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41.
Alternative nonlinear model for estimating second-order rate coefficients for biodegradation. 总被引:2,自引:2,他引:0 下载免费PDF全文
A modification of the second-order model for biodegradation was derived, applied to an example data set, and shown to be superior for describing the anaerobic biodegradation of p-cresol by an enriched bacterial consortium. The modified model circumvents the no-growth assumption implicit in the use of the second-order rate equation, but still requires the assumption of first-order kinetics over the course of substrate depletion. Violation of the no-growth assumption is particularly important since overestimates of the pseudo-first-order rate coefficient lead to underestimates of the time required for the removal of a xenobiotic chemical from a contaminated environment. Our calculations show that the errors introduced into the pseudo-first-order rate coefficient (and the resulting estimates of the second-order rate coefficient) approach 100% if one doubling occurs in activity over the course of substrate depletion. For an exemplary data set, use of a first-order model resulted in a 100% overestimate of the first-order decay coefficient, which would in turn lead to a corresponding overestimate of the second-order rate coefficient. The modified model we describe is a potential alternative to the pseudo-first-order model for the routine estimation of second-order rate coefficients. 相似文献
42.
A structural and dynamic model has been developed for the branch point formed when two DNA double helices exchange strands during genetic recombination. This model, which generalizes most previous structural models, maintains the twofold symmetry inherent in the covalent and hydrogen bonded structure, yet has three degrees of freedom about virtual bonds, constituting a simplified junction. Using this structural model, a three-step dynamic model for branch point migration has been developed: longitudinal diffusion about the virtual bonds to achieve a structure in which the helix axes are approximately parallel; opening of the base pairs; and rotary diffusion about the helix axis to effect a migratory event. The model, which includes the possible role of electrostatic interactions, solves problems inherent in previous treatments. We find that no significant electrostatic torques arise that promote branch point migration. The absence of a kinetic mechanism to circumvent thermodynamic barriers due to mispairing suggests that an energy source is used for those situations in living systems. 相似文献
43.
Summary Fifty four microcosmic communities were assembled over 4 months from a 28-species source pool of phytoplankton using nine different invasion patterns each replicated six times. Three communities from each set of replicates then were invaded with a cladoceran that feeds on phytoplankton. All communities were then treated identically for an additional 4 months. In all nine invasion categories species richness was greater in predated communities. Predation opened communities to invasion by increasing the representation of infrequently sampled species at the expense of more common species. Invasion rate was four times more influential than predation and over eleven times more important than either invasion order or the timing pattern of interspecific arrivals in determining species richness in this system of communitites. 相似文献
44.
Circulating patterns of luteinizing hormone (LH) and prolactin (PRL) were monitored for 5 yr in ewes maintained either outdoors in natural conditions or indoors in a fixed, short photoperiod (8L:16D). The ewes were ovariectomized and each was treated with a Silastic implant containing estradiol to provide a fixed negative feedback signal to the reproductive neuroendocrine axis. Serum concentrations of LH and PRL were subjected to a statistical algorithm developed for the purpose of detecting hormone cycles. In ewes maintained outdoors, serum concentrations of both hormones underwent high amplitude cycles with a period no different from 365 days. Among ewes maintained in the fixed photoperiod, unambiguous cycles of LH and PRL persisted through the 5 yr of exposure to short days. Period of these cycles differed from 365 days. Further, the LH cycles became desynchronized among ewes housed together and desynchronized with respect to the LH cycles in ewes kept outdoors. These findings document the existence of an endogenous circannual rhythm of reproductive neuroendocrine function in ewes. 相似文献
45.
