Copper and cobalt in African species ofAeolanthus Mart. (Plectranthinae,Labiatae)

Summary Hyperaccumulators (here defined as species containing at least 500 μg/g dryweight of either copper or cobalt in leaves), are of interest for the fields of mineral exploration and phytochemistry. Reported hyperaccumulation of copper inAeolanthus biformifolius and the presence of two other species on copper/cobalt mineralization in Shaba (Za?re) led to a survey of these elements in 49 species of the African genusAeolanthus Mart.A. biformifolius appears to be also a hyperaccumulator of Co (2520 μg/g in leaves, 4300 μg/g in corms). Cobalt levels for most species ofAeolanthus were considerably above typical values for phanerogams and indicate the favorable potential of the genus for further study.     

Design of laboratory continuous-culture equipment for accurate gaseous metabolism measurements

The accuracy of kinetic and stoichiometric data obtained from most laboratory-scale continuous-culture equipment, particularly involving gaseous measurements, may be much lower than many workers realize, despite the use of good quality instruments. For example, errors in specific oxygen uptake measurements (QO(2)) easily can be as high as +/-100%. This article assesses the accuracies of individual instruments and of the overall system in greater detail than has previously been reported and suggestions are made as to how the errors can be reduced to acceptable levels.     

Anion and ionic strength effects upon the oxidation of cytochrome c by cytochrome c oxidase

Citrate and other polyanion binding to ferricytochrome c partially blocks reduction by ascorbate, but at constant ionic strength the citrate-cytochrome c complex remains reducible; reduction by TMPD is unaffected. At a constant high ionic strength citrate inhibits the cytochrome c oxidase reaction competitively with respect to cytochrome c, indicating that ferrocytochrome c also binds citrate, and that the citrateferrocytochrome c complex is rejected by the binding site at high ionic strength. At lower ionic strengths, citrate and other polyanions change the kinetic pattern of ferrocytochrome c oxidation from first-order towards zero-order, indicating preferential binding of the ferric species, followed by its exclusion from the binding site. The turnover at low cytochrome c concentrations is diminished by citrate but not the K_m (apparent non-competitive inhibition) or the rate of cytochrome a reduction by bound cytochrome c. Small effects of anions are seen in direct measurements of binding to the primary site on the enzyme, and larger effects upon secondary site binding. It is concluded that anion-cytochrome c complexes may be catalytically competent but that the redox potentials and/or intramolecular behaviour of such complexes may be affected when enzyme-bound. Increasing ionic strength diminishes cytochrome c binding not only by decreasing the ‘association’ rate but also by increasing the ‘dissociation’ rate for bound cytochrome c converting the ‘primary’ (T) site at high salt concentrations into a site similar kinetically to the ‘secondary’ (L) site at low ionic strength. A finite K_m of 170 μM at very high ionic strength indicates a ratio of $\text{K}{}^{\mathrm{\infty }}{}_{\mathrm{<mtext>M</mtext>}}\text{K}{}^0{}_{\mathrm{<mtext>M</mtext>}}$ of about 5000. It is proposed that anions either modify the E¹₀ of cytochrome c bound at the primary (T) site or that they perturb an equilibrium between two forms of bound c in favour of a less active form.     

Selective binding of specific rat epididymal secretory proteins to spermatozoa and erythrocytes

Tissue pieces from the caput epididymidis of the rat were incubated in vitro with (³⁵S) methionine to produce radioactive secretory proteins. The radioactive secretory proteins so formed were tested for their ability to bind to washed rat spermatozoa collected from the rete testis and cauda epididymidis, and to rat erythrocytes. The sperm and erythrocytes bound approximately 5% of the total radioactive protein. Binding was protein-specific in that only selected proteins became associated with the cells. Binding was not cell-specific, however, since testicular spermatozoa, caudal spermatozoa, and erythrocytes all bound the same proteins to a similar degree.     

