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101.
Eleonora Condrea Bruce E. Rapuano Karen R. Soons Philip Rosenberg 《Life sciences》1983,32(13):1455-1461
The effects on some pharmacological and enzymatic properties were determined following methylation of histidine at the enzymatic active site of the basic relatively toxic and the acidic relatively non-toxic phospholipases A2. Following methylation a very low residual enzymatic activity (0.4 -- 1% of control) was accompanied by a parallel loss in intraventricular lethality, anticoagulant potency, direct hemolytic action and ability to block directly and indirectly evoked contractions of the mouse phrenic nerve-diaphragm preparation. Since methylation does not impair the enzyme's ability to bind monomeric of micellar substrates or Ca2+, the results suggest that the pharmacologicallly active region of the molecule is different from the micellular substrate binding site but strongly influenced by the invariant histidine-48 located at the enzymatic active site. 相似文献
102.
Electron Transport-Dependent Chlorophyll-a Fluorescence Quenching by O(2) in Various Algae and Higher Plants 总被引:2,自引:2,他引:0
A comparison of chlorophyll-a fluorescence in brown algae (Macrocystis integrifolia, Fucus vesiculosis), green algae (Scenedesmus obliquus, Ulva sp.) and higher plants (bean, corn) show differences in the relative fluorescence intensities and induction time courses which characterize each type of plant. These differences are not reflected in either the maximum fluorescence emission in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (Fmax) or the nonvariable fluorescence (Fo). Constancy of Fo and Fmax suggests functional similarities of photosystem II and associated antennae pigments in the various classes of plants. The time course differences are observed only in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and appear, therefore, to be electron transport dependent. During induction, the peak in fluorescence (Fp) is much lower in all of the algae studied than in the higher plants. Exogenous O2 strongly quenches Fp in all plants studied and our data indicate that the low Fp in the algae can be partially accounted for by endogenous O2 quenching. 相似文献
103.
Mesophyll cells isolated from Phaseolus vulgaris and Lycopersicon esculentum show decreasing photosynthetic rates when suspended in media containing increasing concentrations of osmoticum. The photosynthetic activity was sensitive to small changes in osmotic potential over a range of sorbitol concentrations from 0.44 M (−1.08 MPa) to 0.77 M (−1.88 MPa). Photorespiration assayed by 14 CO2 release in CO2 -free air and by 14 CO2 release from the oxidation of [1–14 C] glycolate also decreased as the osmotic potential of the incubation medium was reduced. The CO2 compensation points of the cells increased with increasing concentration of osmoticum from approximately 60 μ I−1 1 at −1.08 MPa to 130 μl 1−1 for cells stressed at −1.88 MPa. Changes in photosynthetic and photorespiratory activities occurred at moderate osmotic potentials in these cells suggesting that in whole leaves during a reduction in water potential, non- stomatal inhibition of CO2 assimilation and glycolate pathway metabolism occurs simultaneously with stomatal closure. 相似文献
104.
Comparison of the effects of 1,2-dimethylhydrazine and cyclophosphamide on micronucleus incidence in bone marrow and colon 总被引:2,自引:0,他引:2
An in vivo micronucleus assay has been developed that utilizes colonic epithelial cells. The genotoxic effects of 1,2-dimethylhydrazine (54-07-3), a colon carcinogen, and of the nitrogen mustard, cyclophosphamide (50-18-0), on the bone-marrow polychromatic erythrocytes and on colonic epithelium from mice were compared using micronucleus induction in each organ as the end point. In the bone marrow, cyclophosphamide was a potent inducer of micronuclei, while 1,2-dimethylhydrazine administration had little effect on the micronucleus incidence. In the colon, 1,2-diemthylhydrazine was an effective inducer of micronuclei. Thus, the colonic micronucleus assay appears to be a potentially useful test for the detection of colon carcinogens. 相似文献
105.
Gregory A. Osenga Bruce C. Coull 《Journal of experimental marine biology and ecology》1983,67(3):221-225
Meiofauna were collected from adjacent areas with and without Spartina alterniflora Loisel culms. Environmental complexity resulting from root structure cannot be determined by simply observing S. alterniflora culm distribution. There were no significant differences between the two areas with respect to root density or root sizes; however, the number of live roots was significantly higher in the area containing Spartina culms. Nematodes, the most abundant taxon were positively correlated with live root density. In contrast, the number of dead roots and harpacticoid copepods were higher in the area without culms. This study suggests that nematodes are positively associated with micro-oxygenated zones produced by live S. alterniflora roots. Other measures of biogenic structure, such as total root density or root size fractions are not important in affecting nematode abundance. 相似文献
106.
Aphids of Schizaphis graminum (Rondani) (biotype C) reared on its host-plant, Sorghum bicolor (L.) Moench, sequestered campesterol, stigmasterol and sitosterol. Aphids reared for 72 hr on holidic diets supplemented with [4-14C]-sitosterol contained both [14C]-sitosterol and [14C]-cholesterol, indicating that these aphids are capable of dealkylation at C-24. When aphids were reared on artificial diets containing [2-14C]-mevalonic acid, no detectable amounts of radioactively labelled desmethyl sterols, nor metabolic intermediates in sterol synthesis (i.e. squalene, 2,3-oxidosqualene, 4,4-dimethyl and 4-monomethyl sterols) were found to accumulate in their tissues. The relevance of these findings to previous research suggesting the ability of aphids, via their symbiotes, to synthesize sterols is discussed. 相似文献
107.
