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Mar. Drugs 2013, 11(8), 3025-3045; doi:10.3390/md11083025
Article

Structural Characterization of New Microcystins Containing Tryptophan and Oxidized Tryptophan Residues

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2,* , 1,3
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1 Cawthron Institute, Private Bag 2, Nelson 7042, New Zealand 2 Chemistry Department, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand 3 Department of Biological Sciences, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand 4 Norwegian Veterinary Institute, P.O. Box 750 Sentrum, N-0106 Oslo, Norway 5 Department of Chemistry, University of Oslo, N-0315 Oslo, Norway
* Author to whom correspondence should be addressed.
Received: 8 June 2013 / Revised: 15 July 2013 / Accepted: 15 July 2013 / Published: 21 August 2013
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Abstract

Microcystins are cyclic peptides produced by cyanobacteria, which can be harmful to humans and animals when ingested. Eight of the (more than) 90 microcystin variants presently characterized, contain the amino acid tryptophan. The well-researched oxidation products of tryptophan; kynurenine, oxindolylalanine, and N-formylkynurenine, have been previously identified in intact polypeptides but microcystin congeners containing oxidized tryptophan moieties have not been reported. Liquid chromatography-tandem mass spectrometric analysis of an extract of Microcystis CAWBG11 led to the tentative identification of two new tryptophan-containing microcystins (MC‑WAba and MC-WL), as well as eight other microcystin analogs containing kynurenine, oxindolylalanine and N‑formylkynurenine (Nfk). Investigation of one of these congeners (MC‑NfkA) by nuclear magnetic resonance spectroscopy was used to verify the presence of Nfk in the microcystin. Liquid chromatography-mass spectrometry analysis of a tryptophan oxidation experiment demonstrated that tryptophan-containing microcystins could be converted into oxidized tryptophan analogs and that low levels of oxidized tryptophan congeners were present intracellularly in CAWBG11. MC-NfkR and MC-LNfk were detected in standards of MC-WR and MC-LW, indicating that care during storage of tryptophan-containing microcystins is required.
Keywords: microcystin; tryptophan oxidation; N‑formylkynurenine; mass spectrometry; nuclear magnetic resonance spectroscopy; Microcystis CAWBG11 microcystin; tryptophan oxidation; N‑formylkynurenine; mass spectrometry; nuclear magnetic resonance spectroscopy; Microcystis CAWBG11
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Puddick, J.; Prinsep, M.R.; Wood, S.A.; Miles, C.O.; Rise, F.; Cary, S.C.; Hamilton, D.P.; Wilkins, A.L. Structural Characterization of New Microcystins Containing Tryptophan and Oxidized Tryptophan Residues. Mar. Drugs 2013, 11, 3025-3045.

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