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1.
Gad Avigad Priscilla J. Bohrer 《Biochimica et Biophysica Acta (BBA)/General Subjects》1984,798(3):317-324
6-Phosphofructo-2-kinase (ATP: D-fructose-6-phosphate-2-phosphotransferase) and D-fructose-2,6-bisphosphatase activities have been found in extracts prepared from etiolated mung bean seedlings. The activity of 6-phosphofructo-2-kinase exhibits a sigmoidal shape in response to changes in concentrations of both substrates, D-fructose 6-phosphate and ATP (S0.5 values of 1.8 and 1.2 mM, respectively). Inorganic orthophosphate (Pi) has a strong stimulating effect on the 2-kinase activity (A0.5 at about 2 mM), moderately increasing the Vmax and modifying the response into hyperbolic curves with Km values of 0.4 and 0.2 mM for fructose 6-phosphate and ATP, respectively. 3-Phosphoglycerate (I0.5 about 0.15 mM) partially inhibited the kinase activity by counteracting the Pi activation. In contrast, the activity of D-fructose-2,6-bisphosphatase (Km 0.38 mM) is strongly inhibited by Pi (I0.5 0.8 mM) lowering its affinity to fructose-2,6-P2 (Km 1.4 mM). 3-Phosphoglycerate activites the enzyme (A0.5 at about 0.3 mM) without causing a significant change in its Km for fructose-2,6-P2. The activities of both of these enzymes in relationship to the metabolic role of D-fructose 2,6-bisphosphate in the germinating seed is discussed. 相似文献
2.
Promotion and inhibition of vesicle fusion by polylysine 总被引:1,自引:0,他引:1
Polylysine induced rapid aggregation of large unilamellar vesicles composed of phosphatidylcholine-cardiolipin (1:1 molar ratio) but not their fusion. Application of the terbium-dipicolinic acid fusion assay showed that addition of polylysine at nanomolar concentrations enabled a significant lowering of the Ca2+ threshold concentration for vesicle fusion from 9 to 1 mM. Analysis of the kinetics of fusion with a mass-action kinetic model showed that polylysine enhanced significantly the rate of aggregation but affected only slightly the rate of fusion per se. Maximal enhancement of overall fusion rates occurred at a charge ratio (polylysine/cardiolipin) of about 0.5. At larger polylysine concentrations, e.g., at charge ratios greater than 3, polylysine inhibited vesicle fusion. 相似文献
3.
Evolution of the 28S ribosomal RNA gene in anurans: regions of variability and their phylogenetic implications 总被引:1,自引:0,他引:1
Fifteen restriction sites were mapped to the 28S ribosomal RNA gene of
individuals representing 54 species of frogs, two species of salamanders, a
caecilian, and a lungfish. Eight of these sites were present in all species
examined, and two were found in all but one species. Alignment of these
conserved restriction sites revealed, among anuran 28S rRNA genes, five
regions of major length variation that correspond to four of 12 previously
identified divergent domains of this gene. One of the divergent domains
(DD8) consists of two regions of length variation separated by a short
segment that is conserved at least throughout tetrapods. Most of the
insertions, deletions, and restriction-site variations identified in the
28S gene will require sequence-level analysis for a detailed reconstruction
of their history. However, an insertion in DD9 that is coextensive with
frogs in the suborder Neobatrachia, a BstEII site that is limited to
representatives of two leptodactylid subfamilies, and a deletion in DD10
that is found only in three ranoid genera are probably synapomorphies.
相似文献
4.
