全文获取类型
收费全文 | 8801篇 |
免费 | 753篇 |
国内免费 | 6篇 |
出版年
2022年 | 86篇 |
2021年 | 184篇 |
2020年 | 90篇 |
2019年 | 135篇 |
2018年 | 165篇 |
2017年 | 151篇 |
2016年 | 214篇 |
2015年 | 375篇 |
2014年 | 428篇 |
2013年 | 449篇 |
2012年 | 566篇 |
2011年 | 485篇 |
2010年 | 320篇 |
2009年 | 316篇 |
2008年 | 417篇 |
2007年 | 400篇 |
2006年 | 339篇 |
2005年 | 331篇 |
2004年 | 319篇 |
2003年 | 309篇 |
2002年 | 305篇 |
2001年 | 215篇 |
2000年 | 194篇 |
1999年 | 188篇 |
1998年 | 110篇 |
1997年 | 88篇 |
1996年 | 84篇 |
1995年 | 73篇 |
1994年 | 64篇 |
1993年 | 61篇 |
1992年 | 121篇 |
1991年 | 106篇 |
1990年 | 120篇 |
1989年 | 88篇 |
1988年 | 104篇 |
1987年 | 76篇 |
1986年 | 75篇 |
1985年 | 86篇 |
1984年 | 69篇 |
1983年 | 57篇 |
1982年 | 64篇 |
1981年 | 46篇 |
1980年 | 52篇 |
1979年 | 78篇 |
1978年 | 65篇 |
1977年 | 47篇 |
1976年 | 48篇 |
1975年 | 47篇 |
1974年 | 65篇 |
1973年 | 51篇 |
排序方式: 共有9560条查询结果,搜索用时 578 毫秒
71.
72.
73.
Victor S. Sapirstein Charles E. Nolan Itzhak Fischer Elizabeth Cochary Susana Blau Cheryl J. Flynn 《Neurochemical research》1991,16(2):123-128
Plasmolipin is a plasma membrane proteolipid is a major myelin membrane component (Cochary et al., 1990). In this study we report the phylogenic expression of plasmolipin in the vertebrate nervous system. Using Western blot analysis with polyclonal antibodies, we have analyzed membrane fractions, including myelin, from elasmobranchs, teleosts, amphibians, reptiles, birds and mammals. On the basis of immune detection, plasmolipin appears to be restricted to the mammalian nervous system. Comparison of the central and peripheral nervous systems of mammals showed only minor differences in the level of plasmolipin in these two regions. Within mammals, little quantitative differences were observed when rat, human and bovine membrane fractions were compared. The late evolutionary expression of plasmolipin which results in its restriction to mammals makes it unique among the (major) myelin proteins. The potential physiologic significance of these data are discussed.Abbreviations EDTA
Ethylene diamine N.,NN tetracetic acid
- EGTA
Ethylene glycol bis-(B-Aminoethyl Ether) N,,NN tetracetic acid
- MES
([N-Morpholino] ethane sulfonic acid) DCCD, N, Dicyclohexyl carbodiimide 相似文献
74.
Pyruvate decarboxylase (PDC) contains thiamine pyrophosphate (TPP) and Mg2+ as cofactors. 31P NMR studies with PDC in the presence of added Mn2+ reveal the pyrophosphate moiety of TPP to be a nonaccessible area for the external Mn2+ and thus proving the Mg-P-complex (taking part in the binding of the coenzyme to the protein) to be a nonaccessible area for the medium. Glyoxylic acid, acting as an inhibitor of PDC by forming a noncleavable bond with the catalytic center of TPP causes a steric immobilization of the coenzyme indicated by a line broadening of the pyrophosphate moiety. 相似文献
75.
W Korytowski C C Felix B Kalyanaraman 《Biochemical and biophysical research communications》1988,154(2):781-788
Oxidase electrode measurements as well as optical and electron spin resonance spectroscopic data have shown that synthetic neuromelanin oxidizes the neurotoxin metabolite 1-methyl-4-phenyl-2,3-dihydropyridinium in a dose-dependent manner forming 1-methyl-4-phenylpyridinium and hydrogen peroxide. Hydroxyl radicals are formed in this reaction which is promoted by iron chelates. In contrast, neither 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine nor 1-methyl-4-phenylpyridinium reacts with synthetic neuromelanin in a similar fashion. The mechanism of selective toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in pigmented neuronal cells is discussed in the light of these findings. 相似文献
76.
