were 2.5–5 times higher in people who smoke than in nonsmokers. They were not elevated in exclusive e-cigarette or cannabis users. Two case-control studies were performed to determine whether NAC-BDA-lysine metabolites or NAC-BDA-ethanolamine sulfoxide were associated with smoking-related diseases. In the first study, there was no association between any of the furan metabolites and liver cancer. In the second study, there was a suggestive association between NAC-BDA-ethanolamine sulfoxide and the risk of smoking-related respiratory disease symptoms. However, the overall trend was not significant. Therefore, there was no clear link between this furan metabolite and the risk of smoking-related respiratory disease symptoms.