Investigation of the apparent inefficiency of the coupling between Photosystem II electron transfer and ATP formation

Measurement of chloroplast ATP synthesis activity in Arabidopsis

There are numerous options for monitoring ATP synthesis in chloroplasts using isolated thylakoid membranes, intact chloroplasts, and even whole leaves. Currently, the most commonly used method employs isolated thylakoids coupling the synthesis of ATP to light emission from luciferin in a reaction catalyzed by luciferase. The luciferin-luciferase assay can be highly sensitive and is a direct measure of ATP. Another direct measurement of ATP is the incorporation of 32P into ATP, which, while more technically difficult, has the advantage over the luciferin-luciferase assay of being able to distinguish newly synthesized from total ATP. The phosphorylation of ADP results in a net decrease in pKa (acid disassociation constant) between the reactants and the product ATP, resulting in an increase in the pH of the assay media, which can be used as a convenient, continuous measurement of ATP synthesis. The formation of ΔμH+ across the thylakoid membrane and its concomitant dissipation as ATP is synthesized can be measured by an electrochromic absorption band shift (ECS) of thylakoid pigments measured at 518 nm (Witt, Biochim. Biophys. Acta 505:355-427, 1979; Petty and Jackson, Biochim. Biophys. Acta: Bioenergetics 547:463-473, 1979). The first-order decay time of the ESC can be used to estimate the rate of ATP synthesis providing a noninvasive, indirect method for measuring ATP synthase activity that can be used with intact leaves.

Selenocysteine is selectively taken up by red blood cells

Both organic and inorganic forms of selenium are utilized in the biosynthesis of selenoproteins. Selenite is taken up by red blood cells and then returned to the plasma after reduction, but little is known about the metabolic fate of selenocysteine. We found that selenocysteine was taken up into red blood cells without decomposition into selenide.

ATP formation onset lag and post-illumination phosphorylation initiated with single-turnover flashes. I. An assay using luciferin-luciferase luminescence

The great sensitivity of the luciferin-luciferase ATP detection system allows direct observation of ATP formation derived from single-turnover flashes in a thylakoid reaction mixture. The method can measure the energization threshold--the number of flashes required for the initiation of ATP formation--as well as detect post-illumination ATP formation after the last flash of a flash sequence. We describe the characteristics of this post-illumination phosphorylation which can be observed after a series of phosphorylating flashes (PIP+) or when the assay for ATP formation was performed in a "traditional" manner where the ADP and Pi were added after the flash-energization period (PIP-). Comparing PIP+ yields and kinetics of the PIP+ decay under various treatments can give information about membrane energization events only if it is clearly established that different PIP+ yields and decay rates are not due to limitations of the luciferase-catalyzed reaction. Experiments showing that the PIP+ ATP yield and kinetics were due to membrane-limited deenergization events (proton efflux) rather than luciferase limitations include: (1) An uncoupler, nigericin, added after the last flash reduced the PIP+ yield, but had no effect on the luciferase reaction. (2) The kinetics of the luminescence after adding standard ATP were much faster than the PIP+ kinetics. (3) Valinomycin and K+ stimulated the PIP+ yield but had no influence on the luciferase reaction. (4) Lowering the pH from 8 to 7 increased both the PIP- (an assay independent of luciferase kinetics) and the PIP+ ATP yields, an expected result owing to the greater endogenous buffering power encountered by the proton gradient when the external pH is 7. In spite of the last-mentioned point, the threshold flash number for ATP formation onset was the same for pH 7 and 8 (valinomycin, K+ present) at slow flash frequencies. This is consistent with a membrane-localized rather than a delocalized gradient. The accompanying reports (W. A. Beard, G. Chiang and R. A. Dilley, and W. A. Beard and R. A. Dilley, J. Bioenerg. Biomembr.) show that different conditions can lead to observing either localized or delocalized proton gradient coupling in the PIP+ event and the ATP onset threshold flash number.

Stoichiometry of proton uptake in isolated pea chloroplasts under different light intensities

The number of protons released inside the chloroplast thylakoids per electron which is transferred through the electron transport chain (H+/e- ratio) was measured in isolated pea chloroplasts at pH 6.0 under continuous illumination and with methyl viologen as an electron acceptor. At saturating light intensity (200 W X m-2) ("strong" light) the H+/e- ratio was 3. At low intensity (0.9 W X m-2) ("weak" light) the H+/e- ratio was 2 with dark-adapted chloroplasts, but it was close to 3 with chloroplasts that were preilluminated with strong light. It is shown that the presence of azide in the reaction mixture leads to errors in the determination of the H+/e- ratio due to underestimation of the initial rate of H+ efflux on switching off the light. To explain the above data, we assume that transformation of the electron transport chain occurs during illumination with strong light, namely, the Q cycle becomes operative.

Cooperation among electron-transfer complexes in ATP synthesis in chloroplasts

We have investigated the extent to which redox reactions of thylakoid membranes cooperate in ATP synthesis. This was done by measuring the onset of ATP synthesis following a series of single-turnover light-flashes at various levels of electron transport inhibition. In the presence of antibiotics that prevent the formation of a membrane potential, the onset of ATP synthesis seems to depend entirely on the formation of an adequately large delta pH. Under our conditions, the accumulation of about 60 mmol H+ X mol chlorophyll-1 is then necessary to form the requisite delta pH, which in turn requires about 15 saturating flashes in uninhibited thylakoid samples. Inhibition of some of the electron transfer centers by limiting the light intensity of the flashes, by dichlorophenyldimethylurea, by heat treatment, or by NH2OH-treatment caused an increase in the number of flashes required for the onset of ATP synthesis. The increase in the requisite number of flashes reflected the decreased number of electrons transferred in each flash, almost exactly the same number of electrons being transferred before ATP synthesis could begin. This effect of inhibitors was true when the two photosystems were operating in unison and when either of the two photosystems was acting alone. However, when either photosystem acted alone, there was an increase in the number of flashes required for the onset of ATP synthesis, an increase which was consistent with the observed lower flash-induced proton accumulation. A mathematical analysis of the onset of flash-induced ATP synthesis shows that at least several hundred proton-translocating electron transport complexes must be cooperating to form the threshold delta pH. In spite of this evidence for extensive cooperation among different electron transport complexes in ATP formation, the implied pooling of H+ ions does not seem to involve inner vesicle regions accessible to exogenous buffers. Thus, even when the number of H+ ions accumulated per flash was reduced by 70% through attenuation of the intensity of the flashes, exogenous hydrogen ion buffers present within the lumen of the thylakoid vesicle had no effect on the number of flashes required for the onset of ATP synthesis.