Amplification of D-xylose and D-glucose isomerase activities in Escherichia coli by gene cloning

A recombinant plasmid, designated pUC1002, was constructed by ligation of a HindIII restriction endonuclease fragment of Escherichia coli chromosomal DNA to vector plasmid pMB9. Strains carrying this plasmid were selected by transformation of an E. coli strain bearing the xyl-7 mutation to a xylose-positive (Xyl+) phenotype. Strains containing pUC1002 produced coordinately elevated levels of D-xylose isomerase and D-xylulose kinase. Under appropriate conditions, the isomerase also efficiently catalyzed the conversion of D-glucose to D-fructose.     

Mucopolysaccharidosis VI (Maroteaux-Lamy syndrome). An intermediate clinical phenotype caused by substitution of valine for glycine at position 137 of arylsulfatase B.

The Maroteaux-Lamy syndrome (mucopolysaccharidosis type VI) is a lysosomal storage disease with autosomal recessive inheritance caused by deficiency of the enzyme arylsulfatase B. Severe, intermediate, and mild forms of the disease have been described. The molecular correlate of the clinical heterogeneity is not known at present. To identify the molecular defect in a patient with the intermediate form of the disease, arylsulfatase B mRNA from his fibroblasts was reverse-transcribed, amplified by the polymerase chain reaction, and subcloned. Three point mutations were detected by DNA sequence analysis, two of which, a silent A to G transition at nucleotide 1191 and a G to A transition at nucleotide 1126 resulting in a methionine for valine 376 substitution, were polymorphisms. A G to T transversion at nucleotide 410 causing a valine for glycine 137 substitution (G137V) was identified as the mutation underlying the Maroteaux-Lamy phenotype of the patient, who was homozygous for the allele. The kinetic parameters of the mutant arylsulfatase B enzyme toward a radiolabeled trisaccharide substrate were normal excluding an alteration of the active site. The G137V mutation did not affect the synthesis but severely reduced the stability of the arylsulfatase B precursor. While the wild type precursor is converted by limited proteolysis in late endosomes or lysosomes to a mature form, the majority of the mutant precursor was degraded presumably in a compartment proximal to the trans Golgi network and only a small amount escaped to the lysosomes accounting for the low residual enzyme activity in fibroblasts of a patient with the juvenile form of the disease.     

Characterization of "native" apomyoglobin by molecular dynamics simulation.

We have used molecular dynamics simulation methods to study the structure and fluctuations of "native" apomyoglobin in aqueous solution for a period of greater than 0.5 nanosecond. This work was motivated by the recent attempts of Hughson et al. to characterize the structure and motion of both this molecule and the less compact, acid stabilized I stage, using methods of pulsed H/2H exchange. The study of these systems provides new insights into protein folding intermediates and our simulation has yielded a detailed model for structure and fluctuations in apomyoglobin which complements the experimental studies. We find that local (short-time) fluctuations agree well with fluctuations observed for the holoprotein in aqueous solution, as well as results from the crystallographic B-factors. In addition, the structural features we observe for native apomyoglobin are very similar to the holoprotein, in basic agreement with the findings of Hughson et al. By examining larger-scale motions, developing only over timescales in excess of a 100 picoseconds, we are able to identify conformationally "labile" and "non-labile" regions within native apomyoglobin. These regions correspond extremely well to those identified in the nuclear magnetic resonance experiments as unstable and stable "folding subdomains" in the I state of apomyoglobin. Overall we find that helices A, B, E, G and H show the least amount of motion and helices C, D and F move substantially over the timescales examined. The major motions, and the primary difference between the holo and apo structures as we have observed them, are due to the shifting motion of helices C, D and F into the vacant heme cavity. We also find that motions at the interface of helical segments can be large, with one important exception being the chain segment connecting helices G and H. This segment of chain interacts with the conformationally "non-labile" helix A to form a relatively rigid subdomain composed of helices A, G and H. We believe that these findings provide direct support for the suggestion of Hughson et al. that helices A, G and H constitute a compact subdomain that remains in a native-like conformation as the protein begins to unfold in environments of decreasing pH.