Characterization of cross-bridge elasticity and kinetics of cross-bridge cycling during force development in single smooth muscle cells 总被引:4,自引:2,他引:2 下载免费PDF全文
Force development in smooth muscle, as in skeletal muscle, is believed to reflect recruitment of force-generating myosin cross-bridges. However, little is known about the events underlying cross-bridge recruitment as the muscle cell approaches peak isometric force and then enters a period of tension maintenance. In the present studies on single smooth muscle cells isolated from the toad (Bufo marinus) stomach muscularis, active muscle stiffness, calculated from the force response to small sinusoidal length changes (0.5% cell length, 250 Hz), was utilized to estimate the relative number of attached cross-bridges. By comparing stiffness during initial force development to stiffness during force redevelopment immediately after a quick release imposed at peak force, we propose that the instantaneous active stiffness of the cell reflects both a linearly elastic cross-bridge element having 1.5 times the compliance of the cross-bridge in frog skeletal muscle and a series elastic component having an exponential length-force relationship. At the onset of force development, the ratio of stiffness to force was 2.5 times greater than at peak isometric force. These data suggest that, upon activation, cross-bridges attach in at least two states (i.e., low-force-producing and high-force-producing) and redistribute to a steady state distribution at peak isometric force. The possibility that the cross-bridge cycling rate was modulated with time was also investigated by analyzing the time course of tension recovery to small, rapid step length changes (0.5% cell length in 2.5 ms) imposed during initial force development, at peak force, and after 15 s of tension maintenance. The rate of tension recovery slowed continuously throughout force development following activation and slowed further as force was maintained. Our results suggest that the kinetics of force production in smooth muscle may involve a redistribution of cross-bridge populations between two attached states and that the average cycling rate of these cross-bridges becomes slower with time during contraction. 相似文献
5.
Alexander E. Gad Carmi Lubitz-Omero Nurit Rosenberg Arie Altman 《Plant cell reports》1988,7(5):341-343
Negatively charged large unilamellar vesicles (LUV) were incubated with tobacco (Nicotiana tabacum var. xanthi) cell suspensions and with the cell-free medium of the cell suspensions. The extent of cell-LUV interaction was determined by the leakage of the LUV contents. Cells enhanced the leakage of LUV contents and this effect increased with cell age. Addition of polylysine to the reaction mixture increased even further the leakage of the LUV contents. The cell-free medium of the cell suspension also affected the integrity of the LUV. Cell-free medium, by itself, promoted leakage of LUV contents and caused a reduction in the leakage exerted by polylysine. Centrifugation (8000g) of the cell-free medium decreased its effect, heat treatment (122°C) did not alter its effect and sonication enhanced it. The effects of the cell-free medium are attributed to the presence of cell wall debris of disintegrated cells. 相似文献
6.
Increased lysine synthesis in tobacco plants that express high levels of bacterial dihydrodipicolinate synthase in their chloroplasts 总被引:7,自引:2,他引:5
A major nutritional drawback of many crop plants is their low content of several essential amino acids, particularly lysine. The biosynthesis of lysine in plants is regulated by several feedback loops. Dihydrodipicolinate synthase (DHPS) from Escherichia coli, a key enzyme in lysine biosynthesis, which is considerably less sensitive to lysine accumulation than the endogenous plant enzyme has been expressed in chloroplasts of tobacco leaves. Expression of the bacterial enzyme was accompanied by a significant increase in the level of free lysine. No increase in protein-bound lysine was evident. Free lysine accumulation was positively correlated with the level of DHPS activity in various transgenic plants. Compartmentalization of DHPS in the chloroplast was essential for its participation in lysine biosynthesis as no lysine overproduction was obtained in transgenic plants that expressed the bacterial enzyme in the cytoplasm. The elevated level of free lysine in the transgenic plants was sufficient to inhibit, in vivo, a second key enzyme in lysine biosynthesis, namely, aspartate kinase, with no apparent influence on lysine accumulation. The present report not only provides a better understanding of the regulation of lysine biosynthesis in higher plants but also offers a new strategy to improve the production of this essential amino acid. 相似文献
7.
