The Hill Reaction

In the case of green plants, the electron donor is water, according to Van Niel s theory. This view was supported by the discovery of the Hill reaction (Hill and Scarisbrick, 1940), in 

It had been known for a long time that oxygen was evolved during the illumination of ground leaves, isolated chloroplasts, or aqueous suspensions of dried leaf powders. The amounts of oxygen evolved were small, and it remained for Hill (4) to show that illuminated isolated chloroplasts were able to produce considerable quantities of oxygen if an aqueous extract of an acetone powder of yeast or leaves was added. Subsequently it was shown that the yeast or leaf extract could be omitted if certain ferric salts were added. With ferric iron, the reaction proceeds as follows  

Additional work on this reaction, commonly called the Hill reaction, has led to the conclusion that it probably represents the photochemical phase of photosynthesis. This reaction shares with photosynthesis the conversion of light energy into chemical energy, and the appearance of molecular oxygen. 

In biological oxidations and reductions the reaction can be represented as follows  

The Standard Oxidation-Reduction Potential Eq of Some Biological Compounds at pH 7.0 Referred to the Standard Hydrogen Electrode 

Electron donor Electron acceptor Potential Eq (volts) 

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Ozone causes both quantitative and qualitative changes in carbon dioxide fixation patterns. Wilkinson and Bames, using carbon dioxide-found a reduction in radioactivity in soluble sugars and increases in free amino acids and sugar phosphates in white pine after a 10-min exposure to ozone at 0.10 ppm. Miller observed a decrease in carbon dioxide-fixation in ponderosa pines that correlated with loss of chlorophyll, after exposure to ozone at 0.30-0.35 ppm. The Hill reaction rates of chloroplasts isolated from healthy and ozone-injured ponderosa pine indicated that both light and dark reactions of the chloroplasts from ozone-injured plants were depressed. Barnes found depressed photosynthesis and stimulated respiration in seedlings of four pine species of the southeastern United States after exposure to ozone at 0.15 ppm. 

In the search for more effective post-emergent herbicides, many laboratories have measured the inhibition of photosystem II in chloroplasts i.e., the Hill reaction. In a continuing investigation of this system, ( ) Corwin Hansch s group at Pomona College, in cooperation with BASF in Germany, analyzed two sets of phenyl substituted ureas 17 1,1-dimethyl-3-phenyl, and 38... 

Another example is the herbicidal activity of substituted nitrophenols, expressed as the inhibition of the Hill reaction. 

In 1937 Robert Hill found that when leaf extracts containing chloroplasts were illuminated, they (1) evolved 02 and (2) reduced a nonbiological electron acceptor added to the medium, according to the Hill reaction ... 

ATP synthesis in chloroplasts. The flow of electrons between PSII and PSI (Fig. 23-18) is of great importance for ATP formation. As previously mentioned, plastocyanin is usually the immediate donor to P700 and serves as a mobile carrier to bring electrons to this reaction center. In this function it is analogous to cytochrome c of mitochondrial membranes. The essentiality of plastocyanin was shown by study of copper-deficient Scenedesmus (Fig. 1-11). The photoreduction of C02 by H2 is impaired in these cells, but the Hill reaction occurs at a normal rate. 

Members of an extensive group of sym-triazine herbicides, usually having one or two secondary amine substituents, block the Hill reaction and inhibit photosynthesis in a manner quite similar to that of the urea herbicides. The most widely used, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), (10), is one of several hundred herbicidal analogs... 

Distinct differences in cells with regard to the presence or absence of target structures or metabolic processes also offer opportunities for selectivity. Herbicides such as phenylureas, simazine, and so on, block the Hill reaction in chloroplasts, thereby killing plants without harm to animals. This is not always the case because paraquat, which blocks photosynthetic reactions in plants, is a pulmonary toxicant in mammals, due apparently to analogous free-radical reactions (see Figure 18.4) involving enzymes different from those involved in photosynthesis. 

