全文获取类型
收费全文 | 21341篇 |
免费 | 1496篇 |
国内免费 | 11篇 |
专业分类
22848篇 |
出版年
2024年 | 27篇 |
2023年 | 66篇 |
2022年 | 242篇 |
2021年 | 413篇 |
2020年 | 247篇 |
2019年 | 310篇 |
2018年 | 531篇 |
2017年 | 393篇 |
2016年 | 680篇 |
2015年 | 1126篇 |
2014年 | 1223篇 |
2013年 | 1393篇 |
2012年 | 1825篇 |
2011年 | 1708篇 |
2010年 | 1098篇 |
2009年 | 912篇 |
2008年 | 1346篇 |
2007年 | 1185篇 |
2006年 | 1052篇 |
2005年 | 971篇 |
2004年 | 960篇 |
2003年 | 779篇 |
2002年 | 785篇 |
2001年 | 627篇 |
2000年 | 632篇 |
1999年 | 422篇 |
1998年 | 166篇 |
1997年 | 129篇 |
1996年 | 119篇 |
1995年 | 87篇 |
1994年 | 82篇 |
1993年 | 69篇 |
1992年 | 157篇 |
1991年 | 125篇 |
1990年 | 88篇 |
1989年 | 103篇 |
1988年 | 70篇 |
1987年 | 65篇 |
1986年 | 69篇 |
1985年 | 53篇 |
1984年 | 47篇 |
1983年 | 37篇 |
1982年 | 27篇 |
1981年 | 24篇 |
1978年 | 28篇 |
1976年 | 32篇 |
1975年 | 29篇 |
1973年 | 33篇 |
1971年 | 23篇 |
1969年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Acetate kinase purified from Acinetobacter calcoaceticus was inhibited by diethylpyrocarbonate with a second-order rate constant of 620 M-1.min-1 at pH 7.4 at 30 degrees C and showed a concomitant increase in absorbance at 240 nm due to the formation of N-carbethoxyhistidyl derivative. Activity could be restored by hydroxylamine and the pH curve of inactivation indicates the involvement of a residue with a pKa of 6.64. Complete inactivation of acetate kinase required the modification of seven residues per molecule of enzyme. Statistical analysis showed that among the seven modifiable residues, only one is essential for activity. 5,5'-dithiobis(2-nitrobenzoic acid), p-chloromercuryphenylsulfonate, N-ethylmaleimide and phenylglyoxal did not affect the enzyme activity. These results suggest that the inactivation is due to the modification of one histidine residue. The substrates, acetate and ATP, protected the enzyme against inactivation, indicating that the modified histidine residue is located at or near the active site. 相似文献
52.
S-thiolation of creatine kinase and glycogen phosphorylase b initiated by partially reduced oxygen species 总被引:1,自引:0,他引:1
S-thiolation of cardiac creatine kinase and skeletal muscle glycogen phosphorylase b was initiated by reduced oxygen species in reaction mixtures containing reduced glutathione. Both proteins were extensively modified at similar rates under conditions in which the oxidation of glutathione was inadequate to cause S-thiolation by thiol-disulfide exchange. Creatine kinase was both S-thiolated and non-reducibly oxidized at the same time at low glutathione concentration. The amount of each modification was decreased by adding additional reduced glutathione, and with adequate glutathione oxidation was prevented while S-thiolation was still very active. S-thiolation of glycogen phosphorylase b was not significantly affected by glutathione concentration and non-reducible oxidation of glycogen phosphorylase b was not observed. These experiments suggest that oxyradical or H2O2-initiated processes may be an important mechanism of protein S-thiolation during oxidative stress, and that the cellular concentration of glutathione may be an important factor in S-thiolation of different proteins. Both creatine kinase and glycogen phosphorylase b competed favorably with ferricytochrome c for superoxide anion in the standard xanthine oxidase system for the generation of oxyradicals and H2O2. These proteins were as effective as ascorbate and much more effective than reduced glutathione in this regard. Ascorbate was also an effective inhibitor of oxyradical-initiated S-thiolation of creatine kinase, suggesting a role of superoxide anion in protein S-thiolation. Other experiments showed that both catalase and superoxide dismutase could partially inhibit protein S-thiolation. Thus, reduced oxygen species may react with protein sulfhydryls resulting in S-thiolation by a mechanism that involves the reaction of an activated protein thiol with reduced glutathione. 相似文献
53.
