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M Cohn

Publications and source records attributed to M Cohn.

At least 109 records · Page 6Linked to original sources
PubMed1981

31P NMR of enzyme-bound substrates of rabbit muscle creatine kinase. Equilibrium constants, interconversion rates, and NMR parameters of enzyme-bound complexes.

The reaction catalyzed by rabbit muscle creatine kinase ATP + creatine in equilibrium ADP + P-creatine has been investigated by 31P NMR. At pH 8.0 and 4 degrees C, the equilibrium constant of the overall reaction [P1][P2]/[S1] [S2] is found to be 0.08, while that for the interconversion step between enzyme-bound substrates and products [E.P1. P2]/[E.S1.S2] is estimated to be approximately 1; the latter value is the same for all other kinases investigated. The rate of interconversion of enzyme-bound substrates and products is approximately 90 s-1 and is not the rate-limiting step of the overall reaction. Of the phosphate groups in enzyme complexes of reactants or products, the 31P chemical shifts of beta-P(ADP) and beta-P[MgADP) change by approximately 2 ppm downfield while all others change by less than 0.8 ppm. In the transition state analog complexes E.MgADP.NO3-.creatine and E.MgADP.HCOO-.creatine, the beta-P(MgADP) signal shows a substantial upfield shift in the direction of the beta-P(MgATP) resonance. The pattern of chemical shifts and line shapes of nucleotide complexes of creatine kinase parallel those for the corresponding complexes of arginine kinase, indicating structural and/or conformational similarity of the phosphate chains of nucleotides bound to the two enzymes. However, a difference in active sites is indicated by the pH independence (pH 6.0 to 9.0) of the chemical shift of the beta-P of MgADP bound to creatine kinase, whereas with arginine kinase this resonance showed a pKa approximately 7.5.

Adenosine Diphosphate↗
PubMed1981

Tumorigenicity and lysis by natural killers.

Detailed analysis of the natural killer (NK) activity directed at nontumorigenic cell lines and their transformed tumorigenic derivatives has revealed a paradox. On the one hand, a correlation has been found between the tumorigenic potential of chemically transformed fibroblast cell lines and their sensitivity to NK cells in vitro. Nontransformed cells (N-type cell lines) and cells tumorigenic in normal mice (C-type cell lines) are resistant to NK-mediated lysis. In contrast, cell lines that are tumorigenic in ATxFL mice (these mice are very low in NK activity), but not in normal mice (I-type cell lines) are sensitive to NK-mediated lysis. These findings support the concept that NK activity is involved in host surveillance against tumors. On the other hand, NK-resistant fibroblasts, whether taken directly form animals or derived as tumorigenic or nontumorigenic cell lines, compete with NK-sensitive target cells to inhibit their lysis by NK effectors. Not only are both NK-sensitive and -resistant cells recognized by NK effectors but both receive lytic signals from NK effector cells. Target cell resistance is a result of a protein synthesis-dependent mechanism that prevents lysis such that in the presence of inhibitors of protein synthesis all fibroblasts tested are NK sensitive. Those fibroblasts that are normally sensitive to NK-mediated lysis must be deficient in their ability to produce or respond to this counterlytic mechanism. These findings are in contrast with the general findings when lymphoid cells are studied as NK targets where sensitivity appears to be a result of recognition by NK effectors. Because our findings show that transformed and normal cells express the same recognition determinants, in order for NK activity to play an important in vivo role in tumor surveillance, a mechanism must operate to permit NK effectors to find their targets in vivo. In the absence of a special discrimination mechanism, the killing of NK-sensitive transformants that arise autochronously would be less than optimal as a consequence of competition by the normal, NK-resistant, cells.

Animals↗
PubMed1981

The expression of glial fibrillary acidic protein in a rat cerebellar cell line.

A rat cerebellar cell line, WC5, derived by transformation with Rous sarcoma virus, which is temperature-sensitive for transformation (ts-RSV), can be induced to express glial fibrillary acidic protein (GFAP). Immunofluorescence, radioimmune assay, and electron microscopy studies show that GFAP is expressed in WC5 cells grown at the nonpermissive temperature (NPT), but not at the permissive temperature (PT) for transformation. GFAP is first detectable about 3 days after incubating cells at the NPT, and reaches an apparent plateau by the seventh or eighth day. The expression of GFAP is reversible; shifting cells from the NPT to the PT causes a dramatic decrease in GFAP after 96 hr. In order to determine if the expression of GFAP is linked to the temperature-sensitive transforming activity of the viral src gene product, phenotype revertants of WC5 were established. By the criteria of morphology and growth in agar, the revertant lines, in contrast to the parent cell line WC5, were shown to exhibit a transformed phenotype at both the NPT and PT. Immunofluorescence studies on several of the revertant cell lines show that they do not express GFAP at either the PT or NPT. These findings suggest that the expression of GFAP in WC5 is linked to the expression of the src gene product. The advantage of using ts-RSV to derive neural cell lines which exhibit differentiated properties is discussed.