Identification of the molecular species of lysophosphatidic acid produced when platelets are stimulated by thrombin 总被引:19,自引:0,他引:19
Platelets, when stirred with 3 U thrombin/10(9) platelets, produced significant quantities of palmitoyllysophosphatidic acid (2.17 ng/10(9) platelets), stearoyllysophosphatidic acid (2.11 ng/10(9) platelets), and arachidonoyllysophosphatidic acid (1.06 ng/10(9) platelets). When platelets were pretreated with 100 microM of the phospholipase A2 inhibitor U10029A, there was a significant decrease in thrombin-stimulated production of stearoyllysophosphatidic acid (to 0.16 ng/10(9) platelets), while arachidonoyllysophosphatidic acid production was unchanged. U10029A concomitantly increased thrombin-stimulated production of stearoyl-containing phosphatidic acid species (primarily stearoylarachidonoylphosphatidic acid) from 5.99 to 9.71 ng/10(9) platelets. The results are consistent with the concept that stearoyllysophosphatidic acid production in platelets occurs via phospholipase A2 degradation of phosphatidic acid. 相似文献
46.
The kinetics of cell growth and foreign protein production, as well as factors affecting protein stability, were studied and optimized in batch and fed-batch fermentations of a recombinant strain of Escherichia coli. The pL promoter from bacteriophage lambda under the control of a temperature-sensitive cl represser, with the entire construct integrated into the E. coli chromosome through the use of a defective bacteriophage lambda lysogen, was used to direct the synthesis of T4 DNA ligase. The biphasic fermentations consisted of a primary growth phase at 30 degrees C followed by an induction phase which was initiated by shifting the temperature to 42 degrees C. In the fed-batch fermentations, additional nutrients were added at the time of initiating induction. Maintenance of sufficiently high concentrations of the organic substrates (glucose and casamino acids) during the induction phase was required for continued cell growth at 42 degrees C. Such growth was essential for T4 DNA ligase formation and in vivo stability. Hence, fed-batch fermentations produced the highest yield of the foreign protein Commensurate with providing lower total amounts of substrates. In such cases, high cell densities (6 g dry wt/L) with substantial intracellular levels of T4 DNA ligase (4.6% total cellular protein, or 2.7% of the dry biomass) were achieved. 相似文献
47.
TGF-beta 1-induced inhibition of mouse mammary ductal growth: developmental specificity and characterization 总被引:11,自引:0,他引:11
C W Daniel G B Silberstein K Van Horn P Strickland S Robinson 《Developmental biology》1989,135(1):20-30
TGF-beta 1, implanted into growing mouse mammary glands, was previously shown to inhibit ductal growth in an apparently normal and fully reversible manner. In this report we extend these findings to show that TGF-beta 1 inhibition is highly specific. In pregnant or hormone-treated mice, doses of TGF-beta 1 that were capable of fully inhibiting ductal elongation had little effect on the proliferation of lobuloalveolar structures. Additionally, the inhibitory action of TGF-beta 1 on ducts is epithelium-specific, resulting in cessation of DNA synthesis in the rapidly proliferating epithelium of mammary end buds, but does not inhibit DNA synthesis in the stroma surrounding the end buds. At the cellular level, transplant studies showed that TGF-beta 1 inhibited the regeneration of mammary ductal cells when implanted into mammary gland-free fat pads by suppressing the formation of new end buds, without inhibiting maintenance DNA synthesis in ductal lumenal epithelium; this observation indicates the potential of TGF-beta 1 to maintain patterning by suppressing adventitious lateral branching. The time-course of TGF-beta 1 inhibition of end buds was rapid, with cessation of DNA synthesis by 12 hr, followed by loss of the stem cell (cap cell) layer. The question of glandular exposure to TGF-beta 1 administered in EVAc implants was also investigated. Incorporation of TGF-beta 1 into EVAc was found not to degrade the hormone, while the release kinetics of the ligand from implants, its retention in the gland, and the demonstrable zone of exposure were consistent with observed inhibitory effects. These results support the hypothesis that TGF-beta 1 is a natural regulator of mammary ductal growth. 相似文献
48.