Research had confirmed that no parent simazine residues were found in treated com plants, and additional data on the dissipation pathway of simazine needed to be developed. Research also indicated that triazines interfered with the photosynthetic process on susceptible growing weeds, as evidenced by the appearance of chlorotic leaves. Steps were undertaken to elucidate simazine s dissipation pathway and herbicidal mode of action. In Basel, Dr. Gast (1958) showed that the accumulation of starch by common coleus (Coleus blumei Benth.) plants was inhibited from treatment with 2-chloro-4,6-bis-(alkyl-amino)-triazines due to the inhibition of sugar synthesis. At the same time, Moreland et al. (1958) found weed control activity could be reduced by supplying carbohydrates to the plants through their leaves and that simazine was a strong inhibitor of the Hill reaction in photosynthesis. Exer (1958) found that triazines inhibited the Hill reaction as strongly as urea of the CMU (monuron) type. 

In a study designed to determine the mode of action of atrazine in higher plants, Shimabukuro and Swanson (1969) concluded that atrazine inhibits the Hill reaction and its noncyclic phosphorylation, while being ineffective against cyclic photophosphorylation. Atrazine readily penetrated the chloroplast of resistant as well as susceptible plants. In tolerant plants such as sorghum, the metabolism of atrazine was postulated to occur outside the chloroplasts to form water-soluble and insoluble residues that reduced the concentration of photosynthetic inhibitors in the chloroplasts. 

Shimabukuro et al. (1973) identified 2-chloro-4,6-diamino-.v-triazinc (G-28273), which represented complete dealkylation of the triazine ring, as an organosoluble metabolite in sorghum. This metabolite did not inhibit the Hill reaction and cyclic and noncyclic photophosphorylation in isolated pea chloroplasts. 

Good, N.E. (1961). Inhibitors of the Hill reaction. Plant Physiol., 36 788-803. 

Wessels, J.S.C. and R. van der Veen (1956). Action of some derivatives of phenylurethan and of 3-phenyl-1,1-dimethylurea on the Hill reaction. Biochim. Biophys. Acta, 19 548-549. 

Artificial electron acceptors, such as ferricyanide, can be substituted for NADP these give rise to oxygen evolution but involve only a short segment of the oxidation chain. This partial reaction is known as the Hill reaction and compounds that disrupt it are known as Hill inhibitors. Herbicides that inhibit the Hill reaction, by blocking electron transport, prevent the production of ATP and NADPH required for carbon dioxide fixation. 

Herbicides that inhibit the photochemical reactions of isolated chloroplasts have been called routinely inhibitors of the Hill reaction. This has been done primarily for convenience and because, for many years, their action was evaluated under nonphosphorylating conditions, frequently with ferricyanide as the electron acceptor. In the past few years, more sophisticated studies have been conducted with herbicides and more is known about their differential actions. Consequently, Moreland and Hilton (2) separated herbicidal inhibitors of the photochemically induced reactions into the following classes (a) electron transport inhibitors, (b) uncouplers, (c) energy transfer inhibitors,... 

The Hill reaction has served as a target for structure-activity studies with phenylureas, iV-phenylcarbamates, polycyclic ureas, acylanilides, s-triazines, uracils, dihalogenated benzoni-... 

Substituent Parameters. A significant advance was made, in the structure-activity studies with the Hill reaction, when Hansch and Deutsch (12) evaluated some of the published Hill inhibition data with a multiple regression analysis, an extra-thermodynamic approach, or the so-called sigma, pi (a, T ) regression analysis. The principle of the approach rests on the assumption that changes in biological activity can be correlated with measurable molecular or substituent parameters. This analysis involved equations of the following type ... 

Some good inhibitors of the Hill reaction, however, do not contain the carbonyl oxygen-nitrogen moiety. Examples are the dinitroanilines, diphenylethers, 2,4-dinitrophenols, halogenated benzonitriles, and pyridinols. Hence, the postulates proposed are not all inclusive. Three of these herbicides are phenols. Under physiological pH s, the molecules can be expected to be ionized, and it may be the ionized form of the molecule that binds to the receptor. 

