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71.
Plasma membrane vesicles, isolated from ejaculated ram sperm, were found to contain Ca2+-activated Mg2+-ATPase and Ca2+ transport activities. Membrane vesicles that were exposed to oxalate as a Ca2+-trapping agent accumulated Ca2+ in the presence of Mg2+ and ATP. The Vmax for Ca2+ uptake was 33 nmol/mg protein per h, and the Km values for Ca2+ and ATP were 2.5 μM and 45 μM, respectively. 1 μM of the Ca2+ ionophore A23187, added initially, completely inhibited net Ca2+ uptake and, if added later, caused the release of Ca2+ previously accumulated. A Ca2+-activated ATPase was present in the same membrane vesicles which had a Vmax of 1.5 μmol/mg protein per h at free Ca2+ concentration of 10 μM. This Ca2+-ATPase had Km values of 4.5 μM and 110 μM for Ca2+ and ATP, respectively. This kinetic parameter was similar to that observed for uptake of Ca2+ by the vesicles. The Ca2+-ATPase activity was insensitive to ouabain. Both Ca2+ transport and Ca2+-ATPase activity were inhibited by the flavonoid quercetin. Thus, ram spermatozoa plasma membranes have both a Ca2+ transport activity and a Ca2+-stimulated ATPase activity with similar substrate affinities and specificities and similar sensitivity to quercetin.  相似文献   
72.
Microtubules assembled by the incubation of GTP at 37 °C were disassembled by the action of calmodulin-dependent protein kinase (Kinase II) which occrs only in the brain tissues. This disassembly required the presence of ATP and physiological concentrations of Ca2+ and calmodulin.  相似文献   
73.
A 28 amino acid peptide with diuretic and natriuretic activity has been purified from rat atrial muscle. The primary structure of this atrial peptide is H-Ser-Leu-Arg-Arg-Ser-Ser-Cys-Phe-Gly-Gly-Arg-Ile-Asp-Arg-Ile-Gly- (sequence in text) Ala-Gln-Ser-Gly-Leu-Gly-Cys-Asn-Ser-Phe-(Arg)-Tyr-OH. The biological activity of this peptide is identical to that of atrial natriuretic factor and cardionatrin I isolated from rat atria.  相似文献   
74.
In a previous paper (Van 't Sant, P., Mak, J.F.C. and Kroon, A.M. (1981) Eur. J. Biochem. 121, 21–26) we showed the existence of three elongated precursor proteins (45, 36 and 25 kDa) of mitochondrial translation products in Neurospora crassa. We presented some indications that the largest precursor could be related to subunit 1 of cytochrome c oxidase. Here we present conclusive evidence that the 45-kDa polypeptide is indeed this precursor by demonstrating that an immunodetectable 45-kDa polypeptide displays the same behaviour as the labeled 45-kDa precursor; both accumulate after long incubation with cycloheximide or by decreasing the temperature and both are not tightly membrane bound. Moreover the antibody against subunit 1 of cytochrome c oxidase also recognizes, in immunoadsorption experiments, besides subunit 1, the 45-kDa polypeptide accumulated by cycloheximide incubation. Furthermore, we developed a small scale purification of antibodies against subunit 1 of cytochrome c oxidase. By means of these purified antibodies it is demonstrated that the 45-kDa polypeptide and subunit 1 have corresponding antigenic determinants. Under the various conditions tested, all three precursors are less firmly membrane-bound than the mature subunits. Finally, it is observed that in short incubations in vivo, chloramphenicol inhibits the processing of the mitochondrially synthesized precursors, under conditions where mitochondrial translation is only partially inhibited.  相似文献   
75.
Polysomes from the skeletal muscle of normal and dystrophic hamsters were dissociated into ribosomal subunits by treatment with puromycin and the subunits from both strains were reassociated in all possible combinations. When their protein synthesis activity was assayed in a poly(U)-directed cell-free system at a low magnesium concentration, the reassociated ribosomes from dystrophic hamsters were less active than the ribosomes from control animals. The ribosomal defect is a property of the 60S subunit and is due to a ribosomal component rather than to abnormal binding of a non-ribosomal protein.  相似文献   
76.
The sensitivity of the (Na+ + K+)-ATPase in human red cell membranes to inhibition by Ca2+ is markedly increased by the addition of diluted cytoplasm from hemolyzed human red blood cells. The concentration of Ca2+ causing 50% inhibition of the (Na+ + K+)-ATPase is shifted from greater than 50 microM free Ca2+ in the absence of hemolysate to less than 10 microM free Ca2+ when hemolysate diluted 1:60 compared to in vivo concentrations is added to the assay mixture. Boiling the hemolysate destroys its ability to increase the sensitivity of the (Na+ + K+)-ATPase to Ca2+. Proteins extracted from the membrane in the presence of EDTA and concentrated on an Amicon PM 30 membrane increased the sensitivity of the (Na+ + K+)-ATPase to Ca2+ in a dose-dependent fashion, causing over 80% inhibition of the (Na+ + K+)-ATPase at 10 microM free Ca2+ at the highest concentration of the extract tested. The active factor in this membrane extract is Ca2+-dependent, because it had no effect on the (Na+ + K+)-ATPase in the absence of Ca2+. Trypsin digestion prior to the assay destroyed the ability of this protein extract to increase the sensitivity of the (Na+ + K+)-ATPase to Ca2+.  相似文献   
77.