A carrier-supported mycelial growth of Penicillium chrysogenum was applied to penicillin fermentation system using celite as a support material. Hyphal growth through the pore matrices of the material showed strong anchorages and provided highly stable biofilm growth. With bioparticles developed in such a manner, both cell growth and penicillin production were observed to increase significantly compared to the conventional dispersed filamentous cultures. Maximum values of specific penicillin production rate were found to be constant regardless of the growth form. A three-phase fluidized-bed fermentor was designed and tested for penicillin production using the bioparticles. Two modes of operation, semicontinuous and repeated fed batch, of the fermentor were tried. It was noted that the overgrowth of free mycelia and the development of fluffy loose bioparticles caused poor mixing and made the fermentor operation quite difficult. Control of the bioparticle size and the extension of production phase were therefore considered important to maintain the reactor productivity at a desired level. From the results of repeated fed-batch operation it was found that the control of bioparticle size could be successfully achieved by phosphate-limiting culture condition. Penicillin production under this condition was also observed to be maintained at a high level (about 80% of the maximum) for at least 1 month. 相似文献
54.
B D Hammock G D Prestwich D N Loury P Y Cheung W S Eng S K Park D E Moody M H Silva R N Wixtrom 《Archives of biochemistry and biophysics》1986,244(1):292-309
An affinity purification procedure was developed for the cytosolic epoxide hydrolase based upon the selective binding of the enzyme to immobilized methoxycitronellyl thiol. Several elution systems were examined, but the most successful system employed selective elution with a chalcone oxide. This affinity system allowed the purification of the cytosolic epoxide hydrolase activity from livers of both control and clofibrate-fed mice. A variety of biochemical techniques including pH dependence, substrate preference, kinetics, inhibition, amino acid analysis, peptide mapping, Western blotting, analytical isoelectric focusing, and gel permeation chromatography failed to distinguish between the enzymes purified from control and clofibrate-fed animals. The quantitative removal of the cytosolic epoxide hydrolase acting on trans-stilbene oxide from 100,000g supernatants, allowed analysis of remaining activities acting differentially on cis-stilbene oxide and benzo[a]pyrene 4,5-oxide. Such analysis indicated the existence of a novel epoxide hydrolase activity in the cytosol of mouse liver preparations. 相似文献
55.
Peptidoglycan synthetic activities in membranes of Escherichia coli caused by overproduction of penicillin-binding protein 2 and rodA protein 总被引:27,自引:0,他引:27
F Ishino W Park S Tomioka S Tamaki I Takase K Kunugita H Matsuzawa S Asoh T Ohta B G Spratt 《The Journal of biological chemistry》1986,261(15):7024-7031
Penicillin-binding protein (PBP)-2 and the RodA protein are known to function in determining the rod shape of Escherichia coli cells. Peptidoglycan biosynthetic reactions that required these two proteins were demonstrated in the membrane fraction prepared from an E. coli strain that overproduced both of these two proteins and which lacked PBP-1B activity (the major peptidoglycan synthetase activity in the normal E. coli membranes). The cross-linked peptidoglycan was synthesized from UDP-N-acetylmuramylpentapeptide and UDP-N-acetylglucosamine in the presence of a high concentration of cefmetazole that inhibited all of PBPs except PBP-2. The peptidoglycan was synthesized via a lipid intermediate and showed up to 30% cross-linking. The cross-linking reaction was strongly inhibited by the amidinopenicillin, mecillinam, and by other beta-lactam antibiotics that have a high affinity for PBP-2, but not by beta-lactams that had very low affinity for PBP-2. The formation of peptidoglycan required the presence of high levels of both PBP-2 and the RodA protein in the membranes, but it is unclear which of the two proteins was primarily responsible for the extension of the glycan chains (transglycosylation). However, the sensitivity of the cross-linking reaction to specific beta-lactam antibiotics strongly suggested that it was catalyzed by PBP-2. The transglycosylase activity of the membranes was sensitive to enramycin and vancomycin and was unusual in being stimulated greatly by a high concentration of a chelating agent. 相似文献
56.
