Xenobiotic biotransformation in unicellular green algae. Involvement of cytochrome P450 in the activation and selectivity of the pyridazinone pro-herbicide metflurazon.
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 K(m) 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 piperonylbutoxide, 1-aminobenzotriazole, 1-phenoxy-3-(1H-1,2,4-triol-1yl)-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.
Thies
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F
Department of Biology/Chemistry, University of Bremen, Germany.
Backhaus
T
T
Bossmann
B
B
Grimme
L H
LH
D001711:Biotransformation; D002708:Chlorella / Q000187:drug effects / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006540:Herbicides / Q000378:metabolism*; D007700:Kinetics; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism; D010105:Oxygenases; D011355:Prodrugs / Q000378:metabolism; D011724:Pyridazines / Q000378:metabolism* / Q000633:toxicity; D013045:Species Specificity; D013379:Substrate Specificity; D015262:Xenobiotics / Q000378:metabolism*
D006540:Herbicides; D011355:Prodrugs; D011724:Pyridazines; D015262:Xenobiotics; D003577:Cytochrome P-450 Enzyme System; D010105:Oxygenases; C103647:metflurazon-N-demethylase; D010089:Oxidoreductases, N-Demethylating; C001602:metflurazon