Subcellular distribution of (35)S-sulfur in spinach leaves after application of (35)SO 4 (2-) , (35)SO 3 (2-) , and (35)SO 2

Sulphite-induced changes of sulphur metabolism in the lichen alga Trebouxia sp

Solutions of sodium sulphite have been used to simulate the effect of gaseous sulphur dioxide on the lichen alga Trebouxia sp. The uptake and the metabolism of inorganic and organic [³⁵ S]compounds such as sulphate, sulphite, and cysteine were investigated in cells of Trebouxia under conditions of sulphite preincubation (sulphite-stressed cells: 3 d pretreatment with 50 mmol m^-3 and 5 mol m^-3 solutions of sodium sulphite). Metabolism was similar, regardless of whether the Trebouxia was provided with [³⁵ S]sulphate or -sulphite. In sulphite-stressed cells, uptake and incorporation into ethanol-soluble compounds, especially cysteine and methionine, was significantly decreased. In Trebouxia exogenous [³⁵ S]cysteine is metabolized only at a very low rate.

A light-enhanced metabolism of sulfite in cells of Cucumis sativus L. cotyledons

The effects of light and several photosynthetic inhibitors on the rate of sulfite metabolism in cells obtained from Cucumis sativus L. cotyledons was studied. The cells were treated with 200 μM Na2SO3 and the disappearance of sulfite was monitored using either dithiobisnitrobenzoic acid or fuchsin. The rate of sulfite disappearance in light was double the dark rate. Disalicylidene propanediamine at 1 mM increased this light-enhanced metabolism approx. 50%; neither 1 μM 3,4-dichlorophenyl-N,N-dimethylurea nor 0.1 mM cyanazine, which completely inhibited CO2-dependent oxygen evolution, affected the rate of sulfite metabolism. Addition of 200 μM Na2SO3 to the cells partially inhibited (14)CO2 fixation. The rate of sulfite consumption by the cells did not affect this inhibition. We conclude that light-dependent sulfite metabolism is cucumber cells may utilize reduced ferredoxin generated as a result of photosynthetic electron transport. An injurious interaction between CO2 fixation and sulfite appears to occur independently of the sulfite-metabolism process.

Long-distance transport of sulfur in Nicotiana tabacum

Sulfur reduction in tobacco plants is a light-enhanced process that predominantly takes place in the leaves rather than the roots. The amount of sulfate reduced in mature leaves can exceed their own requirement and enables an export of reduced sulfur, both basipetal toward the roots as well as acropetal toward the growing parts of the stem. Evidence is presented that translocation of reduced sulfur toward the roots occurs in the phloem. TLC and paper chromatography reveal that glutathione is the main transport form of reduced sulfur in tobacco plants; 67-70% of reduced (35)S was confined to glutathione, 27-30% to methionine, and 2-8% to cysteine.

Disulfide-linked peptides in the chloroplast thylakoid membrane

Two peptides move out from the diagonal when dodecyl sulfate gels of pea leaf chloroplast thylakoid membrane proteins are treated with a reducing agent and electrophoresed in the second dimension. Apparently two of the proteins in the thylakoid membrane are disulfide linked dimers. Additional changes in the electrophoretic mobility of certian peptides occur when thylakoid membranes are denatured in the presence of reducing agents: these peptides probably contain intrapeptide disulfide bonds or prosthetic groups covalently attached to the peptide through disulfide bonds.

THE ROLE OF METAL-ION BINDING IN MODIFYING THE TOXIC EFFECTS OF SULPHUR DIOXIDE ON THE LICHEN UMBILICARIA MUHLENBERGII: II. 14 C-FIXATION STUDIES

Total ¹⁴ C fixation in Umbilicaria muhlenbergii was not significantly affected at levels near the capacity of the first binding site (c. 12 μmol g^-1 ) of class A metal ions or borderline metal ions with class A characteristics: i.e. Sr²⁺ , Mg²⁺ , Ca²⁺ , Ni²⁺ , and Zn²⁺ . Although both Cu²⁺ and Pb²⁺ are borderline ions with significant class B character, only Cu²⁺ greatly reduced fixation. However Ni²⁺ and Pb²⁺ , as well as Cu²⁺ , induced increased photosynthate release from lichen samples. Protection against damage due to SO₂ exposure was rendered by the uptake of class A ions and even borderline ions except those normally accumulated intracellularly (Mg²⁺ , Zn²⁺ ) or possessing the capacity to penetrate cells readily (Cu²⁺ ). Of the borderline ions tested, Pb²⁺ , Cu²⁺ and Ni²⁺ , as well as SO₂ , induced changes resulting in the incorporation of less ¹⁴ C into ribitol and more into sucrose and other sugars during photosynthesis by the lichen samples. The presence of the class A ion Ca²⁺ caused a shift in the opposite direction. The presence of Ca²⁺ , Ni²⁺ , Pb²⁺ , but not Cu²⁺ , on the lichen tended to inhibit the large SO₂ -induced shift away from ribitol production. The threshold levels of various metals reported from field studies to result in visible damage are discussed in terms of the observed photosynthetic perturbations, K²⁺ leakage and factors determining metal uptake by lichens. Finally, the ecological significance of the ability of Ca²⁺ to protect lichens against the damaging effects of air pollution is discussed.

