Achim Trebst

Photosynthetische Reaktionen in lyophilisierten Zellen der Blaualge Anacystis

Intact cells of the bluegreen algae Anacystis nidulans photoreduce p-benzoquinone under O2-evolution but not ferricyanide. Catalytic amounts of a quinone catalyze the photoreduction of ferricyanide; the more positive its redoxpotential the better is the stimulation. By the simple procedure of lyophilizing Anacystis in 5% sucrose and resuspending in 4·10—2-m. magnesiumchloride a preparation is obtained which exhibits the same photosynthetic activities as known from isolated chloroplast fragments, i. e. they evolve oxygen and photoreduce ferricyanide, quinones and NADP *; these Hill- reactions are coupled to ATP-formation; they catalyze pseudocyclic and cyclic photophosphorylation. Rates compare favorably with those of isolated chloroplasts except for the NADP-system. A number of known inhibitors like DCMU, KCN, dinitrophenol, salicylaldoxime and ammoniumchloride were checked and found to behave in principal as in photosynthetic chloroplast reactions.

Zur Hemmung photosynthetischer Reaktionen in isolierten Chloroplasten durch Salicylaldoxim

Oxygen evolution and coupled ATP formation in a Hill - reaction with NADP or ferricyanide and cyclic photophosphorylation with vitamin K3 by broken chloroplasts are inhibited by 10-2-m. salicylaldoxim and 2 · 10-2-m. KCN, but photosynthetic reduction of NADP by dichlorphenolindophenol/ascorbate is not. This seems to indicate that salicylaldoxime acts on a site between the two light reactions of photosynthesis and suggests a possible role for copper in photosynthesis.

Untersuchungen über die Beteiligung des Sauerstoffs in photosynthetischen Reaktionen mit Hilfe von Hemmstoffen

Hydrogenperoxide formation in the light in isolated chloroplasts is markedly increased upon the addition of KCN, diethyldithiocarbamate and 3-amino-1.2.4-triazol, whereas photophosphorylation is not influenced by these compounds. Catalase activity of chloroplasts is inhibited by KCN, diethyldithiocarbamate and aminotriazol. Since furthermore the oxidation of chlorogenic acid requires light, we assume that the reduced cofactor in oxygen catalyzed photophosphorylation is not reoxidized by phenoloxidase but by a chlorophyll sensitized photoöxidation accompanied by H2O2-formation. In the absence of inhibitors the H2O2 is decomposed by catalase. The influence of dichlorphenyldimethylurea and o-phenanthroline on cyclic photophosphorylation in the absence of oxygen has been investigated. Photophosphorylation catalyzed by Vitamin K or dichlorphenolindophenol is not inhibited provided that the cofactor is in the reduced state at the beginning of the reaction. The finding that indophenols are catalysts of cyclic photophosphorylation without the participation of oxygen explains former results hitherto hard to fit into the known schemes of electrontransport in chloroplasts.

Photosynthetische Reaktionen in UV-bestrahlten Chloroplasten

Photosynthetic reactions in isolated chloroplast fragments were investigated after they had been exposed to UV-light of 254 mμ, which is found to destroy the endogenous plastoquinone. The oxygen evolution is the most UV sensitive photosynthetic reaction. The ATP formation in the noncyclic and in the cyclic system with vitamin K5 as cofactor is less sensitive; the inactivation goes parallel to the destruction of the plastoquinone of the chloroplasts. Cyclic photophosphorylation with PMS as cofactor is diminished only after longer UV-irradiation. Photosynthetic reduction of NADP at the expense of an artificial electron donor system like DAD/ascorbate is not inactivated even after prolonged UV-exposure of the chloroplasts and therefore requires no plastoquinone. This indicates again, that the site of function of plastoquinone is close to the second light-reaction of photosynthesis.

Über die Konstitution eines weiteren Plastochinons und seines Dimeren aus Kastanienblättern

Small amounts of plastoquinone-20 and the dimeres of plastoquinone-20 and plastoquinone-45 have been isolated from chestnuttree leaves in addition to plastoquinone-45. Their structures have been investigated mainly by spectroscopic means. Dimerisation of pure plastoquinone-45 was brought about by irradiation with UV light.

