Halogenation Enhances the Photosystem II Inhibitory Activity of 4-Hydroxypyridines: Structure-Activity Relationships and Their Mode of Action Tadao Asamia, Masatoshi Babab, Hiroyuki Koikec, Yorinao Inouea, and Shigeo Yoshidaa a The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-01, Japan b Agricultural Chemicals Research Department, Nissan Chemical Ind., Ltd., Tsuboi 722-1, Funabashi, Chiba 274, Japan c Department of Life Science, Faculty of Science, Himeji Institute of Technology, Harima Science Garden City, Hyogo 678-12, Japan Z. Naturforsch. 48c, 152-158(1993); received October 30, 1992 4-Pyridone, 4-Hydroxypyridine, Photosynthetic Electron Transport, Inhibition, Halogenation A range of 4-hydroxypyridines were synthesized and their activity as PET inhibitors were investigated with regard to their structural resemblance to plastoquinone in photosynthetic electron transport (PET). The activity of these compounds was markedly enhanced upon mod ifying their structures: introduction of halogens into both the 3- and 5-positions of the pyridine ring and additional substitution at the a-position of the side chain at 6-position were effective among others in enhancing the activity. Insertion of a phenyl ring into the side chain at 6-position of the pyridine ring also increased the activity. Substituents on the phenyl ring greatly affected the activity: when substituted with an appropriate functional group, the com pounds became 10- to 100-fold more active. The mode of action of both halogenated and non- halogenated 4-hydroxypyridines were investigated by means of thermoluminescence measure ments and cross resistance examination against atrazine-resistant thylakoids having mutation in D 1 protein. It was inferred that upon halogenation, 4-hydroxypyridines changed their mode of action from plastoquinone-pool inhibitors to phenol-type inhibitors. Introduction In the design of inhibitors acting at an enzymat ic level, one important feature is the structural re semblance between inhibitors and substrates. For example, in mitochondrial electron transport sys tem, piericidines and their analogs bearing a close structural resemblance to ubiquinone, act as po tent inhibitors [1]. In photosynthetic electron trans port system, on the other hand, the plastoquinone site is the main target of various photosynthesis inhibiting herbicides. Their mode of inhibition is a consequence of displacement of plastoquinone from its binding site on the protein subunits of pho tosystem II [2]. In view of this, we may reasonably expect that chemicals structurally resembling plas toquinone should inhibit photosynthetic electron flow at the plastoquinone binding site. In fact, di- bromothymoquinone [3] and tetra-halogenated 4-hydroxypyridine [4], both structurally analogous to plastoquinone, are known as potent inhibitors. Reprint requests to T. Asami. Verlag der Zeitschrift für Naturforschung, D-W-7400 Tübingen 0939-5075/93/0300-0152 $01.30/0 4-Pyrones and 4-hydroxypyridines are also known as structurally related inhibitors, but their activity level are low. It has been also reported that 4-hydroxypyridines enhance their activity upon halogenation both on the pyridine ring and a-posi- tion of the side chain at 6-position, while 4-pyrones do not [5]. It is believed that lipophilicity of the compounds plays an important role in enhancing the PET in hibitory activity [6-9]. It has further been suggest ed that the phenyl group in the lipophilic side chain may have interactions with some amino acid residues surrounding the binding site of the D 1 protein [10]. A convenient synthetic route for 4-hydroxypyridines allows us to prepare many types of compounds having various substituents on the pyridine ring and to examine the effect of halogenation, change of lipophilicity, and inser tion of phenyl group into the molecule. We report here the structure-activity relationships of halo genated 4-hydroxypyridine and the possible rea son for the activity enhancement upon halogena tion. We further analyzed comparatively the fea tures of inhibition of the Hill reaction by various compounds in atrazine-resistant and -susceptible