"Metallothionein-like" metal complexes in angiosperms; their structure and function

Evidence for the presence of the metal-binding protein metallothionein, MT, in higher plants is equivocal. Although a number of MT-like metal complexes have been isolated from plants, the chemical structures of most of these compounds have not been fully elucidated. Recently a novel class of plant peptides, poly (γ-glutamylcysteinyl) glycines, (γEC)_nG, have been discovered. These peptides bind metal ions, and in the presence of such ions the amount of (γEC), G in plant cells increases. The presence of peptide bonds through the γ-carboxyl group of glutamate, rather than the α-carboxyl group, suggests that these peptides are not encoded by structural genes but are the products of biosynthetic pathways. Cells which are resistant to supra-optimal concentrations of certain metal ions over-produce (γEC)_n G. (γEC)_n G. may be functional analogues of MT. Whether or not some plants also produce MT is an important question which remains to be answered.     

Concentration and annual accumulation values of heavy metals in lake sediments: Their significance in studies of the history of heavy metal pollution

The first part of the paper discusses the significance of using either concentration or accumulation values for expressing the results of investigations of lake sediment cores aimed at studying the history of heavy metal pollution. Neither the values for heavy metal concentration in the lake sediment, whether expressed per gram dry sediment or per gram soluble (organic) sediment, nor the values for their total annual accumulation per unit area of the lake bottom, can, on their own, provide an accurate picture of past pollution conditions, but when considered in combination they render a fairly reliable and detailed interpretation. The second part of the paper deals with Pb, Cd and Hg analyses of cores of varved sediments from several lakes in N. Sweden. Pb and Cd pollution increased during the second half of the 19th century. In most lakes, Hg pollution seems first to have started during the 20th century. Marked increases in both concentration and accumulation of heavy metals took place during the 20th century. For recent decades, the estimated accumulation rates of heavy metals from anthropogenic sources are: Pb 0.5–1.5 µg cm^–2 yr^–1, Cd 15–30 ng cm^–2 yr^–1 and Hg 1–2 ng cm^–2 yr^–1, Higher values were recorded in lakes affected by local emissions.     

An oxygen-evolving complex with a simple subunit structure - ‘a water-plastoquinone oxidoreductase” - from the thermophilic cyanobacterium Synechococcus sp.

An oxygen-evolving complex has been highly purified from the thermophilic cyanobacterium Synechococcus sp. The complex, which reproducibly showed 5 major polypeptide bands of 47, 40, 35, 30 and 9 kDa on SDS-polyacrylamide gel electrophoresis and contained 3.2 Mn per Q_A, had an oxygen-evolving activity of 300–400 μmol/mg chl per h in the presence of 5 mM MnCl₂; or CaCl₂. The complex most likely represents a minimum functional unit of the photosynthetic oxygen evolution.     

Transition Metals Potentiate Paraquat Toxicity

The involvement of transition metal ions in paraquat toxicity was studied in bacterial model system. We show that the addition of micromolar, or lower, concentrations of copper dramatically enhanced the rate of bacterial inactivation. In contrast, the addition of chelating agents totally eliminated the killing of E. coli. No inactivation was observed under anaerobic exposure to paraquat, both in the absence and presence of copper. However, in the presence of copper, the anaerobic addition of hydrogen peroxide resulted in complete restoration of inactivation as under aerobiosis.

Paraquat either produces superoxide ions or directly reduces bound copper ions in a catalytic mode. The reduced cuprous complexes react with hydrogen peroxide to locally form hydroxyl radicals (OH) which are probably responsible for the deleterious effects.

This study indicates the involvement of a site-specific metal-mediated Haber-Weiss mechanism in paraquat toxicity. It is in agreement with earlier observations that copper unusually enhance biological damage induced by either superoxide or ascorbate.     

An enzyme-probe study of motile domains in the subfragment-2 region of myosin

The temperature-dependence of local melting within the subfragment-2 region of rabbit skeletal muscle myosin has been investigated using an enzyme-probe technique. Rate constants of fragmentation of two long subfragment-2 particles (61,000 Mr and 53,000 Mr per polypeptide chain) and a short subfragment-2 particle (34,000 Mr per polypeptide chain) by three different enzymes (alpha-chymotrypsin, trypsin and papain) have been determined over the temperature range 5 to 40 degrees C. We followed the time-course of digestion at specific sites at high (I = 0.50, pH 7.3) and low (physiological, I = 0.15, pH 7.3) ionic strengths by electrophoresis of the digestion products on sodium dodecyl sulfate-containing gels. All rate constants were corrected for the intrinsic temperature-dependence of the enzymes by comparison with model substrates. Normalized rate constant versus temperature profiles for the three enzyme-probes are similar in showing that local melting in long subfragment-2 (61,000 Mr) occurs in two distinct stages as was observed earlier for the intact myosin rod. Over the temperature range 5 to 25 degrees C a restricted region at Mr = 53,000 to 50,000 from the N terminus of the rod (the light meromyosin/heavy meromyosin junction) shows the highest susceptibility to proteolytic cleavage. At temperatures above 25 degrees C local melting was detected by all three enzymes at several specific sites within the hinge domain (Mr = 53,000 to 34,000). Activation energies for cleavage at the susceptible sites were similar for the three enzyme probes. They suggest that this region of the myosin rod has significantly lower thermal stability than the flanking light meromyosin and short subfragment-2 segments. These results, together with other physico-chemical studies, point to the hinge domain of the myosin cross-bridge as an important functional element in the mechanism of force generation in muscle.     

