PLANT PHYSIOLOGY , Vol 112, Issue 1 361-370, Copyright © 1996 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Xenobiotic Biotransformation in Unicellular Green Algae (Involvement of Cytochrome P450 in the Activation and Selectivity of the Pyridazinone Pro-Herbicide Metflurazon)
F. Thies, T. Backhaus, B. Bossmann and L. H. Grimme
Institute of Cell Biology, Biochemistry and Biotechnology, Department of Biology/Chemistry, University of Bremen, 28334 Bremen, Germany
The N-demethylation of the pyridazinone pro-herbicide metflurazon into
norflurazon implies a toxification in photosynthetic organisms. This is
confirmed by quantitative structure activity relationships determined for
two unicellular green algae, Chlorella sorokiniana and Chlorella fusca;
however, the latter is 25 to 80 times more sensitive to metflurazon. This
sensitivity is linked to differences in the N-demethylase activity of both
algae, as determined by an optimized in vivo biotransformation assay.
Apparent Km values of the metflurazon-N-demethylase indicate a 10-fold
higher affinity for this xenobiotic substrate for Chlorella fusca.
Furthermore, algal metflurazon-N-demethylation is characterized by distinct
variations in activity, depending on the stage of cell development within
the cell cycle. Several well-established inhibitors of cytochrome
P450-mediated reactions, including piperonyl-butoxide,
1-aminobenzotriazole,
1-phenoxy-3-(1H-1,2,4-triol-1yI)-4-hydroxy-5,5-dimethylhexane, and
tetcyclacis, as well as cinnamic acid, a potential endogenous substrate,
inhibited the N-demethylation of metflurazon. The results suggest that the
N-demethylation of metflurazon by both algae is mediated by a cytochrome
P450 monooxygenase. The determination of antigenic cross-reactivity of
algal proteins with heterologous polyclonal antibodies originally raised
against plant P450s, anti-cinnamic acid 4-hydroxylase (CYP73A1),
anti-ethoxycoumarin-O-dealkylase, anti-tulip allene oxidase (CYP74), and an
avocado P450 (CYP71A1) or those of bacterial origin, CYP105A1 and CYP105B1,
suggests the presence of distinct P450 isoforms in both algae.
