Target Sites Theory versus Practice

In this section, the available evidence for the modes of action of current bleaching herbicides with single, multiple, or unknown target sites is discussed. Previous reviews on the subject can be found elsewhere.  

Sandmann and Boger have suggested that the phenylpyridazinones have two modes of action on phytoene desaturase. Firstly, the compound mimics a carotene molecule and acts as an allosteric regulator. Secondly, an irreversible binding of the —CF3 derivative occurs at the active site. Phenylpyridazinones such as Sandoz 9785 inhibit fatty acid desaturation in plants, which may reflect the potential of these compounds to inhibit a range of desaturases (see also Chapter 3). 

Fluorochloridone has also been shown to be effective in vivo and in vitro, as has difunone and its analogues.  

Inhibitor studies with LS 80707 and its isomer LS 80717 (containing JS-groups at C-5 and C-6 of the pyrone ring) have revealed that LS 80717 does not inhibit -carotene desaturase at the same concentration as LS 80707. Another dihydropyrone, LS 780091, binds irreversibly and non-competitively at the active site of the enzyme.  

Finally, a group of phenoxytriethylamines, originally synthesized as potential uncouplers of oxidative phosphorylation, preferentially inhibit lycopene cyclization.  

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