Demonstration that the leukocyte common antigen CD45 is a protein tyrosine phosphatase 总被引:49,自引:0,他引:49
It has been proposed on the basis of amino acid sequence homology that the leukocyte common antigen CD45 represents a family of catalytically active, receptor-linked protein tyrosine phosphatases [Charbonneau, H., Tonks, N. K., Walsh, K. A., & Fischer, E. H. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 7182-7186]. The present study confirms that CD45 possesses intrinsic protein tyrosine phosphatase (PTPase) activity. First, a mouse monoclonal antibody to CD45 (mAb 9.4) specifically eliminated, by precipitation, PTPase activity from a high Mr fraction containing CD45, prepared by gel filtration (Sephacryl S200) of a Triton X-100 extract of human spleen. Second, PTPase activity was demonstrated in a highly purified preparation of CD45 that was eluted with a high pH buffer from an affinity column, constructed from the same antibody. Third, on sucrose density gradient centrifugation, PTPase activity was only found in those fractions that contained CD45 as determined by Western analysis. When CD45 was caused to aggregate, first by reacting it with mAb 9.4 and then adding a secondary, cross-linking anti-mouse mAb, the PTPase activity shifted to the same higher Mr fractions that contained CD45. No shift in CD45 or PTPase was observed following addition of a control IgG2a. On this basis, it is concluded that CD45 is a protein tyrosine phosphatase. 相似文献
77.
Phosphorylase phosphatase isolated from rabbit skeletal muscle can be activated in several ways. Trypsin-Mn2+ treatment of the purified Mr = 70,000 complex or addition of Mn2+ alone to the isolated inactive catalytic subunit gives enzyme species that readily dephosphorylate phosphorylase a and the type 2 regulatory subunit of cAMP-dependent protein kinase as well as synthetic phosphopeptides corresponding to the phosphorylation sites of these proteins. In contrast, enzyme activated by phosphorylation of the regulatory subunit using factor FA (glycogen synthase kinase 3) and Mg2+-ATP and thought to be of physiological significance dephosphorylates the protein substrates but not the phosphopeptides. Likewise, the active catalytic subunit isolated following FA treatment could not act on the peptides unless Mn2+ ions (maximal effect at 250 microM) were added. Mg2+ and Ca2+ could not substitute for Mn2+. Such differences in substrate specificity are not seen with p-nitrophenyl phosphate which is dephosphorylated by all forms of the phosphatase. The results suggest that the primary sequence surrounding the phosphorylation site of the substrate is not all that is necessary for recognition by the FA-activated form of the enzyme. They are interpreted in terms of constraints within the enzyme that are relaxed following exposure to Mn2+ or by the additional determinants present in larger protein substrates. 相似文献
78.
The optical spectra of the reaction center (RC) of Rhodopseudomonas viridis including absorption (A), linear dichrosism (LD), circular dichroism (CD), absorption-detected magnetic resonance (ADMR) and its linear dichroism (LD-ADMR) are simulated by an exciton model. It involves the Qy transitions of six prosthetic groups: four (bacteriochlorophyll-b molecules, two bacteriopheophytin-b molecules and the Soret bands By of the special-pair pigments, which couple most strongly with the corresponding Qy transitions. For the ADMR-spectra additional excitations of the special-pair triplet are introduced. While interactions between the Qy transitions and the By transitions are in principle determined from the structural arrangement of the pigments, the interactions to the other states are adjusted to fit the spectral features for the various transitions. The interaction of the newly introduced states are interpreted in terms of a simple model. 相似文献
79.
Ovulation was induced in rabbits between Days 14 and 18 of pseudopregnancy by an intravenous injection of hCG. Induction of ovulation from Day 16 onwards led to normal progestational endometrial transformation. In rabbits injected on Day 14 or 15, a normal preimplantation endometrial morphology developed, but not earlier than 7 days after hCG (Day 14/15 + 7). Uteroglobin secretion was advanced during the second pseudopregnancy. After mating or artificial insemination, fertility was greatest on Day 18 of pseudopregnancy. Conception failed on Day 14 and embryo transfers were unsuccessful on Day 14 + 1. Transfers performed on Day 14 + 3, however, led to implantation and offspring, even though endometrial morphology did not correspond to the normal Day 3 preimplantational morphology at the time of transfer. We conclude that endometrial transformation typical of normal pseudopregnancy can be induced by ovulation during the regeneration phase of pseudopregnancy from Day 16 onwards; fertilization and implantation can be achieved as early as Day 15 of pseudopregnancy; an oestrous period with high mating activity and fertility occurs about 3 days later; and Day 14 after hCG represents a limited time of functional change from pseudopregnancy to a fertile uterine cycle in the rabbit. 相似文献
80.
J Pohl-Rüling P Fischer D C Lloyd A A Edwards A T Natarajan G Obe K E Buckton N O Bianchi P P van Buul B C Das 《Mutation research》1986,173(4):267-272
Unstable chromosome aberrations induced by in vitro irradiation with zero plus seven low doses of 14.8 MeV D-T neutrons in the range 3.55-244 mGy have been analysed in human peripheral blood lymphocytes. In order to obtain the required large numbers of scored cells for such low doses, fourteen laboratories participated in the experiment. The dose responses for dicentrics, excess acentrics and total aberrations, fitted well to the Y = alpha D model. The alpha coefficient of yield for dicentrics, 1.60 +/- 0.07 X 10(-2) Gy-1, compares well with the values obtained in previous studies with D-T neutrons at somewhat higher doses. Results from a previous collaborative study using 250 kVp X-rays over a comparable dose range indicated the possible existence of a threshold below 50 mGy. In the present study there is no clear evidence for neutrons for such a threshold. However, the data were insufficient to permit the rejection of a possible threshold below approximately 10 mGy. 相似文献