全文获取类型
收费全文 | 13817篇 |
免费 | 1190篇 |
国内免费 | 3篇 |
出版年
2024年 | 16篇 |
2023年 | 146篇 |
2022年 | 92篇 |
2021年 | 216篇 |
2020年 | 205篇 |
2019年 | 211篇 |
2018年 | 567篇 |
2017年 | 466篇 |
2016年 | 506篇 |
2015年 | 398篇 |
2014年 | 425篇 |
2013年 | 782篇 |
2012年 | 1119篇 |
2011年 | 1279篇 |
2010年 | 663篇 |
2009年 | 430篇 |
2008年 | 946篇 |
2007年 | 854篇 |
2006年 | 844篇 |
2005年 | 637篇 |
2004年 | 664篇 |
2003年 | 620篇 |
2002年 | 557篇 |
2001年 | 410篇 |
2000年 | 522篇 |
1999年 | 258篇 |
1998年 | 134篇 |
1997年 | 98篇 |
1996年 | 93篇 |
1995年 | 77篇 |
1994年 | 77篇 |
1993年 | 74篇 |
1992年 | 73篇 |
1991年 | 63篇 |
1990年 | 40篇 |
1989年 | 29篇 |
1988年 | 30篇 |
1987年 | 30篇 |
1986年 | 16篇 |
1985年 | 39篇 |
1984年 | 27篇 |
1983年 | 25篇 |
1982年 | 31篇 |
1981年 | 23篇 |
1980年 | 22篇 |
1979年 | 22篇 |
1978年 | 21篇 |
1977年 | 18篇 |
1974年 | 18篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
31.
Raúl Castaño Angel Ezquerra Nieves Doménech José A. López de Castro 《Immunogenetics》1988,27(5):345-355
The HLA-A2 antigen expressed by donor OZB can be distinguished from the main HLA-A2.1 subtype by isoelectric focusing - it is one charge unit more acidic — and by some alloreactive T-cell clones but not by cytolytic T lymphocyte lines. The structure of variant OZB has been examined by comparative peptide mapping with A2.1 and radiochemical sequence analysis. The two molecules were found to differ in a single tryptic peptide from the 0 region, spanning residues 220–243. The amino acid sequence of this peptide from variant OZB revealed that there was only one amino acid change of Glu instead of Ala at position 236, a hitherto invariant residue in class I HLA antigens. All previously characterized HLA or H-2 natural variants have structural changes restricted to the 1 and/or 2 domains. Thus, variant OZB is unique in that (1) it has one amino acid change in 3 and (2) it has no changes in l and 2. The only detected substitution of this variant may be accounted for by a single base change at the DNA level, suggesting that it might have resulted from a point mutation in the A2.1 gene. The structural features of variant OZB open a novel way to examine the influence of polymorphism in 3 on cytolytic T-cell recognition of naturally occurring class I antigens.Abbreviations CTL
cytolytic T lymphocytes
- HPLC
high performance liquid chromatography
- IEF
isoelectric focusing
- MHC
major histocompatibility complex 相似文献
32.
P. Badia A. Lorenzo T. Gomez A. Bolaños 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1988,157(6):865-871
Summary Ion transport processes in the ileum of the lizard,Gallotia (=Lacerta) galloti was examined in vitro by measuring Na22 and Cl36 fluxes across short-circuited preparations.In Ringer-bicarbonate solution there was both a net sodium flux (
) and a net chloride flux (
) from mucosa to serosa. The inequality between
and short-circuit current (I
sc) suggests that part of the net sodium transport is the result of an electrically neutral transport mechanism or that another electrogenic mechanism opposite in sign is contributing to the short-circuit current.In the absence of sodium, the short-circuit current and net chloride flux were abolished. In the absence of chloride, the net sodium was reduced but not abolished and the short-circuit current was unchanged.From an analysis of the effects of the inhibitors furosemide, amiloride, disulfonic stilbene (DIDS) and acetazolamide, a plausible model was developed to explain the characteristics of these transports. It is proposed that the entry of sodium into the cell across the luminal membrane occurs by two pathways. Part occurs by the antiport Na+H+ and part by an electrogenic pathway. The entry of chloride is by the antiport Cl–HCO
3
–
.Symbols and abbreviations
DIDS 4,4
diisothiocyanatostilbene-2,2-disulfonic acid
-
G
t
tissue conductance
-
I
sc
short circuit current
-
m
mucosal
-
PD
potential difference
-
s
serosal 相似文献
33.
