全文获取类型
收费全文 | 79024篇 |
免费 | 6572篇 |
国内免费 | 4842篇 |
专业分类
90438篇 |
出版年
2024年 | 142篇 |
2023年 | 914篇 |
2022年 | 2083篇 |
2021年 | 3615篇 |
2020年 | 2332篇 |
2019年 | 2842篇 |
2018年 | 2879篇 |
2017年 | 2035篇 |
2016年 | 2879篇 |
2015年 | 4595篇 |
2014年 | 5302篇 |
2013年 | 5972篇 |
2012年 | 6904篇 |
2011年 | 6359篇 |
2010年 | 3820篇 |
2009年 | 3376篇 |
2008年 | 4116篇 |
2007年 | 3657篇 |
2006年 | 3178篇 |
2005年 | 2690篇 |
2004年 | 2285篇 |
2003年 | 1977篇 |
2002年 | 1738篇 |
2001年 | 1560篇 |
2000年 | 1569篇 |
1999年 | 1449篇 |
1998年 | 847篇 |
1997年 | 798篇 |
1996年 | 809篇 |
1995年 | 738篇 |
1994年 | 688篇 |
1993年 | 530篇 |
1992年 | 818篇 |
1991年 | 657篇 |
1990年 | 602篇 |
1989年 | 532篇 |
1988年 | 421篇 |
1987年 | 362篇 |
1986年 | 336篇 |
1985年 | 299篇 |
1984年 | 221篇 |
1983年 | 199篇 |
1982年 | 112篇 |
1981年 | 118篇 |
1980年 | 86篇 |
1979年 | 147篇 |
1978年 | 84篇 |
1977年 | 95篇 |
1975年 | 111篇 |
1974年 | 116篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
F C Wei L H Colony H C Chen C C Chuang M S Noordhoff 《Plastic and reconstructive surgery》1989,84(4):651-661
Historically, restoration of hand function following multiple digital amputation has been unsatisfactory. The evolution of digital reconstruction with toe transfer has enabled surgeons to reestablish prehension in these severely injured hands. A 4-year experience with 26 consecutive combined second and third toe transfers to replace missing adjacent fingers was reviewed in order to delineate the indications and technical considerations and to emphasize prevention of donor-site complications. Combined second and third toe transfer is reserved for adjacent finger amputations proximal to the digital web space with remaining fingers no longer than the small finger. Radial amputations are replaced with contralateral combined toe units, while ipsilateral toes are more ideal for ulnar amputations. Limited dorsal and plantar skin flaps extending only to the midpoint of the first and third digital web spaces allow for direct donor-site closure and uncomplicated healing. Maintenance of the plantar metatarsal arch by avoiding metatarsal shaft osteotomies or bone grafting-shortened metatarsals eliminates potential gait disturbances. When properly applied in selected patients, this single-stage microsurgical procedure can restore prehensile function, improve the appearance of the hand with multiple digital amputations, and preserve near-normal donor-foot function. 相似文献
162.
The effect of aging on glutathione and cysteine levels in different regions of the mouse brain 总被引:5,自引:0,他引:5
T S Chen J P Richie C A Lang 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1989,190(4):399-402
A general glutathione (GSH) deficiency occurs in many tissues of the aging mouse. However, there is no information on GSH in the aging brain even though it has been involved in a number of neurobiologic reactions. To this end, C57BL/6 mice, 3-31 months old, representing the growth, maturation, and aging periods of the life-span were studied. Brain cortex, hippocampus, and stem samples were dissected, processed, and analyzed specifically for reduced and oxidized glutathione (GSH, GSSG) and cyst(e)ine using high performance liquid chromatography with dual electrochemical detection. The GSH content of each brain region varied in the order brain cortex greater than brain hippocampus greater than brainstem. However, the GSH profiles of all regions were the same through the life-span, namely, high values during growth dropping to a maturation plateau and then decreasing 30% during aging. In contrast to GSH, the order of cysteine levels was brain cortex less than brain hippocampus less than brainstem and no life-span changes occurred in any region. In addition, the brain GSSG and cystine contents of all regions were very low and did not change during the life-span. Thus, the GSH loss was not accountable by oxidation to GSSG or degradation to cyst(e)ine. Altogether these results demonstrated a GSH deficiency in brain tissues of aging mice like that found previously in other tissues. These findings suggest an increased susceptibility of the aging brain to oxidative damage. 相似文献
163.
