全文获取类型
收费全文 | 20403篇 |
免费 | 1895篇 |
国内免费 | 12篇 |
出版年
2023年 | 121篇 |
2022年 | 300篇 |
2021年 | 564篇 |
2020年 | 267篇 |
2019年 | 390篇 |
2018年 | 435篇 |
2017年 | 340篇 |
2016年 | 586篇 |
2015年 | 985篇 |
2014年 | 1020篇 |
2013年 | 1399篇 |
2012年 | 1597篇 |
2011年 | 1565篇 |
2010年 | 967篇 |
2009年 | 844篇 |
2008年 | 1212篇 |
2007年 | 1192篇 |
2006年 | 1073篇 |
2005年 | 1015篇 |
2004年 | 914篇 |
2003年 | 853篇 |
2002年 | 823篇 |
2001年 | 198篇 |
2000年 | 153篇 |
1999年 | 180篇 |
1998年 | 169篇 |
1997年 | 141篇 |
1996年 | 117篇 |
1995年 | 111篇 |
1994年 | 117篇 |
1993年 | 112篇 |
1992年 | 120篇 |
1991年 | 107篇 |
1990年 | 98篇 |
1989年 | 85篇 |
1988年 | 93篇 |
1987年 | 72篇 |
1986年 | 66篇 |
1985年 | 89篇 |
1984年 | 96篇 |
1983年 | 62篇 |
1982年 | 81篇 |
1981年 | 72篇 |
1980年 | 67篇 |
1979年 | 77篇 |
1978年 | 54篇 |
1977年 | 61篇 |
1976年 | 67篇 |
1975年 | 70篇 |
1973年 | 55篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
Hydrogen peroxide-mediated inactivation of microsomal cytochrome P450 during monooxygenase reactions
Cytochrome P450 can undergo inactivation following monooxygenase reactions in liver microsomes of untreated, phenobarbital and 3-methylcholanthrene-treated rats and rabbits. The acceleration of cytochrome P450 loss in the presence of catalase inhibitors (sodium azide, hydroxylamine) indicates that hydrogen peroxide is involved in hemoprotein degradation. It was revealed that cytochrome P450 is inactivated mainly by H2O2 formed through peroxy complex breakdown, whereas H2O2 formed via the dismutation of superoxide anions produces a slight inactivating effect. The hydrogen peroxide added outside or formed by a glucose-glucose oxidase system has less of an inactivating effect than H2O2 produced within the cytochrome P450 active center. Self-inactivation of cytochrome P450 during oxygenase reactions is highly specific. Other components of the monooxygenase system, such as cytochrome b5, NADH- and NADPH-specific flavorproteins, undergo no inactivation. The alterations in phospholipid content and in the rate of lipid peroxidation were not observed as well. The inactivation of cytochrome P450 by H2O2 is the result of heme loss or destruction without cytochrome P420 formation. Such. a mechanism operates with different substrates and cytochrome P450 species catalyzing the partially coupled monooxygenase reactions. 相似文献
202.
The marine algaBrachiomonas submarina var.pulsifera (Droop) CCAP 7/2A, is employed as a food organism in aquaculture; it can be cultured heterotrophically or mixotrophically. Growth rates and productivity under heterotrophic conditions were lower than those achieved under mixotrophic conditions. By reducing the osmotic potential of the medium, whilst simultaneously increasing the levels of nitrogen and phosphorus and using sodium acetate as a carbon source, a 20-fold increase in final yield was attained. This corresponded to a maximum culture of 9.02times 106 cells ml–1 and a dry weight of 2.51 g l–1.Author for correspondence 相似文献
203.
Piotr Ceglowski Alexander Boitsov Natalia Karamyan Sunghee Chai Juan C. Alonso 《Molecular & general genetics : MGG》1993,241(5-6):579-585
The low-copy-number and broad-host-range pSM19035-derived plasmid pBT233 is stably inherited in Bacillus subtilis cells. Two distinct regions, segA and segB, enhance the segregational stability of the plasmid. Both regions function in a replicon-independent manner. The maximization of random plasmid segregation is accomplished by the recombination proficiency of the host or the presence of the pBT233 segA region. The segA region contains two open reading frames (or) [ and ]. Inactivation or deletion of or results in SegA– plasmids. Better than random segregation requires an active segB region. The segB region contains two ors (or and or). Inactivation of either of the orfs does not lead to an increase in cell death, but or– plasmids are randomly segregated. These results suggest that pBT233 stabilization relies on a complex system involving resolution of plasmid oligomers (segA) and on the function(s) encoded by the segB region. 相似文献
204.