Low-ultraviolet circular dichroism spectroscopy of oligopeptides 1-95 and 96-168 derived from myelin basic protein of rabbit 总被引:6,自引:0,他引:6
Myelin basic protein (MBP) is a major protein constituent of the myelin sheath of the central nervous system, where it is believed to have functional alpha-helical segments. One element of the function of the protein might be "conformational adaptability" of specific regions of its amino acid sequence, since the purified protein appears to be largely devoid of ordered structure. To pursue this question, low-ultraviolet circular dichroism (CD) spectroscopy was conducted on the sequential thrombic peptides 1-95 and 96-168 of the protein in the presence of 0-92% trifluoroethanol (TFE), a solvent known to promote stable secondary structures in polypeptides. The series of CD spectra of the oligopeptides were subjected to a computerized best-fit analysis of four peptide conformations, the alpha-helix, beta-structure, beta-turn, and nonordered form. Agreement between experimental and best-fit composite spectra was achieved when standard CD curves of peptide conformations were derived from known theoretical spectra and experimental spectra of polypeptides. In dilute buffer alone, oligopeptides 1-95 and 96-168 evidence no alpha-helix but significant beta-structure (18% and 23%, respectively), as well as a predominant, extended nonordered conformation. However, the two parts of the protein differed in conformational adaptability. From 0% to 30% TFE, 96-168 exhibited concomitant transitions to 10% helix and 32% beta-structure from the nonordered form. In contrast, in 10-30% TFE, 1-95 underwent a transition to approximately 21% helix with partial loss of beta-structure as well as nonordered form; higher concentrations of TFE (40-75%) promoted additional transitions to both helix and beta-structure (totaling 33% and 25%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
57.
R A Nicholas F Ishino W Park M Matsuhashi J L Strominger 《The Journal of biological chemistry》1985,260(10):6394-6397
The localization of the active site of penicillin-binding protein 5 from the dacA mutant of Escherichia coli strain TMRL 1222 has been determined. The protein was purified to homogeneity and labeled with [14C] penicillin G. The labeled protein was digested with trypsin, and the active site tryptic peptide was purified by a combination of gel filtration and high-pressure liquid chromatography. Sequencing of the purified [14C]penicilloyl peptide yielded the sequence Arg-Asp-Pro-Ala-Ser-Leu-Thr-Lys, which corresponds to residues 40-47 of the gene sequence (Broome-Smith, J., Edelman, A., and Spratt, B. G. (1983) in The Target of Penicillin (Hakenbeck, R., Holtje, J.-V., and Labischinski, H., eds) pp. 403-408, Walter de Gruyter, Berlin). The catalytic amino acid residue that forms a covalent bond with penicillin was identified by treating the purified [14C]penicilloyl peptide with a mixture of proteases and then separating the radioactive products using high-pressure liquid chromatography. Analysis of the radioactive peaks by amino acid analysis confirmed that it is the serine residue that reacts with the beta-lactam ring of penicillin. 相似文献
58.
Developmental changes in the activities of prostaglandin synthesizing enzymes in the digestive and immune systems of rat 总被引:1,自引:0,他引:1
The developmental changes of prostaglandin (PG) synthesizing enzymes in the digestive system (stomach and small intestine) and the immune system (spleen and thymus) of rats were investigated. In all the digestive organs, the predominant PG produced from PGH2 changed at around 2 weeks after birth to another PG. Further, the predominant activities of PG synthesizing enzymes were different organ by organ in the digestive system. In the case of the immune system, only the activity of PGD2 synthesizing enzyme displayed a significant increase during development and the activities of other PG synthesizing enzymes remained insignificant throughout the development. These results suggest that PGs may play important roles during the development of each organ. 相似文献
59.
Anil K. Sinha V. Ann Linscombe B. Bhaskar Gollapudi George C. Jersey Colin N. Park 《Cell biology and toxicology》1985,1(4):333-342
Sister chromatid exchanges (SCE) were analyzed in peripheral blood lymphocytes from a select group of 71 healthy men, 56 nonsmokers and 15 cigarette smokers. In addition to estimating baseline SCE, data were examined to seek relationships of SCE frequencies to age and smoking. The baseline value of 7.53 SCE per cell from the 56 nonsmokers was within the range (5.60 to 9.10 SCE/cell) reported for other human populations. No relationship was found between the mean SCE frequency per cell and age. However, a significant increase in the SCE mean value was observed in smokers as compared to nonsmokers. The results of this study are compared with those of other reports on SCE effects of age and smoking.Abbreviations BUdR
5-bromo,2-deoxyuridine
- SCE
sister chromatid exchange 相似文献
60.
The analysis of a continuous, aerobic, fixed-film bioreactor is performed by simulating the behavior of penicillin production in a three-phase fluidized bed. Rigorous mathematical models are developed for a fluidized-bed fermentor in which bioparticles are fluidized by the liquid medium and air. The steady-state performance of the fluidized-bed reactor is appraised in terms of penicillin productivity and outlet concentration by considering the two extremes in contacting patterns, complete back-mix and plug flow, in the absence of a growing biofilm. The results show that the complete back-mix contacting pattern is preferred over that of plug flow due to the nature of the penicillin kinetic relationships. It is also shown that for the dual-nutrient (glucose and oxygen) penicillin reaction system the optimum biofilm thickness does not equal the penetration depth of a limiting nutrient, but depends upon the total reactor configuration. 相似文献