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Biomedical subjects

T M Konijn

Publications and source records attributed to T M Konijn.

At least 37 records · Page 2Linked to original sources
PubMed1980

Folic acid deaminase activity during development in Dictyostelium discoideum.

Folic acid attracts vegetative amoebae of Dictyostelium discoideum. Secreted by bacteria, it may act as a food-seeking device. The inactivation of this attractant is catalyzed by a deaminase. As assay has been developed to measure the folic acid deaminase activity. In addition to cell-surface an intracellular deaminase, the amoebae of D. discoideum release the enzyme into the medium. The pH optimum of the extracellular enzyme was 6.0, and higher for the cell-associated deaminases. The extracellular enzyme was secreted maximally by vegetative amoebae, and its activity diminished during cell differentiation. The cell-surface bound enzyme was less active than the extracellular enzyme, and its activity decreased twofold during a 6-h starvation period. The enzyme activity of homogenates and 48,000 x g pellets diminished during this period 35 to 40%. The supernatant of a homogenate had a higher deaminase activity than the homogenate itself or its pellet; this suggests the presence of an inhibitor in the particulate fraction. The underlying mechanism for inactivation of folic acid has similar characteristics as that for inactivation of cyclic adenosine monophosphate.

Aminohydrolases↗
PubMed1979

Intracellular cyclic GMP-binding proteins in cellular slime molds.

The complexity of cyclic GMP-binding activity in the 48,000 X g supernatant of three species of the cellular slime molds (Dictyostelium discoideum, Dictyostelium rosarium, and Polysphondylium violaceum) was studied by gel filtration chromatography on AcA 34 Ultrogel. All these species have in common a cyclic GMP-binding protein of molecular weight of about 2.5 X 10(5) which specifically binds this nucleotide. In addition, Scatchard plots of assays carried out with the 48,000 X g supernatant of these species exhibit cyclic GMP-binding activity with an apparent dissociation constant of about 1 nM. None of the cyclic GMP-binding proteins separated by chromatography on AcA 34 Ultrogel was associated to protein kinase activity stimulation. In view of the cyclic GMP function during chemotactic transduction in the cellular slime molds, the possible molecular function for this 2.5 X 10(5)-dalton cyclic GMP-binding protein is discussed.

Carrier Proteins↗
PubMed1978

A model for cyclic AMP-chemoreceptor interaction in Dictyostelium discoideum.

Based on the chemotactic activity of approximately 50 different adenosine 3',5'-cyclic-monophosphate (cyclic AMP) derivatives with substitutions at the phosphate, ribose and adenine moieties, a model for the cyclic AMP-chemoreceptor interaction in Dictyostelium discoideum is proposed. In this model the cyclic AMP molecule is bound to the receptor by three hydrogen bonds at, respectively, the 3'-oxygen of the ribose and the 6-amino and the 7-nitrogen of the base, and possibly by one ionic interaction of the negatively charged phosphate group. The conformation of the adenine moiety is in the anti range and binds additionally to the receptor by hydrophobic interactions betueen its pi-electron system and a corresponding acceptor at the active site. Although this receptor clearly differs from that involved in protein kinase activation in higher organisms, the existence of striking similarities suggests a basic mechanism for cyclic AMP interaction conserved during evolution.

Binding Sites↗
PubMed1978

Chemotaxis in Dictyostelium discoideum: effect of concanavalin A on chemoattractant mediated cyclic gmp accumulation anlight scattering decrease.

In cells of the cellular slime mold Dictyostelium discoideum concanavalin A (Con A), at a concentration of 100 microgram per ml, inhibits folic acid and cyclic AMP induced decrease in light scattering. Con A has no effect on folic acid mediated cyclic GMP accumulation and increases cyclic AMP mediated cyclic GMP accumulation two-fold. At a lower Con A concentration, 10 microgram per ml, changes in light scattering induced by folic acid are normal and cyclic AMP induces a monophasic instead of a biphasic response. The stimulatory effect of Con A on cyclic AMP mediated cyclic GMP accumulation is still observable at 10 microgram Con A per ml. When cells are repeatedly stimulated with cyclic AMP, a decrease in light scattering without being accompanied by changes in cyclic GMP concentration is observed. Based on these results a model for chemotaxis is proposed.

Chemotaxis↗
PubMed1978

Purification and preliminary characterization of an aggregation-sensitive chemoattractant of Dictyostelium minutum.