Animals↗
PubMed1980

Properties of two temperature-sensitive Rous sarcoma virus transformed cerebellar cell lines.

Cells from the cerebellum of 3-day-old BD-IX rats were obtained as permanent lines by transforming them with temperature-sensitive Rous sarcoma virus. The presence or absence of veratridine-stimulated Na+-uptake (voltage-dependent channels) was used to operationally classify them as neuronal or glial. When incubated at 34 degrees C, the permissive temperature for transformation, the cerebellar cells exhibit a transformed phenotype determined by anchorage independence, rounded morphology, high growth rate and absence of density-dependent inhibition of growth. In contrast, when the transformed cerebellar cell lines are kept at a temperature (38 degrees C) non-permissive for transformation, they exhibit a normal cellular phenotype with respect to the above properties. Moreover, changes toward neuronal morphology, increase in veratridine-stimulated Na+-uptake, decreased growth rate and the expression of the astrocyte specific protein, glial fibrillary acidic protein, suggest that a degree of differentiation is expressed at the non-permissive temperature.

Animals↗
PubMed1980

31P NMR quantitation of the displacement of equilibria of arginine, creatine, pyruvate, and 3-P-glycerate kinase reactions by substitution of sulfur for oxygen in the beta phosphate of ATP.

31P NMR measurements have been found to be a convenient means for simultaneously measuring the concentrations of several species in the equilibrium mixtures of the reactions catalyzed by arginine kinase and creatine kinase. MgATP + X in equilibrium MgADP + XP where X = arginine or creatine and XP = P-arginine or P-creatine. The free energy of phosphorylaton of various metabolites by adenosine 5'-O-(2-thiotriphosphate) at pH 8.0 and 30 degrees C is more exergonic than the corresponding phosphorylations by ATP by about 2.5 kcal/mol, resulting in a displacement of the equilibrium toward the nucleoside diphosphates by a factor of approximately 60. Since this factor does not depend on the nature of the metabolite, the equilibrium constants of thionucleotide reactions may be used to determined the equilibrium constants of corresponding oxynucleotide reactions which lie too far toward ATP. The equilibrium constants of the oxynucleotide reactions catalyzed by pyruvate kinase and 3-P-glycerate kinase calculated by this method from the experimentally determined equilibrium constants of the corresponding thionucleotide reactions are 3.1 x 10(-4) and 2.9 x 10(-4), respectively, under the experimental conditions used. The equilibrium constants and degree of stereoselectivity of the arginine kinase reaction are altered when Ca2+ replaces Mg2+ as the activating metal ion.

Adenosine Triphosphate↗
PubMed1980

Shift of the equilibrium constant of the 3-P-glycerate kinase reaction towards 1,3-bis-P-glycerate with adenosine 5'-O-(2-thiotriphosphate) (ATP beta S) as substrate.

It has been demonstrated that the reaction of ATP beta S and 3-P-glycerate catalyzed by 3-P-glycerate kinase, unlike the reaction with ATP, can form a readily detectable amount of 1,3-bis-P-glycerate as observed by 31P NMR. By quantifying production of 1,3-bis-P-glycerate from the phosphorothioate analogue of ATP as a function of time as the reaction approaches equilibrium, Keq for the reaction was estimated to be approximately 400, about 1 order of magnitude less than the equilibrium constant previously reported for the analogous reaction of the normal nucleotide substrates.

Adenosine Triphosphate↗
PubMed1979

Determination of the kappa anti-alpha(1,3) dextran immune response difference by A gene(s) in the VKappa-locus of mice.

Mice lacking the V(alpha(1,3) (h gamma1)-gene do not produce a gamma1 anti-alpha(1,3) dextran response. However, on hyperimmunization some strains mount a kappa-anti-alpha(1,3) dextran response, whereas other remain nonresponder. Responsiveness in dominant. The kappa-anti-alpha(1,3) response difference is linked to the Ly-3 locus on chromosone 6 and is likely the result of a structural Vkappa-gene(s). In conjunction with previous work, three Vkappa-allogroups can now be distinguished. At present, this is the only example of an immune responsiveness difference associated with the Vkappa-locus.

Animals↗