Toxicity and mutual interactions of cadmium and zinc ions in normal and carcinogen-transformed mouse cells

The toxicity of chloride salts of physiological (zinc, manganese, nickel) and non physiological (cadmium) bivalent metal ions was studied in normal or carcinogen-transformed mouse embryo fibroblast cells. The dose response curves for toxicity to both types of cells exhibited similar shapes. The transformed cells, however, were about twice as sensitive to zinc toxicity as normal cells. When normal and transformed cells were grown together and incubated for several hours with an appropriate concentration of zinc, the malignant cells were selectively killed. Cadmium was much more toxic than the three other metal ions in both types of cells. Its toxic effect was reversed by simultaneous addition of zinc at nontoxic concentrations.Abbrevications CFA colony forming ability - MCA 3-methylcholanthrene     

Enzymatic studies on the interaction of myosin and heavy meromyosin with 1,N 6 -ethenoadenosine triphosphate ( ATP), a fluorescent analog of ATP

The fluorescent analog of adenosine triphosphate (ATP)¹ 1,N⁶-ethenoadenosine triphosphate, (εATP), has been utilized as a substitute for ATP in the myosin and heavy meromyosin ATPase systems. For myosin, the analog εATP replaced ATP with a somewhat larger K_m (2.6 × 10^?4 mole ?^?1 for εATP as opposed to 8.8 × 10^?5 mole ?^?1 for ATP), indicating that the apparent affinity of the enzyme for εATP is less than for ATP. Perhaps of more interest, further comparison yielded a V_max for εATP about two and one half times the value for ATP (20 μmole PO₄ sec^?1 g protein^?1 as opposed to 8.1 μmole sec^?1 g protein^?1). Results for the HMM-εATPase system were similar, yielding a K_m value of 1.47 × 10^?4 mole ?^?1 and a V_max of 54.2 μmole PO₄ sec^?1 g protein^?1, as opposed to corresponding K_m and V_max values of 1.23 × 10^?4 mole ?^?1 and 20.4 μmole PO₄ sec^?1 g protein^?1, respectively for the HMM-ATP interaction. The pH dependence of εATPase for both systems was comparable to ATP, suggesting a similarity in the mechanism of hydrolysis of the two nucleotides. Activation of εATPase by Ca²⁺ in the presence of 0.5 M KCl was comparable to ATPase for both systems, but inhibition by Mg²⁺ seemed to be more effective for εATPase. These results indicate that εATP is an excellent substitute for ATP in the myosin and heavy meromyosin systems and because of its insertion into the active site of these muscle proteins, it promises to be a very useful probe for conformation studies at this level.     

Copper uptake by free and immobilized cyanobacterium

Abstract Copper uptake in free and immobilized cells of the cyanobacterium Nostoc calcicola has been examined. The immobilized cells invariably maintained a higher profile of Cu intake rate (12.7 nmol mg⁻¹ protein min⁻¹) over the free cells (6.0 nmol mg⁻¹ protein min⁻¹). The total Cu uptake in immobilized cells was almost two and a half-times more than their free cell counterpart under identical experimental conditions. Also, the immobilized cells showed a stronger positive correlation between Cu adsorption and uptake. The results have been discussed in terms of improved metabolic efficiency of immobilized cells.     

Poly(γ-glutamylcysteinyl)glycines or phytochelatins and their role in cadmium tolerance of Silene vulgaris

Abstract. The effect of cadmium on growth of Cd-tolerant and -sensitive plants of Silene vulgaris and on the production of metal-binding compounds in both types of plants was studied. The Cd-content of the roots and the Cd-root/shoot ratio was higher in Cd-tolerant plants. A Cd-binding compound (Cd-BC) with an apparent molecular mass of 14.5 kD was isolated from the roots of Cd-tolerant and -sensitive plants, grown in 40 mmol m⁻³ Cd for 21 d. More than 60% of the total Cd in the roots was associated with this compound. Determination of the amino-acid content of the purified Cd-containing compound from both types of plants showed that they possessed a similar amino-acid composition to that of phytochelatins. Only the bis- and tris-forms were present. The amount of Cd and sulphide associated with phytochelatin was greater in tolerant plants than in sensitive ones suggesting that an increased sulphide content of complexes containing peptide, sulphide and Cd may form the basis of evolved Cd-tolerance in Silence vulgaris.