Aggregative amoebae of Dictyostelium minutum are not attracted by cyclic AMP; they are sensitive to various attracting sources from which yeast extract was chosen to purify the chemoattractant. A small acrasin-like species-specific molecule which contains glycine and C5H5N5 has been purified 30,000-fold. Several characteristics of this chemotactic molecule, which is inactivated by an enzyme that is not species specific, are described.

Chemotaxis↗
PubMed1977

3':5'-cyclic AMP-dependent 3':5'-cyclic GMP accumulation in Dictyostelium discoideum.

Suspensions of 3':5'-cyclic AMP (cAMP)-sensitive cells of Dictyostelium discoideum responded to a cAMP pulse with increased 3':5'-cyclic GMP (cGMP) levels. Under the assay conditions used (2 x 10(8) cells per ml in 10 mM phosphate buffer, pH 6.0) cAMP (5 x 10(-8) M final concentration) increased cGMP levels from 1 pmol per 10(7) cells to 7 pmol per 10(7) cells in 10 sec and basal levels were recovered in 20-25 sec. cGMP accumulation did not occur when cells were in the cAMP-insensitive stage. cAMP-sensitive cells responded with increased cGMP levels when triggered by 5 x 10(-8) M 5'-CH(2)-cAMP or 10(-5) M adenosine-5'-methylmonophosphate (5'-AMPMe) but not after addition of 5 x 10(-8) M 3':5'-cyclic IMP (cIMP) or 5 x 10(-8) M 5'-AMP. As agonists of cAMP, 5'-CH(2)-cAMP and 5'-AMPMe have, respectively, more than 10% and 1% the chemotactic activity of cAMP, while cIMP has 0.01% the activity of cAMP and 5'-AMP is inactive up to a concentration of 10(-3) M. cAMP-mediated cGMP formation was dependent upon cAMP concentration, with a half-maximal cAMP concentration of about 10(-8) M. This cAMP concentration agrees closely with that necessary for half-maximal receptor occupation. cAMP-mediated cGMP formation was independent of the presence of extracellular Ca(2+); cell aggregation and chemotaxis were also independent of the presence of external Ca(2+). Therefore, cAMP action does not depend on stimulation of the Ca(2+) influx. cAMP was found to mediate desensitization of cAMP-dependent cGMP formation. Addition of 5 x 10(-8) M cAMP to sensitive cells induced a desensitization period that lasted 1-5 min. Desensitization was dependent on the cAMP concentration. Finally, we propose that the translation of a chemotactic signal from the cell surface to pseudopod formation in Dictyostelium involves changes in the levels of cGMP.

Adenosine Monophosphate↗
PubMed1975

Chemotaxis and binding of cyclic AMP in cellular slime molds.

To obtain more information about how cyclic AMP mediates cell aggregation as found in some species of the cellular slime molds, we determined the maximal binding activity of cyclic AMP in different species under various environmental conditions. The binding of cyclic AMP is limited to amoebae using this cyclic nucleotide as chemotactic agent. Maximal binding activity proved to coincide with a maximal chemotactic response and to be related to the length of the period between the vegetative and the aggregative phase. Of the species studied, Dictyostelium discoideum has the highest cellular density of cyclic AMP receptors and is the most sensitive to cyclic AMP as attractant. At 15 degrees C, aggregation begins later, chemotaxis takes effect over a greater distance, and the maximal binding activity is higher than 22 degrees C. The number of cyclic AMP receptors is independent of temperature. The delay in the onset of aggregation and the increased chemotactic response in darkness is not due to a change in the maximal binding activity. The binding of cyclic AMP and its inactivation is discussed in the light of cell aggregation.

Cell Aggregation↗
PubMed1975

Signal input for a chemotactic response in the cellular slime mold Dictyostelium discoideum.

Drops with different concentrations of 3':5'-cyclic AMP were deposited at various distances from small populations of Dictyostelium discoideum amoebae, and the distances over which 50% of the amoebae drops reacted positively were determined. The linear regression analysis of a bouble logarithmic plot of distance against concentration gives a straight line with a slope of 1/4.25, which value suggests that the amoebae respond to a spatial gradient of cyclic AMP concentration. The threshold value for the signal is 3.6 X 10(-9) M/mm with a sensitivity of measurement of about 1%. These findings are discussed in relation to our present knowledge of cyclic AMP receptors.

Chemotaxis↗