Inhibition of the Hill reaction by fatty acids and metal chelating agents

On the mechanism of linolenic acid inhibition in Photosystem II

Recent studies in our laboratory have reexamined the interaction of the unsaturated fatty acid, linolenic acid, with Photosystem II and have documented two principal regions of inhibition: one associated with the donor complex (Signal 2f or D1) to the reaction center, and the other located on the reducing side between pheophytin and Qa (Golbeck, J.H. and Warden, J.T. (1984) Biochim. Biophys. Acta 767, 263-271). A further characterization of fatty acid inhibition of secondary electron transport in Photosystem II at room and cryogenic temperatures is presented in this paper. These studies demonstrate that linolenic acid, and related fatty acid analogs, eliminate the transient absorption increase at 320 nm, attributed to Qa-; abolish the production, either chemically or photochemically, of the ESR signal (Q-Fe) associated with the bound quinone acceptor, Qa-; and prevent the photooxidation of Signal 2(1t)(D1) at cryogenic temperature. Linolenic-acid-treated samples are characterized by a high initial fluorescence yield (Fi) equivalent to the maximum level of fluorescence (Fmax); however, the spin-polarized triplet, associated with the reaction-center electron donor, P-680, is observed only in inhibited samples that have been prereduced with sodium dithionite. These results suggest the presence of an additional acceptor intermediate between pheophytin and Qa. The donor-assisted photoaccumulation of pheophytin anion in Photosystem II particles, as monitored by the decline of fluorescence yield, is inhibited by linolenic acid. Redox titrations of the fluorescence yield in control and inhibited preparations demonstrate that the midpoint potential for the primary acceptor for Photosystem II is insensitive to the fatty acid (Em approximately -583 mV) and thus indicate that primary photochemistry is functional during linolenic-acid inhibition. These data are consistent with the hypothesis that unsaturated fatty acids inhibit secondary electron transport in Photosystem II via displacement of endogenous quinone from quinone-binding peptides.

Interaction of linolenic acid with bound quinone molecules in Photosystem II. Time-resolved optical and electron spin resonance studies

Time-resolved spectroscopic techniques, including optical flash photolysis and electron spin resonance spectroscopy, have been utilized to monitor electron-transport activity in Photosystem II subchloroplast particles. These studies have indicated that in the presence of 100 microM linolenic acid (1) a high initial fluorescence yield (Fi) is observed upon steady-state illumination of the dark-adapted sample; (2) flash-induced absorption transients (t greater than 10 mus) in the region of 820 nm, attributed to P-680+, are first slowed, then abolished; and (3) electron spin resonance Signal IIs and Signal IIf (Z+) are not detectable. Upon reversal of linolenic acid inhibition by washing with bovine serum albumin, optical and electron spin resonance transients originating from the photooxidation of P-680 are restored. Similarly, the variable component of fluorescence is recovered with an accompanying restoration of Signal IIs and Signal IIf. The data indicate that linolenic acid affects two inhibition sites in Photosystem II: one located between pheophytin and QA on the reducing side, and the other between electron donor Z and P-680 on the oxidizing side. Since both sites are associated with bound quinone molecules, we suggest that linolenic acid interacts at the level of quinone binding proteins in Photosystem II.

The mechanisms of fatty-acid inhibition of electron transport in chloroplasts

Unsaturated fatty acids at concentrations of 1-2 μmol mg(-1) chlorophyll decrease the intensity of long-lived delayed fluorescence and inhibit the Hill reaction in Pisum sativum L. chloroplasts in a pH-dependent and reversible manner. A charged form of the fatty acids is two times more effective than an undissociated form. Fatty acids, anionic and cationic detergents and urea inhibit activity and decrease the temperature of heat inactivation of the water-spilitting system. Sucrose at a concentration of 2.5 M protects chloroplasts against the effects of these compounds. It is concluded that their action can be explained by the denaturation of the water-splitting protein.

Photosynthetic apparatus in chilling-sensitive plants : I. Reactivation of hill reaction activity inhibited on the cold and dark storage of detached leaves and intact plants

Chloroplast isolated from the detached leaves of chilling-sensitive plants-Phaseolus vulgaris L., Cucumis sativus L., and Lycopersicon esculentum Mill.-stored in the cold for 2-4 days in the dark exhibit an almost complete loss of Hill reaction activity, which on illumination of leaves is restored to almost the original level. In contrast, illumination of either chloroplast suspensions or homogenates from leaves stored in the cold in the dark does not cause restoration of electron transport. Cold and dark storage of leaves of chilling-sensitive plants affects the electron transport before the site of electron donation by diphenylcarbazide and results in an increased sensitivity of the Hill reaction of isolated chloroplasts to exogenous linolenic acid. Illumination of leaves reverses these processes. When tomato plants are exposed to 0°C in intermittent light, Hill reaction activity is not affected while dark storage either at 0°C or 25°C results in a significant decrease of Hill reaction activity after 2-3 days followed by the restoration of electron transport to the original level after 1 or 2 days of the prolonged dark storage of plants. When tomato plants are stored either at 0°C in intermittent light, at 0°C in dark, or at 25°C in dark the sensitivity of the Hill reaction to exogenous linolenic acid remains increased despite a significant restoration of this activity. In conclusion, both darkness and the detachment of leaves from the plant are more effective than cold treatment in damaging photosystem II whereas both light and intact structure of the cell are required for restoration of Hill reaction activity in chloroplasts following cold and dark storage of detached leaves.

Requirement of galactolipids for photosystem I activity in lyophilized spinach chloroplasts

1. The effect of monogalactosyl diacylglycerol and digalactosyl diacylglycerol on reconstitution of Photosystem I activity in heptane-extracted and galactolipase-treated spinach chloroplasts was investigated. 2. Both galactolipids, in a molar ratio with chlorophyll of 2.5, partially restored Photosystem I activity in heptane-extracted chloroplasts. An addition of saturating amounts of plastocyanin caused complete reactivation of Photosystem I. 3. Similarly, with galactolipase-treated chloroplasts, both galactolipids partially restored Phostosystem I activity and additional amounts of plastocyanin were required for complete reactivation. 4. The action of galactolipids on partial reconstitution of Photosystem I supports the suggestion of their structural role in the restoration of thylakoid membranes.

[Assimilation of CO2, NADP and PGA reduction and ATP synthesis in intact leaf cells in relation to water content]

1. Photosynthesis, NADP and PGA reduction and ATP synthesis in strongly dehydrated intact leaves or parts of leaves of fodder beet were investigated. 2. Water stress leads to inhibition of photosynthesis. However, even loss of a major proportion of the water content of cells (up to 80%) does not completely suppress photosynthesis. Under these conditions the rate of photosynthesis is still comparable to that of respiration. Loss of water exceeding about 80% of the total water of leaf cells depresses photosynthesis to a level lower than that of respiration, but respiration is also reduced. Photosynthesis of leaf cells which survived strong dehydration is not restored parallel to rehydration and exhibits a pronounced lag phase in restoration. 3. In vivo in the light, NADP and PGA reduction and ATP synthesis are observed even if up to nearly 85% of the total water content of the leaf cells is removed by wilting. This finding is consistent with the results obtained in vitro with isolated chloroplasts which were dehydrated by exposure to solutions of different tonicity, and it agrees with measurements of photosynthesis in intact leaves. 4. The significance of the results in relation to drought resistance of plants is discussed.