Air-grown cells of a marine, small-celled (2 m diameter) strain of Stichococcus bacillaris contained appreciable carbonic-anhydrase activity but this was repressed when cells were grown on air enriched with 5% (v/v) CO2. Assay of carbonic-anhydrase activity using intact cells and cell extracts showed all activity was intracellular in this Stichococcus strain. Measurement of inorganic-carbon-dependent photosynthetic O2 evolution at pH 5.0, where CO2 is the predominant form of inorganic carbon, showed that the concentration of inorganic carbon required for half-maximal rate of photosynthetic O2 evolution [K0.5(CO2)] was 4.0 M for both air- and CO2-grown cells. At pH 8.3 the K0.5(CO2) was 0.3 mM for air-grown and 0.6 mM for CO2-grown cells. Sodium ions did not enhance bicarbonate utilization. Measurement of the internal inorganic-carbon pool (HCO
3
–
+CO2) by the silicone-oil-layer centrifugal filtering technique showed that air- and CO2-grown cells were able to concentrate inorganic carbon up to 20-fold in relation to the external medium at pH 5.0 but not at pH 8.3. In this alga the high affinity for CO2 and inorganic-carbon accumulation in CO2- and air-grown cells results from active CO2 transport that is not dependent on carbonic-anhydrase activity.Abbreviation Hepes
4-(2-hydroxyethyl)-1-piperazine ethanesulfonic acid 相似文献
34.
F. Javier Caballero M. Isabel Igeño Rafael Quiles Francisco Castillo 《Archives of microbiology》1992,158(1):14-18
Rhodobacter capsulatus strains E1F1 and B10 and Rhodobacter sphaeroides DSM 158 did not use hydroxylamine as nitrogen source for growth but metabolized it mainly through the glutamine synthetase reaction. Hydroxylamine had a high toxicity for cells growing either under phototrophic or dark-aerobic conditions. l-methionine-d,l-sulfoximine partially inhibited hydroxylamine uptake and increased the inhibition time of nitrogenase activity by this nitrogen compound. Nitric oxide was also a powerful inhibitor of nitrogenase in intact cells of R. capsulatus. Since low amounts of NO were produced from hydroxylamine, short-term inhibition of nitrogenase in the presence of this compound could be mediated in vivo by nitric oxide.Abbreviations GS
glutamine synthetase
- MSX
l-methionine-d,l-sulfoximine
- MTA
mixed alkyltrimethylammonium bromide 相似文献
35.
The genetic structure of six populations of Iran (Turks, Kurds, Lurs, Zabolis, Baluchis and Zoroastrians) was examined using
data on blood groups, serum proteins and cell enzymes. Our results show conclusively that there are genetic differences among
the six populations and the analysis of superimposed R and S matrices defined by Harpending & Jenkins (1973) show that the
dispersion of some of the alleles correspond to the dispersion of the populations. The FST estimates are not large enough to favour selection on any of the loci studied. The FIT and FIS estimates are positive and moderately high suggesting that the genetic differentiation to some extent is influenced by inbreeding. 相似文献
36.
J. H. Medina C. Peña M. Piva C. Wolfman M. L. de Stein C. Wasowski C. Da Cunha I. Izquierdo A. C. Paladini 《Molecular neurobiology》1992,6(4):377-386
Great progress has been made in the last 5 yr in demonstrating the presence of benzodiazepines (BDZs) in mammalian tissues,
in beginning studies on the origin of these natural compounds, and in elucidating their possible biological roles. Many unanswered
questions remain regarding the sources and biosynthetic pathways responsible for the presence of BDZs in brain and their different
physiological and/or biochemical actions. This essay will focus on recent findings supporting that: (1) BDZs are of natural
origin; (2) mammalian brain contains BDZs in concentrations ranging between 5.10−10–10−8
M; (3) dietary source of BDZs might be a plausible explanation for their occurrence in animal tissues, including man; (4) the
formation of BDZ-like molecules in brain is a possibility, experimentally supported; (5) BDZ-like molecules including diazepam
andN-desmethyldiazepam are elevated in hepatic encephalopathy; and (6) natural BDZs in the brain are involved in the modulation
of memory processes. Future studies using the full range of biochemical, physiological, behavioral, and molecular biological
techniques available to the neuroscientist will hopefully continue to yield exciting and new information concerning the biological
roles that BDZs might play in the normal and pathological functioning of the brain. 相似文献
37.