Matthias Liebergesell Alexander Steinbüchel 《Applied microbiology and biotechnology》1993,38(4):493-501
From a genomic library of Thiocystis violaceae strain 2311 in L47, two adjacent EcoRI restriction fragments of 5361 base pairs (bp) and of 1978 bp were cloned. The 5361-bp EcoRI restriction fragment hybridized with a DNA fragment harbouring the Alcaligenes eutrophus poly(3-hydroxyalkanoate) (PHA) synthase operon (phbCAB) and restored the ability to synthesize and accumulate PHA in PHA-negative mutants derived from A. eutrophus. The nucleotide sequence analysis of both fragments revealed five open-reading frames (ORFs); at least three of them are probably relevant for PHA biosynthesis. The amino acid sequences of the putative proteins deduced from these genes indicate that they encode a -ketothiolase [phbA
Tv, relative molecular mass (Mr) 40850], which exhibited 87.3% amino acid identify with the -ketothiolase from Chromatium vinosum. The amino acid sequences of the putative proteins deduced from ORF2Tv (Mr 41 450) and phbC
Tv (Mr 39 550), which were located upstream of and antilinear to phbA
Tv, exhibited 74.7% and 87.6% amino acid identify, respectively, with the corresponding gene products of C. vinosum. Downstream of and antilinear to phbC
Tv was located ORF5, which encodes for a protein of high relative molecular mass (Mr 76428) of unknown function. With respect to the divergent organisation of ORF2Tv and phbC
Tv on one side and of phbA
Tv on the other side and from the homologies of the putative gene products, this region of the T. violaceae genome resembled very much the corresponding region of C. vinosum, which was identified recently.
Correspondence to: A. Steinbüchel 相似文献
205.
A cosmid gene bank of partially EcoRI-digested genomic DNA from Methylobacterium extorquens IBT no. 6 was screened for DNA fragments restoring polyhydroxyalkanoic-acid (PHA) accumulation in the PHA-negative mutant Alkaligenes eutrophus H16 PHB–4. The M. extorquens PHA-synthase structural gene phaC
Mex
was mapped on a 23-kbp EcoRI fragment by complementation studies, by hybridization experiments with heterologous DNA probes from A. eutrophus H16 encoding for phaA, phaB and phaC and by nucleic acid sequence analysis. Evidence for the presence of genes for a -ketothiolase or an acetoacetyl-coenzyme A reductase on this fragment was not obtained. The nucleotide sequence of a 3.7-kbp region was obtained. It contained the entire 1.815-kbp phaC
Mex plus approximately each 900-bp upstream and downstream of phaC
Mex. PhaC
Mex
encoded a protein of 605 amino acods with a relative molecular mass (Mr) of 66742, which exhibited 38.1% amino acid identity with the A. eutrophus PHA synthase. Determination of the N-terminal amino acid sequence of an Mr 65 000 protein, which was enriched concomitantly with the purification of PHA granules in sucrose gradients, revealed a sequence that was identical with the amino acid sequence deduced from the most probable translation start codon except for a valine, which was obviously removed post-translationally. Enzyme analysis, which was done with the native gene and a phaC
Mex
-lacZ fusion gene, gave no evidence for expression of phaC
Mex
in Escherichia coli. 相似文献
206.
Alexander A. DiIorio Pamela J. Weathers Ronald D. Cheetham 《Applied microbiology and biotechnology》1993,39(2):174-180
Transformed root tissue of Beta vulgaris (Detroit Dark Red) was permeabilized to stimulate the release of intracellularly stored betanin without adverse affects on tissue viability as measured by biomass accumulation. Product release of up to 15% (w/w) was achieved by heat treatment at 42°C for 45 min with minimal effect on viability. Higher levels of product release were obtained with increasing temperature and exposure, but at the expense of viability. Viability was measured by comparing dry weight increases of permeabilized tissue 3 days after treatment vs non-permeabilized tissue over the same time interval. Recovery of heat-treated tissue was improved by addition of CaCl2 (20 mm for 10 min) post-heat treatment. Betanin release up to 15% was also obtained at ambient temperature (25°C) by addition of up to 20 mm (NH4)2SO4 in the presence of 1 mm ethylenediaminetetraacetic acid (EDTA).
