Bacterial degradation of microcystins and nodularin.
Microcystins and nodularins produced by cyanobacteria are potent hepatotoxins and tumor promoters. They are, respectively, cyclic heptapeptides and cyclic pentapeptides containing a characteristic beta-amino acid residue, (2S,3S,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4(E ),6(E)-dienoic acid (Adda). Strain B-9 isolated from Lake Tsukui, Japan, degrades microcystin-LR, which is the most toxic among the microcystins, to nontoxic Adda as an end product. In the present study, we characterized the bacterial degradation process of the cyclic peptide hepatotoxins by liquid chromatography/ion trap tandem mass spectrometry. The use of protease inhibitors with a B-9 cell extract indicated that the degradation process of microcystin-LR consists of sequential enzymatic hydrolyses of Arg-Adda, Ala-Leu, and then Adda-Glu peptide bonds into two known nontoxic intermediate degradation products and then Adda, respectively. Subsequently, additional microcystins and nodularin were compared with microcystin-LR on substrate specificity. The cyclic peptides containing the Arg-Adda peptide bond were almost completely degraded to Adda as well as microcystin-LR, whereas microcystin-LF containing the Phe-Adda peptide bond instead of Arg-Adda peptide bond and 6(Z)-Adda-microcystin-LR and -RR which are geometrical isomers of the Adda residue were barely degraded. These results indicated that the degrading enzymes selectively hydrolyzed the Arg-Adda peptide bond as the initial ring opening of the cyclic peptide hepatotoxins, microcystins and nodularin.
Imanishi
Susumu
S
Graduate School of Environmental and Human Sciences, Meijo University, 150 Yagotoyama, Tempaku, Nagoya 468-8503, Japan. d6020004@ccmailg.meijo-u.ac.jp
Kato
Hajime
H
Mizuno
Masayoshi
M
Tsuji
Kiyomi
K
Harada
Ken-ichi
K
D001427:Bacterial Toxins / Q000378:metabolism*; D001673:Biodegradation, Environmental; D002851:Chromatography, High Pressure Liquid; D000458:Cyanobacteria / Q000201:enzymology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008387:Marine Toxins; D052998:Microcystins; D010456:Peptides, Cyclic / Q000032:analysis / Q000378:metabolism*; D021241:Spectrometry, Mass, Electrospray Ionization
D001427:Bacterial Toxins; D004791:Enzyme Inhibitors; D008387:Marine Toxins; D052998:Microcystins; D010456:Peptides, Cyclic; C063998:nodularin; C057862:cyanoginosin LR