Zum Mechanismus des photosynthetischen Elektronentransportes in isolierten Chloroplasten

The behavior of 2,3,5,6-tetramethyl-p-phenylendiamine (DAD) in photosynthetic reactions of isolated chloroplast fragments was compared with that of N-tetramethyl-p-phenylendiamine (TMPD). Both reverse the DCMU-inhibition of photosynthetic NADP-reduction. The DAD-system (at high concentrations of DAD), is coupled to a stoichiometric ATP-formation, whereas the TMPD-system is not. This shows that p-phenylendiamines, depending on their constitution, may react with components of the electron transport chain of chloroplasts before or after the phosphorylation site, and locates the phosphorylation step of photosynthetic phosphorylation between two endogenous compounds in that part of the electron transport chain, which connects the two light reactions of photosynthesis. At lower concentrations of DAD the diminished NADP-reduction is no longer coupled to ATP-formation, indicating a second point of entry of electrons from DAD into the electron transport chain. DAD, furthermore, is a cofactor of cyclic photophosphorylation. It therefore behaves like DCPIP, but the rates of the DAD-system are higher.

Intramolekulare Wasserstoffverschiebung in der Hexosephosphatisomerase-Reaktion bei der photosynthetischen Stärkebildung in Chlorella

Chlorella pyrenoidosa was illuminated in the presence of glucose and fructose echa having carbon-14 and tritium markers. Starch and sucrose were degraded to obtain the intramulocular distribution of 14C and tritium. Glucose-1-14C-2-T was incorporated into the glucose of starch and into the glucose moiety of sucrose without change of the T/14C ratio. Fructose-1-14C-1-T (labelled stereospecifically in the isomerase position) was incorporated into starch, yielding predominately glucose-1-14C-2-T, whereas fructose-1-14C-1-T (labelled in the nonisomerase position) gave mainly glucose-1-14C-1-T. The results, in particular the transfer of tritium from C-1 of fructose to C-2 of glucose, are interpreted as being a strong indication for an intramolecular hydrogen transfer in the hexosephosphat-isomerase reaction in a multienzyme system of the photosynthetic starch biosynthesis in vivo, shielding the hexoses of T-exchange reactions with water.

Über eine p-Hydroxylierung in isolierten Chloroplasten

Salicylaldoxime at a concentration of 10-2-m. is an inhibitor of the Hill - reaction and therefore also of aerobic photophosphorylation in isolated chloroplasts. This might indicate a functional role for copper in the electron transport system of photosynthesis. At a concentration of 10-3-m. and lower, salicylaldoxime is not an inhibitor, but a cofactor of aerobic photophosphorylation. This is due to its hydroxylation to the p-hydroquinone, which is the actual cofactor. This p-hydroxylation, which is probably catalyzed by a peroxidase, takes place only with salicylaldehyde and its oxime, but not with other phenols, whether they have a carbonyl function attached to the phenyl ring or not. A number of o- and p-hydroquinones with a carbonyl function at the phenyl ring and two naphthohydroquinones, of which the corresponding quinone cannot be prepared by chemical means, are reversibly oxidized and reduced in photosynthetic phosphorylation in chloroplasts. A possible explanation is that these hydroquinones are oxidized only to the semiquinone level and rapidly reduced again.

Photosyntheseversuche in Tritiumwasser mit Chlorella

The incorporation of tritium into carbohydrates during photosynthesis with Chlorella was investigated. The fixation of 14C and T was compared by providing 14CO2 and HOT simultaneously. The intramolecular distribution of T in hexose and glyceric acid was determined by degradation. The incorporation of T during the carboxydismutase, the aldolase and triosephosphate-isomerase reaction was studied in enzyme systems. The results indicate that - in general - incorporation of T into the products of photosynthesis is due to isomerisation reactions. The influence of isotope effects at various stages of the photosynthetic cycle on the final T-content is discussed. The intramolecular T-distribution in the glucose from starch is different from that of soluble glucose derivatives. To interpret this, it is assumed that the isomerisation of fructose-6-phosphate to glucose-6-phosphate firstly is completely intramolecular in intact Chlorella and secondly shows a large discrimination for tritium-containing (at C-1) molecules. Thirdly, it is proposed that the sequence of reactions from fructose-6-phosphate to starch occurs in a multienzyme system without exchange with free sugar phosphate-pools.

Chinone in photosynthetischen Phosphorylierungen

The Hill- reaction with benzoquinones as electronacceptors is coupled to a phosphorylation. All examined benzo-, naphtho- and anthraquinones are cofactors of cyclic photophosphorylation in air. Quinones with redoxpotentials lower than zero are also cofactors of cyclic photophosphorylation without oxygen. It is assumed, that those hydroquinones are able to react with an oxidized cytochrome. Substituted benzoquinones related to plastoquinone, naphtho- and anthraquinones with potentials lower than zero, chlorogenic acid and DOPA are particularly active.