Activation of mercuric reductase by the substrate NADPH

Preincubation of the oxidized form of the flavoenzyme mercuric reductase with the reducing substrate, NADPH, or with a high concentration of cysteine (30 mM) results in a substantial increase of the catalytic activity as measured in a standard spectrophotometric assay. Also NADH has some activating effect but NADP+ or EDTA have no effect. In the presence of 1 mM cysteine only one equivalent of NADPH per FAD seems to be required for full activation which occurs after an incubation time of about 10 min. Activated mercuric reductase appears to be stable under anaerobic conditions but eventually returns to the original level of activity in the presence of oxygen. The activated state seems to be stabilized by 1 mM cysteine. Activation of mercuric reductase does not seem to be correlated with a change in the number of reactive thiol groups. The chemical nature of the activation process is not yet understood. Stopped-flow studies have shown that the nonactivated enzyme is practically inactive prior to contact with the substrates. The enzyme is gradually activated during the assay. The kinetics of activation of the 'native' enzyme is biphasic but 'clipped' enzyme, lacking an 85-residue N-terminal domain, is activated in a single first-order process. The progress curves obtained with preactivated enzyme are approximately exponential even at saturating concentrations of NADPH (Km = 0.4 microM at 25 degrees C, pH 7.3) and Hg2+ (Km = 3.2 microM in the presence of 1 mM cysteine). The initial rates yield kcat values of about 13 s-1 per FAD molecule (25 degrees C, pH 7.3). We find no evidence for a thiol-dependent change from a rapid to a slow kinetic phase. The shape of the progress curves presumably depends on product inhibition, but NADP+ is not a sufficiently effective inhibitor to explain the effect fully.     

Identification of proximal tubular transport functions in the established kidney cell line, OK

OK cells, derived from an American opossum kidney, were analyzed for proximal tubular transport functions. In monolayers, L-glutamate, L-proline, L-alanine, and alpha-methyl-glucopyranoside (alpha-methyl D-glucoside) were accumulated through Na+-dependent and Na+-independent transport pathways. D-Glucose and inorganic sulfate were accumulated equally well in the presence or absence of Na+. Influx of inorganic phosphate was only observed in the presence of Na+. Na+/alpha-methyl D-glucoside uptake was preferentially inhibited by phlorizin and D-glucose uptake by cytochalasin B. An amiloride-sensitive Na+-transport was also identified. In isolated apical vesicles (enriched 8-fold in gamma-glutamyltransferase), L-glutamate, L-proline, L-alanine, alpha-methyl D-glucoside and inorganic phosphate transport were stimulated by an inwardly directed Na+-gradient as compared to an inwardly directed K+-gradient. L-Glutamate transport required additionally intravesicular K+. D-Glucose transport was similar in the presence of a Na+- and a K+-gradient. Na+/alpha-methyl D-glucoside uptake was inhibited by phlorizin whereas cytochalasin B had no effect on Na+/D-glucose transport. An amiloride-sensitive Na+/H+ exchange mechanism was also found in the apical vesicle preparation. It is concluded that the apical membrane of OK cells contains Na+-coupled transport systems for amino acids, hexoses, protons and inorganic phosphate. D-Glucose appears a poor substrate for the Na+/hexose transport system.     

Synthesis of methyl 2-acetamido-2-deoxy-3-O-(β-d-galactopyranosyl)-α-d-galactopyranoside from a corresponding thioglycoside derivative

The title disaccharide glycoside was synthesized by halide ion-promoted glycosidation, using methanol and the disaccharide bromide derived from methyl 2-azido-3-O-(2,3,4,6-tetra-O-benzoyl--d-galactopyranosyl)-4,6-O-benzylidene-2-deoxy-1-thio--d-galactopyranoside. This derivative in turn was prepared by silver triflate-promoted condensation of monosaccharide derivatives.     

Carbohydrate receptor specificity of K99 fimbriae of enterotoxigenicEscherichia coli

K99 Fimbriae from enterotoxigenicEscherichia coli (ETEC) were found to bind specifically to sialic acid, as measured in a haemagglutination inhibition assay using the intact bacteria and human erythrocytes. The affinity forN-glycolylneuraminic acid was about twice that ofN-acetylneuraminic acid (NeuAc), and other monosaccharides were found to be at least ten-fold less effective as inhibitors. The specificity was found to depend on electrostatic interaction where the carboxyl group and its orientation plays an important role. 2--Benzyl-NeuAc was a better inhibitor than 2--methyl-NeuAc suggesting a hydrophobic patch near the binding site on the protein. Axially oriented hydroxyl groups as in 4-epi-NeuAc and 3-hydroxy-NeuAc seemed to participate in binding since these derivatives were better inhibitors thanN-acetylneuraminic acid. K99 was found to have a higher affinity for 4-O-acetyl-NeuAc and lower affinity forN-acetylneuraminic acid withO-substituents at C7-C9 as compared toN-acetylneuraminic acid. Hence, the degree ofO-acetylation of sialic acid in the mucosa of the small intestine may influence colonization and determine susceptibility to infection.     

