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At least 19 recordsLinked to original sources
PubMed1976

Multi-temperature effects on Hill reaction activity of barley chloroplasts.

1. The relationship between temperature and Hill reaction activity has been investigated in chloroplasts isolated from barley (Hordeum vulgare L. cv. Abyssinian). 2. An Arrhenius plot of the photoreduction of 2,6-dichlorophenolindophenol (DCIP) showed no change in slope over the temperature range 2--38degreesC. The apparent Arrhenius activation energy (Ea) for the reaction was 48.1 kJ/mol. 3. In the presence of an uncoupler of photophosphorylation, methylamine, the Ea for DCIP photoreduction went through a series of changes as the temperature was increased. Changes were found at 9, 20, 29 and 36degreesC. The Ea was highest below 9degreesC at 63.7 kJ/mol. Between 9 and 20degreesC the Ea decreased to 40.4 kJ/mol and again to 20.2 kJ/mol between 20 and 29degreesC. Between 29 and 36degreesC there was no further increase in activity with increasing temperature. The temperature-induced changes at 9, 20 and 29degreesC were reversible. At temperatures above 36degreesC (2 min) a thermal and largely irreversible inactivation of the Hill reaction occurred. 4. Temperature-induced changes in Ea were also found when ferricyanide was substituted for DCIP or gramicidin D for methylamine. The addition of an uncoupler of photophosphorylation was not required to demonstrate temperature-induced changes in DCIP photoreduction following the exposure of the chloroplasts to a low concentration of cations. 5. The photoreduction of the lipophilic acceptor, oxidized 2, 3, 5, 6-tetramethyl-p-phenylenediamine, also showed changes in Ea in the absence of an uncoupler. 6. The temperature-induced changes in Hill activity at 9 and 29degreesC coincided with temperature-induced changes in the fluidity of chloroplast thylakoid membranes as detected by measurements of electron spin resonance spectra. It is suggested that the temperature-induced changes in the properties and activity of chloroplast membranes are part of a control mechanism for regulation of chloroplast development and photosynthesis by temperature.

2,6-Dichloroindophenol↗
PubMedDate not supplied

[Generalization of activation reaction extinction during consecutive stimulation of different parts of the brain].

The process of extinction of the brain activation reactions ("arousal") was studied in chronic experiments on cats with implanted electrodes during repeated electrical stimulation of alternated points in the cortex and in the brain stem reticular formation. Extinction of the reactions achieved by stimulation of one point resulted in the loss of excitability both at this point and in other activating structures at different levels of the brain. The sequence of stimulated structures was of no importance. A possibility is suggested of development of a generalized inhibition in the neural net of the non-specific reticular system of the brain which may be inciated in any point of this system: in the brain stem, in the thalamus or in the cortex.

Animals↗
PubMed1977

Methionyl-tRNA synthetase from Escherichia coli: substituting magnesium by manganese in the L-methionine activating reaction.

While Mg2+ can be efficiently replaced by Ni2+, Co2+ and Mn2+ in the ATP-PPi isotopic exchange reaction catalysed by methionyl-tRNA synthetase from Escherichia coli, the latter ion was selected for detailed analysis of the L-methionine activation reaction. In order to avoid artefactual results due to the slow aggregation of Mn2+ with pyrophosphate, this process was investigated by electron paramagnetic resonance and conditions were determined where it does not interfere with enzymic experiments. The thermodynamic parameters derived from steady-state (ATP-PPi isotopic exchange, fluorescence at equilibrium) or prestationary (fluorescence stopped-flow) experiments are compared to those obtained in the presence of Mg2+ [Hyafil et al. (1976) Biochemistry, 15, 3678-3685]. While the standard deltaG for the reaction (E-Met-ATP-Me2+equilibriumE-Met approximately AMP-PPi-Me2+) is close to zero in the case of Mg2+, Mn2+ slows down the rate of adenylate reversion and thus shifts the reaction towards the latter species. The deltaG for the formation of the E-Met approximately AMP complex does not depend on the metal used, suggesting that the divalent ion does not participate in the structuration of this complex. Substituting Mn2+ for Mg2+ decreases notably the dissociation constant of PPi-Me2+ from the E-Met approximately AMP-PPi-Me2+ species and from its abortive analog E-Met-Ado-PPi-Me2+. Similarly the dissociation constant of ATP-Me2+ from another dead-end analog E-methioninol-ATP-Me2+ is decreased by Mn2+. Involvement of the purine N7 atom in the binding of the metal ion to the active site of methionyl-tRNA synthetase is ruled out by the use of 7-deaza-adenosine. The role of the metal in the catalytic process of methionine activation and its relevance to the specificity of the reaction is then discussed in the light of the results obtained without metal and with Mg2+ and Mn2+.

