Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1998-01-30
2003-04-29
Gitomer, Ralph (Department: 1651)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C435S025000, C435S184000
Reexamination Certificate
active
06555714
ABSTRACT:
p-Hydroxyphenylpyruvate dioxygenase (HPPD; p-hydroxyphenylpyruvate: oxygen oxidoreductase, EC 1.13.11.27) in plant tissues is an enzyme central to the biosynthesis of the quinoid compounds derived from the amino acid tyrosine, such as plastoquinones or tocopherols. During the biosynthesis of quinone, tyrosine is first transaminated by a transaminase to give p-hydroxyphenylpyruvate, which is then decarboxylated and hydroxylated by the enzyme HPPD in a complex sequence. The product of this reaction is homogentisic acid. These first two steps of quinone biosynthesis can analogously also be detected in animal tissues and microorganisms. While, in plants, the homogentisic acid formed can be reacted to give plastoquinones and tocopherols, however, it is an intermediate in the catabolism of the amino acid tyrosine in animals and microorganisms. Plastoquinones and tocopherols are essential structures for plants. Inhibitors of the biosynthesis of plastoquinones and tocopherols should therefore be potential herbicides.
Catabolic p-hydroxyphenylpyruvate dioxygenases have previously been purified and characterized from animal tissues (Lindstedt, S. and Odelhög, B. (1987) Meth. Enzymol. 142, 139-142) and microorganisms (Lindstedt, S. and Odelhög, B. (1987) Meth. Enzymol. 142, 143-148). (Anabolic) p-hydroxyphenylpyruvate dioxygenases from plants, in contrast, have been described in the literature (Fiedler, E., Soll, J. and Schultz, G. (1982) Planta 155, 511-515), but no method for enriching the enzyme has been described to date.
Specifically, there has been no information to date in the literature on simple methods of measuring the plant enzyme which, of course, are indispensable for purification of the enzyme and detecting inhibitors of the enzyme.
The method described by Fiedler et al. is relatively complicated and does not allow the quantitative assays to be carried out which are required for identifying potential herbicides.
What follows describes a method for enriching the anabolic p-hydroxyphenylpyruvate dioxygenases from plant tissues and a method by means of which the enzymatic activity of the enzyme can be measured in a simple manner without complete purification of the enzyme being necessary.
The invention therefore relates to:
A method for enriching an anabolic p-hydroxyphenylpyruvate dioxygenase from plant cells, which comprises isolating the enzyme directly from a buffer in which the cells were homogenized.
The invention furthermore relates to an assay system for identifying inhibitors of p-hydroxyphenylpyruvate dioxygenase from plants, which comprises incubating an enriched HPPD from plants with a test substrate to be examined and determining the enzymatic activity of the enzyme in comparison with the activity of the uninhibited enzyme.
In particular, the invention relates to a method in which the plant tissue or cells are homogenized in extraction buffer, the extract is filtered, the supernatant is subjected to fractional ammonium sulfate precipitation, and the precipitate formed is redissolved.
Suitable extraction buffers are the extraction buffers conventionally used for plant cells, in particular the extraction buffer which is composed as follows:
20 mM phosphate buffer pH 7.0;
0.14 M KCl;
0.1 mg/ml glutathione and
1% of insoluble polyvinylpyrrolidone.
Suitable test substrates are all compounds which are potential HPPD inhibitors. These potential inhibitors are preferably stucturally similar to the natural substrate of HPPD. However, other substances which do not bind in the active center of the enzyme, but which inhibit the enzyme in a different manner, can also be used.
The enzymatic activity can be determined by means of the methods known from the literature (see Bergmeyer, H. U., Methoden der enzymatischen Analyse [Methods in enzymatic analysis], Volumes 1 and 2, Verlag Chemie, Weinheim, (1974) and Suelter, C. H., Experimentelle Enzymologie: Grundlagen für die Laborpraxis [Experimental Enzymology: Fundamentals of Laboratory Practice], Fischer Stuttgart (1990)).