The effect of monensin on thyroglobulin secretion

The effect of monensin on the secretion of thyroglobulin was studied in open follicles isolated from pig thyroid tissue; in this system, thyroglobulin is secreted into the incubation medium. When monensin was present during a 4-h chase incubation after pulse-labelling with 3H-leucine, the secretion of labelled thyroglobulin was reduced by about 85%; in electron-microscopic autoradiographs of rat thyroid lobes labelled and chase-incubated under similar conditions the relative number of grains over follicle lumina was strongly reduced when monensin was present during the chase. These observations are in agreement with the consensus that monensin arrests transport of secretory proteins in the Golgi complex. In other experiments, pulse-labelled follicles were chase-incubated for 1.5 h whereby labelled thyroglobulin was transported from the RER to exocytic vesicles. Monensin present during a subsequent chase of 0.5 h caused only a moderate decrease of labelled thyroglobulin secretion. TSH present during the second chase-stimulated secretion in both control and monensin-exposed follicles. TSH also caused a drastic reduction of exocytic vesicles in rat thyroid lobes, and the number of vesicles remaining in the cells was the same in controls and lobes exposed to the ionophore. The observations are interpreted to show that monensin does not inhibit the basal or TSH-stimulated transport of thyroglobulin from the site of monensin-induced arrest in the Golgi complex to the apical cell surface or the exocytosis of thyroglobulin.     

Testing the “methanochondrion concept”: are nucleotides transported across internal membranes in Methanobacterium thermoautotrophicum?

In order to test the Methanochondrion concept, uptake of adenine nucleotides in various membrane preparations of Methanobacterium thermoautotrophicum was studied. The uptake showed properties which are in general interpreted as indicative of a transport mechanism: (i) kinetics in the time range of minutes, (ii) temperature dependence, (iii) substrate specificity and (iv) failure to remove the substrate by extensive washing.However, nucleotide transport as an interpretation of this uptake can definitely be excluded. Not only an exchange mechanism of the mitochondrial type, but also a general exchange or an uniport mechanism was ruled out. In contrast, the nucleotide uptake was shown to be actually a tight and specific binding of ADP and ATP to binding sites at the interior side of the cell membrane. This was conclusively demonstrated in protoplasts obtained from M. thermoautotrophicum cells. In these protoplasts which do not contain internal membranes also nucleotide binding was observed, but only after disruption of the plasma membrane by osmotic lysis, which leads to the exposure of binding sites.     

Osmoregulation in Escherichia coli by accumulation of organic osmolytes: betaines,glutamic acid,and trehalose

Like other cyanobacteria, Chlorogloeopsis fritschii contained as major lipid classes monogalactosyldiacyl-glycerols, digalactosyldiacylglycerols, sulfoquinovosyldiacylglycerols and diacylglycerophosphoglycerols. In addition to these lipid classes this cyanobacterium also contained small amounts of diacylglycerophosphocholines and sterols, predominantly lanosterol, thus showing similarity to photosynthetic eukaryotes. Dark incubated cells contained larger proportions of the latter two lipid classes than light grown cells. Like other prokaryotes, C. fritschii lacked linolenic acid (18:3) in its lipids. Lipids from the thylakoids were richer in palmitoleic acid (16:1) than those of whole cells. There was no effect of light on the patterns of constituent fatty acids of lipids from C. fritschii, in contrast to photosynthetic eukaryotes.Abbreviations MGDG Monogalactosyldiacylglycerols - PA Phosphatidic acids - PE Diacylglycerophosphoethanolamines - PG Diacylglycerophosphoethanolamines - DGDG Digalactosyldiacylglycerols - SQDG Sulfoquinovosyldiacylglycerols - PC Diacylglycerophosphocholines     

Chromosome analysis of human sperm

Summary A modified technique has been developed for the visualization of the chromosomes in human sperm. The cytogenetic analysis of 129 G-banded human sperm metaphases of 6 normal donors showed an incidence of structural and numerical chromosome abnormalities of 7.8%. Two out of 129 spermatozoa were aneuploid (1.6%). The frequency of sperms with chromatid-type aberrations was 2.3% (3/129). Chromosome-type aberrations were found in 5 out of 129 (3.9%) spermatozoa. X to Y ratio did not differ significantly from the expected one-to-one ratio. Twenty-six sperm complements from a patient 18–20 months after testes exposure to 30 Gy were examined. A significant increase of numerical and structural chromosome abnormalities was not observed. Chromatidtype aberrations were found in two sperm complements (7.7%) and chromosome-type aberrations in one sperm complement (3.9%). The cytogenetic analysis of 15 human sperms from a cancer patient 26 months after chemotherapy showed an increased frequency of aberrant sperm complements (33.4%). One chromatid-type (6.7%), three chromosometype aberrations (20.0%) and one (6.7%) hyperploid sperm complement could be observed. The sample size is still too small to answer the question whether chemical mutagens may increase the frequency of chromosomal abnormalities in human sperm.