The primary structure of the HLA-A2 subtype A*0204 (isoelectric focusing variant A2.A) has been determined. cDNA encoding this subtype was amplified by the polymerase chain reaction. Four independent full-lenght cDNA clones encoding A*0204 were analyzed to obtain a consensus sequence for this subtype. A*0204 differs from A*0201 by a single nucleotide change of G to T through the coding regions, resulting in an Arg to Met change at position 97. This substitution accounts for the isoelectric focusing pattern of the subtype. The same change occurs in other HLA-A specificities in association with other changes in its vicinity. The absence of additional substitutions in A*0204 suggests that it could have arisen from A*0201 by point mutation, and that recurrent mutations may take place during HLA diversification. The spatial location of this change implies that A*0204 must be a functional variant. Comparison of its sequence with other HLA-A2 subtypes reveals that much of the HLA-A2 subtype polymorphism is generated by variations in four neighboring positions, including position 97, which are located in two adjacent -strands on the floor of the peptide binding site of the molecule.The nucleotide sequence data reported in this paper have been submitted to the EMBL nucleotide sequence database and have been assigned the accession number X57954.
Address correspondence and offprint requests to: J. A. López de Castro. 相似文献
38.
G. Egea J. M. Ureña X. Graña J. Marsal J. Carreras F. Climent 《Histochemistry and cell biology》1992,97(3):269-275
Summary We have previously reported (Ureña et al. Eur. J. Cell Biol. 1990) that in skeletal muscle, type MM phosphoglycerate mutase isozyme is present in the nucleus as well as in the cytosol. To determine whether type BB phosphoglycerate mutase isozyme is also present in nucleus, the subcellular location of this isozyme was studied in different rat tissues by cell fractionation and immunogold techniques. With the aid of high affinity-purified anti-phosphoglycerate mutase BB isozyme antibodies, the isozyme was located in the nucleus of neuronal, astroglial and liver cells but not in the nucleus of oligodendroglial and endothelial cells. Biochemical studies on purified nuclear fractions also demonstrated the presence of phosphoglycerate mutase activity in the nucleus. Both immunocytochemical and biochemical techniques showed that nuclear phosphoglycerate mutase-specific activity depended on the type of cell.Abbreviations PGAM
phosphoglycerate mutase
- PGAM-M(M)
muscle specific subunit (isozyme) of PGAM
- PGAM-B(B)
brain type subunit (isozyme) of PGAM
- ssDNA
single stranded DNA
- PBS
0.001 M phosphate buffer, pH 7.4, containing 0.15 M NaCl
- kDa
kilodalton 相似文献
39.
Rita Maria Ulloa Hector Norberto Torres Claudia M. Ochatt Maria Teresa Téllez-Iñón 《Molecular and cellular biochemistry》1991,102(2):155-163
DEAE-cellulose column chromatography of Neurospora crassa soluble mycelial extracts leads to the resolution of three major protein kinase activity peaks designated PKI, PKII, and PKIII.PKII activity is stimulated by Ca2+ and Neurospora or brain calmodulin. Maximal stimulation was observed at 2 µM-free Ca2+ and 1 µg/ml of the modulator. The stimulatory effect of the Ca2+-calmodulin complex was blocked by EGTA and by some calmodulin antagonists such as phenothiazine drugs or compound 48/80.PKII phosphorylates different proteins, among which histone II-A at a low concentration and CDPKS, the synthetic peptide specific for Ca2+-calmodulin dependent protein kinases, are the best substrates. Some phosphorylation can be detected in the absence of any exogenous acceptor. PKII activity assayed in the presence of histone II-A or in the absence of exogenous phosphate acceptor (autophosphorylation) co-elute in a DEAE-cellulose column at 0.28 M NaCl. As result of the autophosphorylation reaction of the purified enzyme a main phosphorylated component of 70 kDa was resolved by SDS-polyacrylamide gel electrophoresis. It is possible that this component is an active part of this enzyme. 相似文献
40.
Rafael Balaña Fouce Maria I. Escribano Jose M. Alunda 《Molecular and cellular biochemistry》1991,107(2):127-133
Summary The putrescine uptake/efflux regulation and their regulatory role on intracellular polyamine pools have been studied in the parasitic protozoa Leishmania infantum. Putrescine uptake was age-dependent with maximal values in logarithmic phase promastigotes and minimal in stationary phase. Moreover, putrescine uptake was activated in response to depletion of intracellular polyamines by alpha-difluoromethylornithine (DFMO) — a well known irreversible enzyme-activated inhibitor of ornithine decarboxylase. Kinetic studies of putrescine uptake induction showed a notable rise in Vmax without Km changes, suggesting a de novo synthesis of putrescine carriers. Putrescine uptake was able to replenish polyamine content and also to recover the proliferative rate in cells treated during 24 hours with DFMO. 相似文献