LETTERS TO THE EDITOR pubs.acs.org/crt
Metabolic Detoxication Pathways for Sterigmatocystin in Primary Tracheal Epithelial Cells: Structural Identification of Glutathione Adducts o the Editor: A recent article published in Chemical Research in Toxicology by Cabaret et al.1 studied the in vitro metabolism of the mycotoxin sterigmatocystin. In this article, the authors describe the metabolic fate of sterigmatocystin incubated with both recombinant P450s and in porcine tracheal epithelial cells (PTEC). Their results in PTEC suggest that sterigmatocystin is mainly detoxified via glucuronide and sulfonate conjugation and undergoes no bioactivation to glutathione reactive metabolites. My concern, however, is in regards to their identification of a series of sterigmatocystin metabolites obtained upon exposure to recombinant CYP1A1, 1A2 and 3A4. The structures of these metabolites (Scheme 1 in their article) are only identified through their electrospray mass spectrometry (ESI-MS) data, and although several of the proposed metabolites agree with the mass spectrometric data, I believe these structures to be incorrect. The authors suggest that an initial metabolite of sterigmatocystin is a transient epoxide which is subsequently hydrolyzed or forms a glutathione adduct. My first concern is with metabolite M2, which is proposed to be derived from hydrolysis of the initial transient epoxide. M2 has an ESI-MS( ) m/z 355 and is drawn as an unsaturated 1,2-diol, but epoxide hydrolysis should result in the formation of a saturated 1,2-diol with ESI-MS( ) m/z 357 (Scheme 1). My second concern is with the assignment of metabolite M3 (ESI-MS(+) m/z 644), which is the result of glutathione addition to the epoxide to form a glutathione adduct. Nucleophilic addition of glutathione to the proposed sterigmatocystin epoxide should produce a metabolite with ESI-MS(+) m/z 646 (Scheme 2), which does not agree with the reported value. The structure of the adduct in their scheme appears to suggest the formation of a sulfur oxygen bond, which is unlikely and requires the incorporation of an oxygen atom during glutathione conjugation. Furthermore, their structure as shown, would have an ESI-MS(+) m/z 660, not 644 as reported. There are a number of more plausible structures that could account for the observed results. I have suggested several structures in Scheme 3 that seem more likely and would be in agreement with the mass spectral data. It should be noted, however, that this is not an exhaustive list of structures and that a number of additional structures are possible. In order to determine the exact structure of the metabolites, a rigorous structural analysis would still need to be performed. One example in Scheme 3 would result in CYP1A1-mediated hydroxylation of the parent compound to either a catechol or a para-hydroquinone.2 Either of these structures would have a ESI-MS(+) m/z 341, which is in agreement with that of metabolite M1. Further aromatic hydroxylation on either ring would result in a metabolite with ESI-MS( ) m/z 355, which is consistent with that of M2. Oxidation of M1 to a quinone (either an ortho-quinone or a para-quinone) followed
T
r 2011 American Chemical Society
Scheme 1. Proposed Epoxidation and Hydrolysis Reaction of Sterigmatocystina
a
Reprinted from ref 1. Copyright 2010 American Chemical Society.
Scheme 2. Proposed Reaction for Glutathione Addition to a Sterigmatocystin Epoxide
by glutathione conjugation would yield a glutathione adduct on the aromatic ring (M3).3 The exact position of glutathione conjugation would require further structural analysis for confirmation. While this shortcoming detracts somewhat from the manuscript, I believe that the overall observation that none of the glutathione reactive species observed in recombinant P450s were formed in PTEC is nevertheless important. Future studies as proposed by the authors to investigate sterigmatocystin metabolism in human primary epithelial cells may or may not indicate the presence of glutathione adducts. While I hope that in the future the authors do indeed determine the structure of the glutathione conjugates and their precursors, determining whether sterigmatocystin glutathione adducts are present in human primary epithelial cells is arguably a good first step.
Published: August 25, 2011 1339
dx.doi.org/10.1021/tx200298a | Chem. Res. Toxicol. 2011, 24, 1339–1340
Chemical Research in Toxicology
LETTERS TO THE EDITOR
Scheme 3. Proposed Metabolic Pathway for Sterigmatocystin in Recombinant CYP1A1
Ed S. Krol* College of Pharmacy and Nutrition, University of Saskatchewan, 110 Science Place, Saskatoon SK S7N 5C9, Canada
’ AUTHOR INFORMATION Corresponding Author
*Phone: (306) 966-2011. Fax: (306) 966-6377. E-mail: ed.krol@ usask.ca.
’ ABBREVIATIONS GSH, glutathione; PTEC, porcine tracheal epithelial cells; ESI-MS, electrospray ionization mass spectrometry. ’ REFERENCES (1) Cabaret, O., Puel, O., Botterel, F., Pean, M., Khoufache, K., Costa, J. M., Delaforge, M., and Bretagne, S. (2010) Metabolic detoxication pathways for sterigmatocystin in primary tracheal epithelial cells. Chem. Res. Toxicol. 23, 1673–1681. (2) Badawi, A. F., Cavalieri, E. L., and Rogan, E. G. (2001) Role of human cytochrome P450 1A1, 1A2, 1B1, and 3A4 in the 2-, 4-, and 16alpha-hydroxylation of 17beta-estradiol. Metabolism 50, 1001–1003. (3) Bolton, J. L., Acay, N. M., and Vukomanovic, V. (1994) Evidence that 4-allyl-o-quinones spontaneously rearrange to their more electrophilic quinone methides: potential bioactivation mechanism for the hepatocarcinogen safrole. Chem. Res. Toxicol. 7, 443–450.
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dx.doi.org/10.1021/tx200298a |Chem. Res. Toxicol. 2011, 24, 1339–1340