The maturation of the ability of the B-cell population to re-express surface immunoglobulin (sIg) after its removal by treatment with rabbit anti-mouse immunoglobulin (RAMIg) was studied in LAF1, C57BL/6, and C57L mice. As demonstrated by previous workers, the B-cell population from immature mice failed to re-express sIg after treatment with RAMIg. We have shown that the age at which the B-cell population acquires the capacity to re-express sIg is different in different strains and that the order in which the B-cell population of the different strains acquires the capacity to re-express sIg is different from the order in which their B-cell populations acquire the capacity to produce high-affinity antibodies. This suggests that these represent distinct differentiation events in the development of the B-cell population. In all of the strains studied the maturation of the capacity to re-express sIg occurred in two steps. After the first maturation step the B-cell population was able to re-express sIg after treatment with RAMIg for 1 hr but did not re-express sIg after treatment with RAMIg for 24 hr. After the second maturation step the B-cell population could re-express sIg even after 24 hr treatment with RAMIg. It has been suggested by previous workers that the inability of the immature B-cell population to re-express sIg could represent one of the mechanisms responsible for the development of B-cell self-tolerance. It is suggested here that the existence of a period during which cells become tolerant only upon prolonged exposure to antigen could protect the developing B cells from becoming unresponsive to transiently experienced foreign antigens but still permit them to become tolerant to self antigens which are continuously present.  相似文献   
78.
Metallothionein (MT) bound to zinc and copper was detected in high concentration in fetal and newborn rat livers by a cadmium saturation method. The levels of both hepatic zinc and MT remained high for the first 14 days after birth and decreased to adult levels by 24 days of age. There was a direct linear relationship between hepatic metallothionein and zinc concentrations during the first 31 days after birth. The ratio of MT to zinc levels also decreased with age suggesting a rapid degradation of MT during postnatal development. Immunohistochemical localization of MT by peroxidase-antiperoxidase technique, using a specific antibody to MT, showed intense intranuclear staining for MT in fetal and newborn rat liver which persisted until Day 9. The nuclear MT staining decreased with age; at 11 days it was equal both in nucleus and cytoplasm and at 14 days, MT was localized mainly in the cytoplasm, similar to adult rat liver pattern. The intranuclear localization of MT in neonates could be considered as a typical fetal-neonatal morphological pattern and its subsequent presence in the cytoplasm, an adult pattern.  相似文献   
79.
Forests losing large quantities of nitrogen have elevated 15N:14N ratios   总被引:1,自引:0,他引:1  
Peter Högberg 《Oecologia》1990,84(2):229-231
Summary Urea (U) and ammonium nitrate (AN) had been applied to a Scots pine (Pinus sylvestris L.) forest in northern Sweden for 18 consecutive years at four doses resulting in total N applications ranging from 0 to 1980 kg ha–1. The 15N abundance ( 15N) of the grass Deschampsia flexuosa (L.) Trin. increased linearly (from –0.7 to 11.0) with application rate in the case of U. The response to AN was in the same direction but smaller. While others have shown that the initial response of nitrogen-limited systems to additions of N is a change of 15N abundance towards that of added N, this study shows that further and excessive additions leads to a retention of 15N. Monitoring 15N abundance over time in dose-response trials of this type thus opens new possibilities to estimate critical loads of N and the point of nitrogen saturation.  相似文献   
80.
Nitrogen fixed in 13 provenances of Acacia albida and 11 isolines of Leucaena leucocephala inoculated with effective Rhizobium strains was measured by 15N techniques and the total N difference method. In the test soil, on the average, L. leucocephala derived about 65% of its total N from atmospheric N2 fixation compared to about 20% by A. albida. Significant differences in the percentage of N derived from atmospheric N2 (% Ndfa) occurred, between provenances or isolines within species. The % Ndfa ranged from 37 to 74% within L. leucocephala and from 6 to 37 within A. albida; (equivalent to 20–50 mg N plant–1 and 4–37 mg N plant–1 for the two species over three months, respectively) and was correlated with the nodule mass (r=0.91). The time course of N2 fixation of three selected provenances (low, intermediate and good fixers) was followed at 12 weekly intervals over a 36 week period. The % Ndfa of all provenances and isolines increased with time; and except for one of the L. leucocephala provenances, % Ndfa was similar within species at the 36 weeks harvest. There was a significant correlation between % Ndfa and the amount of N2 fixed (r=0.96). Significant interactions occurred between provenances and N treatments and often growth of uninoculated but N fertilized plants was less variable than for inoculated unfertilized plants.  相似文献   
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