Two monensin-sensitive bacteria which utilized carbohydrates poorly and grew rapidly on amino acids were isolated from the bovine rumen. The short rods (strain SR) fermented arginine, serine, lysine, glutamine, and threonine rapidly (greater than 158 nmol/mg of protein per h) and grew faster on casein digest containing short peptides than on free amino acids ().34 versus 0.29 h(-1)). Gelatin hydrolysate, an amino acid source containing an abundance of long peptides, was unable to support growth or ammonia production, but there was a large increase in ammonia production if strain SR was cocultured with peptidase-producing ruminal bacteria (Bacteroides ruminicola or Streptococcus bovis). Cocultures showed no synergism with short peptides. Strain SR washed out of continuous culture ().1 h(-1)) at pH 5.9. The irregularly shaped organisms (strain F) deaminated glutamine, histidine, glutamate, and serine rapidly (greater than 137 nmol/mg of protein per min) and grew faster on free amino acids than on short peptides ().43 versus 0.21 h(-1)). When strain F was provided with casein or gelatin hydrolysate and cocultured with peptidase-producing bacteria, there was a more than additive increase in ammonia production. Strain F grew in continuous culture (0.1 h(-1)) when the pH was as low as 5.3. The irregularly shaped cells and short rods were present at less than 10(9)/ml in vivo, but they ahd very high specific activities of ammonia production (greater than 310 nmol of ammonia/mg of protein per min) and could play an important role in ruminal amino acid fermentation. 相似文献
164.
Human atrial natriuretic peptide receptor defines a new paradigm for second messenger signal transduction. 总被引:18,自引:2,他引:16 下载免费PDF全文
D G Lowe M S Chang R Hellmiss E Chen S Singh D L Garbers D V Goeddel 《The EMBO journal》1989,8(5):1377-1384
We isolated cDNAs encoding a 115 kd human atrial natriuretic peptide (alpha ANP) receptor (ANP-A receptor) that possesses guanylate cyclase activity, by low-stringency hybridization with sea urchin Arbacia punctulata membrane guanylate cyclase probes. The human ANP-A receptor has a 32 residue signal sequence followed by a 441 residue extracellular domain homologous to the 60 kd ANP-C receptor. A 21 residue transmembrane domain precedes a 568 residue cytoplasmic domain with homology to the protein kinase family and to a subunit of the soluble guanylate cyclase. COS-7 cells transfected with an ANP-A receptor expression vector displayed specific [125I]alpha ANP binding, and exhibited alpha ANP stimulated cGMP production. These data demonstrate a new paradigm of cellular signal transduction where extracellular ligand binding allosterically regulates cyclic nucleotide second-messenger production by a receptor cytoplasmic catalytic domain. 相似文献
165.
Sequence divergence and open regions of RNA secondary structures in the envelope regions of the 17 human immunodeficiency virus isolates. 总被引:4,自引:2,他引:2 下载免费PDF全文
Genetic variation during the course of infection of an individual is a remarkable feature of the acquired immune deficiency syndrome (AIDS) disease. This variation has been studied for the envelope protein encoding regions of seventeen different sequences from various isolates of human immunodeficiency virus (HIV) using multiple sequence comparison and calculation of variability. The open regions with little intramolecular base pairing in these envelope sequences are predicted by a recently developed statistical method. The minimum length L for a run of hypervariable sites, conserved sites, or open regions that gives significance at the 1% (or 0.1%) level is then determined by a scan statistical method. The results show that significant clusters of open regions predicted at the RNA levels correlate with significant clusters of hypervariable sites in the HIV envelope gene. Those significant genomic variations in HIVs seem to be manifested mainly in the extracellular portion of the envelope protein. Twelve potential antigenic determinants are predicted using an antigenic index method. Interestingly, the majority of the significant hypervariable regions in the exterior envelope protein (gp120) were predicted potential epitopes. 相似文献
166.