An example of geometric isomerism is provided by the dimethylpyrrolidinecarboxanilides (18). Only when the methyl groups substituted on the pyrrolidine ring are in the ois-conformation is electron transport inhibited and phytotoxicity produced. The trans-isomer neither inhibits the Hill reaction, nor is it phytotoxic. 

The regression analyses have shown that inhibitory potency expressed against the Hill reaction can be correlated with physico-chemical parameters within a particular group of herbicides. However, no investigator has successfully correlated mathematically activities between different chemical families (11). 

Insofar as they have been studied, all herbicides that inhibit the Hill reaction of isolated chloroplasts also inhibit photosynthesis of intact plants and photosynthetic microorganisms (2, 3). Phy to toxicity is produced only in the light, and severity of symptoms is proportional to light intensity. Studies with light quality have indicated that the chlorophylls are the principal absorbing pigments involved in the production of phytotoxicity. 

In 1937 Robin Hill demonstrated that isolated chloroplasts, when placed in an aqueous solution in the presence of a suitable electron acceptor, can evolve oxygen in the light, a process that has become known as the Hill reaction. Oxygen evolution proceeds in the absence of CO2, suggesting that CO2 fixation and O2 evolution are separate processes, contrary to the then prevailing belief. Using lsO-labeled H20 and lsO-labeled CO2 in different experiments, Laurens Ruben and Martin Kamen showed in 1941 that the evolved O2 comes from water and not from CO2. Subsequent studies have elucidated the steps intervening between O2 evolution and CO2 fixation in photosynthesis. 

It is not enough for a multiple parameter equation just to work and to give an improved correlation as Higuchi and Davis have emphasized recently (64). The correlation must have a physicochemical significance. Recently, Equation 31 has been presented by Biichel and his co-workers for the inhibition of the Hill reaction with 1,2,4-triazinone herbicides (65). 

The diphenyl ether herbicides are active only in the presence of light and canse chlorosis of leaf tissue. They inhibit the Hill reaction in photosynthesis and photophosphorylation. However, the primary mode of action probably involves the photosynthetic rednction to form radicals, which initiate destructive reactions in lipid membranes leading to cell leakage. 

Triazines act by interfering with photosynthesis and it seems clear that, like the substituted urea herbicides, the primary site of action is inhibition of the Hill reaction of photosynthetic electron transport. 

Uracils and substituted uracils owe their herbicidal activity to inhibition of photosynthesis by blocking the Hill reaction. This activity also present in the ureas and triazines. 

There are at least four known actions of pyridazines on plants involving a one or more of the following effects (a) inhibition of the Hill reaction (photolysis of water during the photosynthesis), (b) inhibition of pigment formation (chlorophylls, carotenoids), (c) change in the linolenic/linoleic acid ratio, and (d) influence on the chloroplast ribosomes. In addition, pyridazines may be used in plant protection as biocides. 

Hill reaction. In 1939, Robert Hill discovered that chloroplasts evolve O2 when they are illuminated in the presence of an artificial electron acceptor such as ferricyanide [Fe3+(CN)g]3-. Ferricyanide is reduced to ferrocyanide [Fe +(CN)5]4- in this process. No NADPH or reduced plastocyanin is produced. Propose a mechanism for the Hill reaction. 

Photosystem II splits water (the Hill reaction). Its reaction centre absorbs maximally at 680 run, and its is found mostly on grana, associated with light-harvesting complex II. It also contains more chlorophyll-b than PSI. 

Greco, G., Novellino, E., Pellecchia, M., Silipo, C. and Vittoria, A. (1993). Use of the Hydrophobic Substituent Constant in a Comparative Molecular Field Analysis (CoMFA) on a Set of Anilides Inhibiting the Hill Reaction. SAR QSAR Environ.Res., 1,301-334. 

Soskic, M. and Sabljic, A. (1995). QSAR Study of 4-Hydroxypyridine Derivatives as Inhibitors of the Hill Reaction. Pestic.ScL, 45,133-141. 

The triazine herbicides are selective inhibitors of the Hill reaction in plant photosynthesis. In mammals, atrazine disrupts luteinizing hormone and prolactin secretion through direct action on the hypothalamus-pituitary axis. 

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