Identification\nof Novel Peptides Originated from Biodegradation\nof Microcystins Using UHPLC-HRMS/MS

Biodegradation\nof toxic cyanobacterial cyclic peptides microcystins\n(MCs) by heterotrophic bacteria isolated from freshwater has gained\nglobal interest as an eco-friendly approach for water treatment. This\nstudy focused on elucidating the structures of MC-LR breakdown products\ngenerated by a group of five bacteria previously isolated from Lake\nErie. Three isomers of a tetrapeptide NH<sub>2</sub>-Adda-Glu-Mdha-Ala-OH\n(Adda-TetraP-1, 2, and 3) and a cyclic tripeptide NH<sub>2</sub>-Adda-Glu-Mdha-OH\n(Adda-TriP) were identified during the MC-LR degradation process.\nOf these peptides, only linear Adda-TetraP-1 was described previously.\nUHPLC-high-resolution (HR)-MS and MS/MS data acquired for tetrapeptides\nand the tripeptide and their thiol derivatization showed that Adda-TetraP-2,\nAdda-TetraP-3, and Adda-TriP were cyclic peptides that contained modified\nMdha and Adda moieties. The experimental evidence suggests that the\nAdda amine group of Adda-TetraP-1 underwent intramolecular aza-Michael\nreaction with the Mdha double bond to form a cyclic tetrapeptide,\nwhich appeared in two diastereomeric forms, of which Adda-TetraP-2\nwas formed faster than Adda-Tetra-P-3. The Mdha-Ala peptide bond of\nAdda-TetraP-2 appeared to be hydrolyzed by bacteria to form Adda-TriP.\nIndependent of our laboratory cultures, Adda-TetraP-1, 2, and 3 were\ndetected in Lake Erie water during cyanobacterial blooms, and identifying\nthem will assist in elucidating biodegradation pathways of MC congeners\nand genes relevant to MC degradation.