Sulfite: Preferential sulfur source and modifier of CO2 fixation in Chlorella vulgaris

Sulfite was added at the time of inoculation to a standard and to a sulfate deficient medium of Chlorella vulgaris. It was not only used as a sulfur source, but besides this, at concentrations <1.0 mmol l(-1), the growth yield was enhanced up to 30% compared to sulfate saturated conditions. Higher sulfite concentrations increasingly inhibited cell growth. Growth rate determinations indicated that the enhancement, and the inhibition respectively, were confined to the very beginning of culture growth; the time period during which the sulfite was not yet oxidized (5-10 h). In contrast, an increased CO2 fixation rate/unit of protein, occurring up to 5.0 mmol l(-1) sulfite and a shift towards the β-carboxylation pathway, are persisting at least during the growth period of 4 days. The preferential uptake of sulfite, also indicated by a marked increase in methionine content of algal protein, presumably causes an increase in thylakoidal sulfolipids, and is such modifying the CO2 fixation.

Sulfate and sulfite translocation via the phosphate translocator of the inner envelope membrane of chloroplasts

The permeability of the inner envelope membranes of spinach (Spinacia oleracea) chloroplasts to sulfite and sulfate was investigated in vitro, using the technique of silicone oil centrifugal filtration. The results show that there is a permeability towards both ions, resulting in rates of uptake of about 1.0 (SO 3 (2-) ) and 0.7 (SO 4 (2-) ) μmol mg chlorophyll(-1) h(-1) respectively (external concentration 2 mmol l(-1)). The rates depend on the external concentration of the anions. Anion exchange experiments with (35)S-preloaded chloroplasts indicate that sulfite and sulfate are exchanged for inorganic phosphate, phosphoglyceric acid, and dihydroxyacetone phosphate with rates up to 14 nmol mg chlorophyll(-1) min(-1). There is no exchange for glucose-6-phosphate and malate. Because of the similarities to the transport of inorganic phosphate and triose phosphates the results give evidence that the phosphate translocator of the inner envelope membrane of chloroplasts is also involved in sulfite and sulfate transport - at least in part.

Control of (35)SO 4 (2-) and (35)SO 3 (2-) incorporation into spinach chloroplasts during photosynthetic CO2 fixation

In addition to membrane translocation, measured in the dark, it was found that pre-illumination of the chloroplasts resulted in an enhancement of sulfate uptake by 25% and of sulfite uptake by 55% as soon as the concentration of the ion in the incubation medium exceeded 2 mmol l(-1). This amount which is additionally taken up after pre-illumination is less readily exchanged for other ions. Kinetics of the uptake in relation to pre-illumination time and to light intensity closely parallel those of titration of SH-groups by 5,5'-dithiobis (2-nitrobenzoic acid). As a consequence, 10(-6) mol l(-1) DCMU completely inhibits the light triggered increase of uptake of both ions. Uncoupling with 10(-6) mol l(-1) CCCP increases the light induced (35)SO 3 (2-) binding, but decreases that of (35)SO 4 (2-) , demonstrating the need of ATP formation to initiate sulfate reduction. Rates of uptake, measured at different intensities of pre-illumination under nitrogen or in the presence of bicarbonate, suggest that the presence of a carbon skeleton increases the binding rate for both ions. With respect to (35)SO 4 (2-) , the data further indicate a rate limiting step (ATP sulfurylase or adenosine 5'-phosphosulfate sulfotransferase) which is activated by light, thus representing a control step to harmonize the rate of CO2 fixation and of sulfate incorporation. On the contrary, (35)SO 3 (2-) is directly bound in relation to the amount of SH-groups, which in turn are created by the photosynthetic electron transport, resulting in Car-S-SO 3 (-) . Since the formation of SH-groups is maximal already at low light intensities, no effective control step for SO 3 (2-) incorporation is indicated.