P-hydroxyphenylpyruvate dioxygenase inhibitor

4-Hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors (HPPD inhbitors) are a class of herbicides that prevent plants by blocking 4-Hydroxyphenylpyruvate dioxygenase, an enzyme in plants that breaks down the amino acid, tyrosine into components that are used by plants to create other molecules that plants need. This process of breakdown, or catabolism, and making new molecules from the results, or biosynthesis, is something all living things do. HPPD inhibitors were first brought to market in 1980, although their mechanism of action was not understood until the late 1990s. They were originally used primarily in Japan in rice production, but since the late 1990s have been used in Europe and North America for corn, soybeans, and cereals, and since the 2000s have become more important as weeds have become resistant to glyphosate and other herbicides. Genetically modified crops are under development that include resistance to HPPD inhibitors. There is a pharmaceutical drug on the market, nitisinone, that was originally under development as an herbicide as a member of this class, and is used to treat an orphan disease, Type I tyrosinemia.

HPPD inhibitors can be classified into three fundamental chemical frameworks: pyrazolones, triketones, and diketonitriles. The triketone class is based on a chemical that certain plants make in self-defense called leptospermone; the class was developed by scientists at companies that eventually became part of Syngenta. Bayer CropScience has also been active in developing new HPPD inhibitors.

The mechanism of action for HPPD inhibitors was misunderstood for the first twenty years that these products were sold, starting in 1980. They were originally thought to be inhibitors of protoporphyrinogen oxidase (protox).

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