The preferential uptake of very-low-density lipoprotein cholesteryl ester by rat liver in vivo. 下载免费PDF全文
The removal from the blood and the uptake by the liver of injected very-low-density lipoprotein (VLDL) preparations that had been radiolabelled in their apoprotein and cholesteryl ester moieties was studied in lactating rats. Radiolabelled cholesteryl ester was removed from the blood and taken up by the liver more rapidly than sucrose-radiolabelled apoprotein. Near-maximum cholesteryl ester uptake by the liver occurred within 5 min of the injection of the VLDL. At this time, apoprotein B uptake by the liver was only about 25% of the maximum. Maximum uptake of the injected VLDL apoprotein B label was not achieved until at least 15 min after injection, by which time the total uptakes of cholesteryl ester and apoprotein B label were very similar. The results suggest that preferential uptake of the lipoprotein cholesteryl ester by the liver occurred before endocytosis of the entire lipoprotein complex. The fate of the injected VLDL cholesteryl ester after its uptake by the liver was also monitored. Radiolabel associated with the hepatic cholesteryl ester fraction fell steadily from its early maximum level, the rate of fall being faster and more extensive when the fatty acid, rather than the cholesterol, moiety of the ester was labelled. By 30 min after the injection of VLDL containing [3H]cholesteryl ester, over one-third of the injected label was already present as [3H]cholesterol in the liver. When VLDL containing cholesteryl [14C]oleate was injected, a substantial proportion (about 25%) of the injected radiolabelled fatty acid appeared in the hepatic triacylglycerol fraction within 60 min: very little was present in the plasma triacylglycerol fraction at this time. 相似文献
49.
P R Robinson S F Wood E Z Szuts A Fein H E Hamm J E Lisman 《The Biochemical journal》1990,272(1):79-85
Previous biochemical and electrophysiological evidence suggests that in invertebrate photoreceptors, a GTP-binding protein (G-protein) mediates the actions of photoactivated rhodopsin in the initial stages of transduction. We find that squid photoreceptors contain more than one protein (molecular masses 38, 42 and 46 kDa) whose ADP-ribosylation by bacterial exotoxins is light-sensitive. Several lines of evidence suggest that these proteins represent distinct alpha subunits of G-proteins. (1) Pertussis toxin and cholera toxin react with distinct subsets of these polypeptides. (2) Only the 42 kDa protein immunoreacts with the monoclonal antibody 4A, raised against the alpha subunit of the G-protein of vertebrate rods [Hamm & Bownds (1984) J. Gen. Physiol. 84. 265-280]. (3) In terms of ADP-ribosylation, the 42 kDa protein is the least labile to freezing. (4) Of the 38 kDa and 42 kDa proteins, the former is preferentially extracted with hypo-osmotic solutions, as demonstrated by the solubility of its ADP-ribosylated state and by the solubility of the light-dependent binding of guanosine 5'-[gamma-thio]triphosphate. The specific target enzymes for the observed G-proteins have not been established. 相似文献
50.
Summary Previous studies have indicated that the asymmetric form of acetylcholinesterase (collagen-tailed) is localized in the basal lamina of the neuromuscular junction of skeletal muscle. The present study shows localization of the asymmetric acetylcholinesterase in the heart of the rat. Antiserum to 14+18 S acetylcholinesterase of the electric eel was raised in rabbits. The purified antibody did not react with collagen type I or laminin. Collagenase reduced the immunoreactivity of the enzyme with the purified antibody. Isolated cardiomyocytes and frozen sections of the heart were stained for acetylcholinesterase with the antibody. Diffuse immunofluorescence appeared over the surface of the cardiomyocytes. In the frozen sections, the immunofluorescence was most intense at the cell boundaries. These data suggest that collagenase-sensitive acetylcholinesterase in the heart is present in the myocytes and occurs in the vicinity of the basal lamina.Abbreviations
AChE
acetylcholinesterase
-
BSA
bovine serum albumin
-
PBS
phosphate-buffered saline
-
DME
Dulbecco's Modified Eagle Medium 相似文献