Adenosine↗
PubMed1979

Plasminogen activator from cells transformed by an oncogenic virus: inhibitors of the activation reaction.

This paper describes an assay for direct measurement of plasminogen activation and its application for determining the kinetic constants and for screening potential inhibitors of the reaction. The assay is based on the conversion of the single chain of 125I-labelled plasminogen to the two chains of 125I-labelled plasmin (EC 3.4.21.7), the latter then being separated from each other and from the plasminogen substrate by electrophoresis under reducing conditions in SDS-polyacrylamide gels. The Km of activator from transformed murine cells for human plasminogen was 180 nM. A broad range of compounds was tested as potential inhibitors of plasminogen activation and of plasmin-catalyzed fibrinolysis respectively, and the two reactions differed qualitatively and quantitatively in their response to previous agents. The principal qualitative difference was in the susceptibility of the reactions to a spectrum of naturally-occurring macromolecular inhibitors: all of the macromolecular inhibitors that blocked the action of plasmin were without effect on murine activator or human urokinase (EC 3.4.99.26). A variety of small molecules inhibited both of the reactions tested, and showed significant quantitative differences; some of these were active at micron concentrations. The exacting specificity of plasminogen activators for macromolecules, both substrates and inhibitors, encourages the expectation that effective inhibitors of great specificity may be isolated from as yet undiscovered natural sources.

Amino Acids↗
PubMed1975

The mechanism of action of methionyl-tRNA synthetase from Escherichia coli. Inhibition by adenosine and 8-aminoadenosine of the amino-acid activation reaction.

Adenosine and 8-aminoadenosine, both competitive inhibitors of ATP-Mg2+ in the ATP-PPi exchange reaction catalyzed by methionyl-tRNA synthetase, are used to investigate the active center for methionyl-adenylate formation. Resolution of the kinetics parameters of the reaction indicates that methionine markedly enhances the affinity of the nucleosides for the enzyme, providing evidence for coupling between the sites for amino acid and the nucleoside moiety of ATP. Furthermore, occupation of both of these sites is a prerequisite for binding of pyrophosphate. Introduction of an amino group in position 8 of the adenine ring strongly increases the affinity constants for the nucleoside and for pyrophosphate in the coupled reactions described above.

Adenosine↗
PubMed1976

Ultrastructural cytochemistry of p-N,N-dimethylamino-beta-phenethylamine (DAPA) oxidation reactions. "activation" and inhibition studies.

The oxidation of p-N,N-dimethylamino-beta-phenethylamine (DAPA) by amine oxidase(s) (AO), i.e., diamine oxidase (DAO), monoamine oxidase (MAO) and/or possibly other oxidases, has been previously demonstrated. This study reports the results of variations in fixation procedures and the incorporation of a series of possible "activators" and inhibitors into the DAPA oxidase (DAPAO) and DAPAO-BSPT [2-(2'-benzothiazolyl)-5-styryl-3-(4'-phthalhydrazidyl) tetrazolium chloride] reaction media in an attempt to elucidate the oxidase(s) involved. Results of these studies are indicative of at least two different oxidases acting preferentially on one or the other of the two systems. The presence of MAO, especially in unfixed tissue, and DAO, especially in fixed tissue, is denoted and that of other oxidase(s) is connoted.

Amine Oxidase (Copper-Containing)↗
PubMed1978

Separation of helper and suppressor T lymphocytes. III. Positive and negative effects of mixed lymphocyte reaction-activated T cells.

Using a Ficoll velocity sedimentation gradient, we provide evidence to show that stimulatory and inhibitory activities induced in mixed lymphocyte culture belong to two distinct subpopulations of T cells. Slow- to medium-sedimenting, nonproliferating cells enhance the humoral response of normal cells to SRBC whereas fast-sedimenting cells inhibit that response. Cytotoxic lymphocytes are found in the same fractions as suppressor cells but appear to be differentiable from the latter in terms of maturation kinetics and specificity.

Animals↗
PubMed1975

The effect of shock on the inflammatory response. A reevaluation of the role of platelets in the active arthus reaction.

The active Arthus reaction can be inhibited by hypovolemic shock or the infusion of catecholamines. A reevalution of previous work with platelet antiserum indicates that shock rather than thrombocytopenia was responsible for preventing the active Arthus reaction. Immunofluorescent studies of the inhibited Arthus sites reveal that immune precipitates are not present in the extravascular tissues. Since leukocyte aggregates can be seen within venules at the inhibited sites, and they phagocytize BSA-anti-BSA complexes, their failure to migrate out of the vessels is due to the absence of complexes in the extravascular spaces. (Am J Pathol 78:159-170, 1975)

Animals↗