A modified form of the method for measuring catabolic HPPDs from human liver has been described by Lindblad (Lindblad, B. (1971) Clin. Chim. Acta 34, 113-121). End-point measurements are used in this case for detecting
14
C—CO
2
, which is liberated from
14
C-p-hydroxyphenylpyruvate by the enzymatic activity of HPPD. If HPPD inhibitors are present in the reaction batch, the enzyme reaction and hence the liberation of CO
2
are suppressed. This assay method allows inhibitors of the enzyme to be found.
It is preferred to carry out the assay in such a way that the enzymatic activity of the HPPD is started up after preincubation of the enriched HPPD with the potential inhibitor by adding the radiolabeled
14
C-p-hydroxyphenylpyruvate, stopping the reaction after a suitable incubation time, and measuring the enzymatic activity indirectly via the radioactivity which has been liberated.
It is self-evident that the assay according to the invention can also be carried out with the purified enzyme. The enzyme can be further enriched in a manner known per se by means of chromatography on anion exchangers, such as, for example, Q-Sepharose, by Pharmacia, followed by gel permeation chromatography, such as, for example, Superdex 200, by Pharmacia, but this is not necessary for carrying out the assay.
Surprisingly, a new class of HPPD inhibitors has now also been identified using this assay system. They are herbicides from the group of the 2-benzoylcyclohexane-1,3-diones. This is surprising since an entirely different mechanism of action is suggested in the literature for this compounds class on the basis of its herbicidal symptoms (they lead to bleaching of the plants). It is assumed that, analogously to other bleaching herbicides, they inhibit phytoene desaturase (Soeda, T. and Uchida, T. (1987) Pestic. Biochem. Physiol. 29, 35-42; Mayonada, D. J., Hatzios, K. K., Orcutt, D. M. and Wilson, H. P. (1989) Pestic. Biochem. Physiol. 35, 138-145). It was possible to demonstrate that this compounds class, while not inhibiting phytoene desaturase, does inhibit HPPD; under the assay conditions, 4.5×10
−8
M 2-(2-chloro-4-methanesulfonylbenzoyl)-1,3-cyclohexanedione (SC-0051, a typical representative of this class of herbicides) showed a 50% inhibition on the anabolic plant HPPD from maize (which corresponds to the IC50 value). Bleaching herbicides which have a different structure do not, in contrast, inhibit HPPD, but inhibit the enzyme phytoene desaturase.
However, the method according to the invention allows not only the activity of known herbicides to be demonstrated at the molecular level, but also novel, previously unknown herbicidal active substances which inhibit HPPD to be identified.
The invention therefore also relates to the novel herbicidal active substances found by the assay method according to the invention.
The invention therefore also relates to compounds of the formula (I)
in which
R
1
is H, halogen, OH, alkoxy, CN, NO
2
or-haloalkyl,
R
2
is H, halogen, OH, alkyl, alkoxy, haloalkyl, haloalkoxy or (alkoxy)-carbonyl,
R
3
is H, halogen, OH, CN, NO
2
, haloalkyl, haloalkoxy or R
4
S(O)
m
—, where R
4
is alkyl and m is zero, one or two,
Q is a radical selected from the group of the formula
where
R
5
, R
6
, R
7
, R
8
and R
9
independently of one another are H, halogen, OH, CN, NO
2
, SO
3
H, SO
3
R
4
, SO
2
NR
14
R
15
, COOH, COOR
4
, CONR
14
R
15
, O—COOR
4
, O—COR
4
, -alkyl, alkoxy, haloalkyl or haloalkoxy,
R
10
, R
11
and R
12
independently of one another are H, halogen, alkyl, alkoxy, haloalkyl or haloalkoxy,
R
13
is H, phenylsulfonyl, alkylsulfonyl or alkylcarbonyl, and
R
14
and R
15
independently of one another are H, alkyl, aryl or benzyl.
In the formula (I), the radicals alkyl, alkoxy, haloalkyl and haloalkoxy can in each case be straight-chain or branched. Alkyl radicals are, for example, methyl, ethyl, n- or i-propyl or n-, i-, t- or 2-butyl. Aryl embraces aromatic and heteroaromatic radicals, such as, for example, phenyl, 2- or 3-naphthyl or 2-, 3- or 4-p
Frommer & Lawrence & Haug LLP
Gitomer Ralph
Hoechst Aktiengesellschaft
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