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K F Jensen

Publications and source records attributed to K F Jensen.

98 records · Page 6Linked to original sources
PubMed1976

Purine-nucleoside phosphorylase from Salmonella typhimurium and Escherichia coli. Initial velocity kinetics, ligand banding, and reaction mechanism.

Purine nucleoside phosphorylase from Salmonella typhimurium has been subjected to kinetic analysis i.e. determination of initial velocity patterns and product inhibition studies. The kinetic results suggest that the enzyme works by a sequential reaction mechanism, where the nucleoside, phosphate, and pentose 1-phosphate are all able to bind to the free enzyme, whereas it appears that the purine base binds after addition of the pentose 1-phosphate. The proposed mechanism is confirmed by substrate binding studies. In addition to the enzyme-substrate complexes suggested by the kinetics, the binding studies revealed a 'dead end' complex, consisting of enzyme, phosphate, and purine base. Similar binding experiments were carried out using the enzyme from Escherichia coli. The results suggest that this enzyme works by an identical reaction mechanism. The binding data are in agreement with the presence of six binding sites per native enzyme molecule, one binding site per subunit, for each ligand. Both enzymes show normal Michaelis-Menten kinetics for their substrates with the exception of phosphate, for which the double-reciprocal plots are concave down. This behaviour is seen in both binding and velocity curves, and most likely is a result of negative cooperativity in the binding of phosphate to the enzyme.

Binding Sites↗
PubMed1976

Iodination as a probe for small regions of disrupted secondary structure in double-stranded DNAs.

Conditions were established where the thallium-catalyzed iodination of random coil DNA proceeded 100-200 times faster than for native DNA. This reaction was explored as a probe for localized regions of disrupted base pairs in duplex DNA. A heteroduplex was constructed between DNA fragments produced by Hind II + III cleavage of phi80 plac DNA and phi80 plac DNA containing the Ll deletion (73 nucleotides in length). This heteroduplex incorporated twelve times as much iodine as the parent homoduplex fragments. Hence the technique could reveal the presence of a few (two or more) nonpaired cytosines, if they existed within an otherwise helical DNA fragment 789 base pairs long. Iodination studies were performed on superhelical SV40 DNA and on linear lambdaplac DNA. Analysis of the relative amount of iodine in restriction endonuclease fragments of these DNA's revealed the absence of localized single-stranded regions.

Coliphages↗
PubMed1975

Purine nucleoside phosphorylase from Escherichia coli and Salmonella typhimurium. Purification and some properties.

The purine nucleoside phosphorylases from Escherichia coli and from Salmonella typhimurium have been purified to electrophoretic homogeneity and crystallized. Comparative studies revealed that the two enzymes are very much alike. They obey simple Michaelis-Menten kinetics for their substrates with the exception of phosphate for which they show negative cooperativity. Gel filtration on Sephadex G-200 of the native enzymes revealed a molecular weight for both enzymes of 138000 plus or minus 10%. By use of dodecylsulphate gel electrophoresis a subunit molecular weight of 23700 plus or minus 5% was determined, suggesting that both enzymes consist of six subunits of equal molecular weight. When the subunits were partially crosslinked with dimethyl suberimidate before dodecylsulphate electrophoresis six protein bands were observed in agreement with the proposed oligomeric state of the enzyme, consisting of six subunits of equal molecular weight. Analysis of the amino acid composition also indicates that the subunits are identical. 6M guanidinium chloride dissociates the enzymes; association experiments with native and succinylated enzymes suggested that only the hexameric form is active. Both enzymes could be dissociated into subunits by p-chloromercuribenzoate; this dissociation is prevented by the substrates: the nucleosides, the pentose 1-phosphates, and mixtures of phosphate and purine bases.

Amino Acids↗
PubMedDate not supplied

Glutamate neurotoxicity in rat auditory system: cochlear nuclear complex.

In other systems such as the hypothalamus and hippocampus, it has been shown that cells postsynaptic with respect to glutamatergic inputs degenerate when exposed to large doses of glutamate ("glutamate neurotoxicity"). We have shown that large doses of glutamate administered intraperitoneally are toxic to spiral ganglion cells in the inner ear of the rat. In the present study, we have investigated whether similar levels of glutamate cause alterations in the neurons of the cochlear nuclei. Specifically, we have studied the morphology and size of the cochlear nuclear complex and its subdivisions as well as the size and density of cochlear nucleus neurons following administration of glutamate. The morphological evidence indicates that glutamate caused severe anatomical alteration of the cochlear nuclei. The changes were most pronounced in the anteroventral cochlear nucleus, especially in the neurons that receive terminals of the end bulbs of Held from the cochlear nerve. This could be a direct effect of glutamate in the cochlear nuclei or secondary to degeneration of cochlear nerve fibers in the inner ear.

Animals↗
PubMedDate not supplied

Neonatal exposure to trimethyltin disrupts spatial delayed alternation learning in preweanling rats.

Trimethyltin is an organotin compound that produces marked neurotoxicity in both adult and developing animals. The limbic system is a primary CNS target site for this toxicity, and a prominent behavioral effect of TMT is disruption of learning and memory. Impairment of cognitive development has also been suggested by studies showing that rats neonatally exposed to TMT cannot perform spatial working memory tasks during adulthood. However, the question of how early in ontogeny such deficits can be detected has not been addressed. The present study examined this question with a T-maze delayed alternation learning paradigm. Long-Evans rat pups, injected IP on Postnatal Day 10 (PND 10) with 6 mg/kg TMT and tested on PND 18, were unable to learn delayed alternation in the manner shown by vehicle control pups. However, TMT- and vehicle-treated groups were both able to learn a simple position discrimination. These findings indicate a selective impairment of spatial working memory by neonatal TMT exposure and show that this impairment can be demonstrated during the preweanling period in the rat.

Aging↗
PubMedDate not supplied

Three genes preceding pyrE on the Escherichia coli chromosome are essential for survival and normal cell morphology in stationary culture and at high temperature.

Previous studies of the upstream region of the pyrE gene in Escherichia coli revealed three genes of unknown function. Inactivation of these genes (designated orfE, orfX and orfY) by crossing the KmR-cassette-disrupted orf into the chromosome indicated that they were not required during exponential growth (Poulsen et al., Mol., Microbiol., 1989 b). Here we report that the three genes are of importance in the stationary phase. Thus, cultures of the mutants grown to a stationary state in rich media contained bacterial filaments of abnormal morphology. In addition, flow cytometric analyses showed that outgrown cultures of the orf mutants have anomalous size distribution and DNA content, and that rifampicin treatment of exponentially growing mutants results in cell populations with chromosome numbers in the range from about 1 to 10, compared with wild type strains that end up with 4 and 8 full chromosomes. Finally, it appeared that the three orf's are indispensable at high temperatures since the insertion mutants were unable to form colonies above 45 degrees C and since cultures of exponentially growing mutants lysed upon a temperature shift from 37 degrees C to 45 degrees C.

Bacterial Proteins↗