Calcium-Dependent Transglutaminase of Rat Sympathetic Ganglion in Development and After Nerve Injury 总被引:6,自引:6,他引:0
The activity of transglutaminase (TG) was examined in the rat superior cervical ganglion (SCG) during development and after postganglionic nerve crush. During postnatal development the enzyme activity is increased by sevenfold in parallel to protein content of the ganglion and reaches adult levels by day 35 after birth. The endogenous activity (enzyme activity assayed in the absence of the exogenous substrate) during development is transiently elevated with a peak at day 21 postnatal. In the adult ganglion the enzyme specific activity is evenly distributed in all subcellular compartments, but most of it is contained in the cytosol. Within the first hour after axotomy TG activity is rapidly and transiently elevated. The peak value, 80% above control levels, is attained by 30 min postoperative. At this time the activity is increased in all subcellular fractions, but the endogenous activity is selectively increased in the fraction containing nuclei. The enhanced TG activity after axotomy can be prevented by topical treatments with verapamil, an inhibitor of voltage-dependent calcium fluxes across excitable membranes, or with the calcium chelator EGTA. The results show that intracellular TG activity is present in the SCG and that it increases with postnatal growth of the ganglion. After axotomy the enzyme activity is rapidly and transiently increased in the ganglion and this elevation critically depends on calcium fluxes. 相似文献
8.
On the Relation between Effector Concentration and the Rate of Induced Enzyme Synthesis 总被引:6,自引:0,他引:6 下载免费PDF全文
The Jacob and Monod scheme for the regulation of enzyme formation leads to the following relation between the relative rate of enzyme synthesis α and cellular effector concentration E (the lower sign is for repressible systems): log (α/1 - α - αb) = ± n log [E] + log αb ± log K1. This equation permits linear plotting of experimental data and the evaluation of three quantities: n, the number of effector molecules combining with a repressor molecule, K1, the dissociation constant of this interaction and K2/Rt, the ratio of repressor-operator dissociation constant to total repressor concentration. Measurements on the repression of alkaline phosphatase in Escherichia coli as a function of phosphate concentration are reported and fit the proposed equation with n = 1, indicating that the binding of a single phosphate to the repressor species may be sufficient to cause repression. K1 of this interaction was found to be 0.58 ±0.11 × 10-3 M. The available data regarding the enzymes of the lac operon in a variety of E. coli strains, and several other enzymes are analyzed. It is confirmed that the lac repressor interacts with 2 isopropyl thiogalactoside (IPTG) molecules to relieve repression with a K1 = 50 ±20 × 10-12 M2. In some strains, separate binding constants for the first and second IPTG molecules can be evaluated. 相似文献
9.
On the nature of the high iron diamine method for sulfomucins 总被引:1,自引:0,他引:1
10.
Excess amino acid polymorphism in mitochondrial DNA: contrasts among genes from Drosophila, mice, and humans 总被引:13,自引:3,他引:10
Recent studies of mitochondrial DNA (mtDNA) variation in mammals and
Drosophila have shown an excess of amino acid variation within species
(replacement polymorphism) relative to the number of silent and replacement
differences fixed between species. To examine further this pattern of
nonneutral mtDNA evolution, we present sequence data for the ND3 and ND5
genes from 59 lines of Drosophila melanogaster and 29 lines of D. simulans.
Of interest are the frequency spectra of silent and replacement
polymorphisms, and potential variation among genes and taxa in the
departures from neutral expectations. The Drosophila ND3 and ND5 data show
no significant excess of replacement polymorphism using the
McDonald-Kreitman test. These data are in contrast to significant
departures from neutrality for the ND3 gene in mammals and other genes in
Drosophila mtDNA (cytochrome b and ATPase 6). Pooled across genes, however,
both Drosophila and human mtDNA show very significant excesses of amino
acid polymorphism. Silent polymorphisms at ND5 show a significantly higher
variance in frequency than replacement polymorphisms, and the latter show a
significant skew toward low frequencies (Tajima's D = -1.954). These
patterns are interpreted in light of the nearly neutral theory where mildly
deleterious amino acid haplotypes are observed as ephemeral variants within
species but do not contribute to divergence. The patterns of polymorphism
and divergence at charge-altering amino acid sites are presented for the
Drosophila ND5 gene to examine the evolution of functionally distinct
mutations. Excess charge-altering polymorphism is observed at the carboxyl
terminal and excess charge-altering divergence is detected at the amino
terminal. While the mildly deleterious model fits as a net effect in the
evolution of nonrecombining mitochondrial genomes, these data suggest that
opposing evolutionary pressures may act on different regions of
mitochondrial genes and genomes.
相似文献