Factor IXPortland: a nonsense mutation (CGA to TGA) resulting in hemophilia B 总被引:5,自引:0,他引:5 下载免费PDF全文
S H Chen C R Scott J Schoof E W Lovrien K Kurachi 《American journal of human genetics》1989,44(4):567-569
Male siblings with severe hemophilia b were studied for the molecular defect responsible for their disorder. To define the precise DNA alteration, a 362-bp fragment in the first part of exon VIII of the factor IX gene was amplified and sequenced. A single-base-pair substitution of C----T at the nucleotide sequence 30875 was found which resulted in a nonsense mutation (TGA) and terminated the protein synthesis of factor IX at amino acid residue 252. The single-base change occurred as a classic CG dinucleotide alteration to TG (or CA), a common mechanism for point mutations in mammals. 相似文献
167.
A new compound, withangulatin A, promotes type II DNA topoisomerase-mediated DNA damage 总被引:1,自引:0,他引:1
J K Juang H W Huang C M Chen H J Liu 《Biochemical and biophysical research communications》1989,159(3):1128-1134
Withangulatin A, a new compound with a known chemical structure and from the antitumor Chinese herb Physalis angulata L, was found to act on topoisomerase II to induce topoisomerase II-mediated DNA damage in vitro. It has two effective dosage ranges of approximate 0.5 and 20 microM, with about one-third the activity of 20 microM VM-26. 相似文献
168.
Liver mitochondrial respiratory functions decline with age 总被引:14,自引:0,他引:14
T C Yen Y S Chen K L King S H Yeh Y H Wei 《Biochemical and biophysical research communications》1989,165(3):944-1003
Human liver mitochondrial respiration rates in Chinese populations of various ages were assayed with an oxygraph. In this study, State 3 and State 4 respiration rates, respiratory control ratio (RCR), and ADP/O ratio were measured for 35 Chinese subjects of 31 to 76 years old. We found a significant negative correlation between age and respiratory control and ADP/O ratios tested. Moreover, the respiratory control and ADP/O ratios decreased with the increase of age. These findings suggest that a substantial fall in mitochondrial oxidative capacity in ageing liver may be an important contributor to the ageing process. 相似文献
169.
D I Crane N H Chen C J Masters 《Biochemical and biophysical research communications》1989,160(2):503-508
Peroxisomal enoyl-CoA hydratase was purified from livers of mice treated with di-(2-ethylhexyl)phthalate and its properties compared with those of the 70 kDa protein present in the membranes prepared by carbonate extraction of peroxisomes. The two proteins had identical subunit molecular masses, of about 70,000 daltons. Limited proteolysis of these proteins using the V8 proteinase of S. aureus yielded identical peptide maps, with these peptides crossreacting with antiserum raised against the 70 kDa membrane protein. These data are consistent with the proposal that the peroxisomal 70 kDa membrane protein and the peroxisomal enoyl-CoA hydratase are the same protein. 相似文献
170.
G C Chen D A Hardman R L Hamilton C M Mendel J W Schilling S Zhu K Lau J S Wong J P Kane 《Biochemistry》1989,28(6):2477-2484
The distribution of lipid-binding regions of human apolipoprotein B-100 has been investigated by recombining proteolytic fragments of B-100 with lipids and characterizing the lipid-bound fragments by peptide mapping, amino acid sequencing, and immunoblotting. Fragments of B-100 were generated by digestion of low-density lipoproteins (LDL) in the presence of sodium decyl sulfate with either Staphylococcus aureus V8 protease, pancreatic elastase, or chymotrypsin. Particles with electron microscopic appearance of native lipoproteins formed spontaneously when detergent was removed by dialysis from enzyme digests containing fragments of B-100 and endogenous lipids, or from incubation mixtures of delipidated B-100 fragments mixed with microemulsions of exogenous lipids (cholesteryl oleate and egg phosphatidylcholine). Fractionation of the recombinant particles by isopycnic or density gradient ultracentrifugation yielded complexes similar to native LDL with respect to shape, diameter, electrophoretic mobility, and surface and core compositions. Circular dichroic spectra of these particles showed helicity similar to LDL but a somewhat decreased content of beta-structure. Most of the fragments of B-100 were capable of binding to lipids; 12 were identified by direct sequence analysis and 14 by reaction with antisera against specific sequences within B-100. Our results indicate that lipid-binding regions of B-100 are widely distributed within the protein molecule and that proteolytic fragments derived from B-100 can reassociate in vitro with lipids to form LDL-like particles. 相似文献