David M. Helfman Bruce D. Appelbaum William R. Vogler J.F. Kuo 《Biochemical and biophysical research communications》1983,111(3):847-853
Phospholipid-sensitive Ca2+-dependent protein kinase (PL-Ca-PK) was found to be present at a high level in human neutrophils, with its activity localized in the particulate fraction. In contrast, cyclic AMP-dependent protein kinase (A-PK) and cyclic GMP-dependent protein kinase (G-PK), present at lower levels compared to PL-Ca-PK, were localized in the cytosolic fraction. Phosphorylation of several endogenous proteins (mol. wts. 89,000, 38,000, 34,000, 17,000 and 15,000), also localized in the particulate fraction, was stimulated specifically by a combination of phosphatidylserine and Ca2+, whereas no substrate proteins were observed for the calmodulin-sensitive Ca2+-dependent protein kinase system under the same incubation conditions. Although no substrate proteins for G-PK were detected, one substrate (mol. wt. 19,000) for A-PK was observed. Phosphorylation of substrates for PL-Ca-PK, but not that for A-PK and for enzymes independent of Ca2+ or cyclic AMP, was inhibited by a variety of agents, including trifluoperazine, W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide], adriamycin, palmitoylcarnitine, and melittin. The present findings suggest that the protein phosphorylation system may be important in the membrane associated functions of human neutrophils. 相似文献
108.
Kinetic behaviour and allosteric regulation of human deoxycytidylate deaminase derived from leukemic cells 总被引:1,自引:0,他引:1
Deoxycytidylate deaminase has been highly purified (1232-fold) from human leukemia CCRF-CEM cells. The native molecular weight of the enzyme is 108 000 and subunit molecular weight 50 500, suggesting that the native enzyme exists as a dimer. The enzyme exhibits a sigmoidal initial velocity vs substrate concentration curve and is regulated by allosteric effectors, dCTP and TTP. The curve relating substrate concentration to initial velocity was changed from a sigmoidal shape to a hyperbolic one by the activator dCTP, while the inhibitor TTP increased the sigmoidicity of the curve. The molecular weight of deoxycytidylate deaminase was unchanged in the presence of allosteric effectors, indicating that aggregation-disaggregation is not the basis of regulation. Deoxycytidylate deaminase exhibited the greatest affinity for the substrate dCMP, with lesser affinity for ara-CMP, and least affinity for CMP. Ara-CMP was an effective substrate in the presence of dCTP concentrations exceeding 4 microM. These data indicate that human neoplastic cell deoxycytidylate deaminase is a highly regulated allosteric enzyme, which is likely to have a significant influence on cellular dUMP, dCTP and TTP pools. These findings further suggest, that the enzyme through its influence on dUMP levels is likely to modulate the biochemical effects of pyrimidine antimetabolites active against the thymidylate synthetase reaction and in the presence of elevated dCTP pools will promote deamination of ara-CMP to the inactive ara-UMP. 相似文献
109.
Summary Ghosts of human erythrocytes prepared by hypotonic hemolysis were assayed for aldolase, triosephosphate isomerase, glyceraldehyde phosphate dehydrogenase, phosphoglycerate kinase, pyruvate kinase, lactate dehydrogenase, and glutathione peroxidase and reductase. Cryptic activity of the enzymes was demonstrated by an increase in activity on dilution with water, which caused fragmentation of the ghosts. Aldolase and glyceraldehyde phosphate dehydrogenase were classed as firmly bound; phosphoglycerate kinase was intermediate; the others were loosely bound. Triton X-100 increased the activities of aldolase, glyceraldehyde phosphate dehydrogenase, and phosphoglycerate kinase. The pH of the medium had little effect upon the firmly bound enzymes but it markedly affected the retention of hemoglobin and the activities of the loosely bound enzymes. The presence of Mg or Ca ions enhanced the retention of hemoglobin and the activity of lactate dehydrogenase and pyruvate kinase, with little effect on aldolase and glyceraldehyde phosphate dehydrogenase. Ghosts diluted in water disintegrated into fragments and tubules or vesicles; Mg or Ca at 1mm afforded protection against this. When ghosts were treated with Triton X-100 and adenosine triphosphate, they contracted to about one-seventh of their volume. The shrunken ghosts had lost a considerable proportion of their cholesterol and protein to the medium. 相似文献
110.
Antiserum was made against a single 30S protein, 30S-7. The amount of complement fixed by total 30S protein in the presence of this antiserum indicated that protein 30S-7 was the major antigen in the mixture of proteins. Each of the 30S ribosomal proteins was tested for cross-reactivity with anti-30S-7. This was done by determining if any of the other 30S proteins inhibited complement fixation by 30S-7. None of the other 30S proteins was found to inhibit complement fixation by 30S-7, indicating that 30S-7 is antigenically distinct from the other proteins. 相似文献