Correspondence to: A. A. DiIorio 相似文献
207.
Robert de Lorimier Sigurd M. Wilbanks Alexander N. Glazer 《Plant molecular biology》1993,21(2):225-237
R-phycocyanin II (RPCII) is a recently discovered member of the phycocyanin family of photosynthetic light-harvesting proteins. Genes encoding the and subunits of RPCII were cloned and sequenced from marine Synechococcus sp. strains WH8020 and WH8103. The deduced amino acid sequences of RPCII were compared to two other types of phycocyanin, C-phycocyanin (CPC) and phycoerythrocyanin (PEC). These three types vary in the composition of their covalently bound bilin prosthetic groups. In terms of amino acid sequence identity RPCII is highly homologous to CPC and PEC, suggesting that the known three-dimensional structures of the latter two are representative of RPCII. Thus the amino acid residues contacting the three bilins of RPCII could be inferred and compared to those in CPC and PEC. Certain residues were identified among the three phycocyanins as possibly correlating with specific bilin isomers. In overall sequence RPCII and CPC are more homologous to one another than either is to PEC. This probably reflects functional homology in the roles of RPCII and CPC in the transfer of light energy to the core of the phycobilisome, a function not attributed to PEC. The genomes of Synechococcus sp. strains WH8020, WH8103 and WH7803 share homologous open reading frames in the vicinity of RPCII genes. The nucleotide sequence extending 3 from RPCII genes in strain WH8020 revealed two open reading frames homologous to components of an CPC phycocyanobilin lyase. These open reading frames may encode a lyase specific for the attachment of phycoerythrobilin to RPCII. 相似文献
208.
Jrn Elsner Johannes Norgauer Gustav J. Dobos Andreas Emmendrffer Erwin Schpf Alexander Kapp Joachim Roesler 《Journal of cellular physiology》1993,157(3):637-643
Flow cytometric analyses were performed to study intracellular single-cell calcium transients ([Ca2+]i) in suspended human neutrophils during the initial phase of N-formyl peptide stimulation. Thereby, two neutrophil populations became apparent. Early maximally Ca2+-responding (high fluorescence) neutrophils and not-yet Ca2+-responding (low fluorescence) neutrophils, but no neutrophils with intermediate levels of [Ca2+]i, were detected. Within 7 s the number of low fluorescence neutrophils decreased and the number of high fluorescence neutrophils increased maximally. This suggests that [Ca2+]i transients occurred abruptly in individual neutrophils within a time interval below 1 s. At lower N-formyl peptide concentrations the lag times of individual neutrophils and the interval time of maximal activation of the [Ca2+]i-responding neutrophil population increased, however the percentage of [Ca2+]i-responding cells decreased. Surprisingly, no influence of the N-formyl peptide concentration on the [Ca2+]i-induced fluorescence signal of the individual cell was observed: it was always in an almost maximal range or not responding. In parallel, binding studies performed with fluorescein-labeled N-formyl peptide revealed that the heterogeneity of [Ca2+]i-responding cells cannot be explained by different receptor occupancy. In summary, this study demonstrates that [Ca2+]i transients induced by N-formyl peptides in suspended individual human neutrophils occur very rapidly in an almost “all-or-none manner” and that the mean increasing fluorescence signal of a calcium indicator within a whole neutrophil population results from varying lag times of the individual cells, rather than from the mean simultaneous progress of many cells. © 1993 Wiley-Liss, Inc. 相似文献
209.
Polarized secretion of IGF-I and IGF-I binding protein activity by cultured aortic endothelial cells
W. Robert Taylor Robert M. Nerem R. Wayne Alexander 《Journal of cellular physiology》1993,154(1):139-142
Insulin-like growth factor-I (IGF-I) secretion by the vascular endothelium has been proposed to play a role in the regulation of vascular smooth muscle cell proliferation. Because vascular smooth muscle cells are adjacent to the abluminal surface of the endothelium, we tested the hypothesis that secretion of IGF-I by endothelial cells is polarized. Porcine aortic endothelial cells were cultured on permeable membranes and IGF-I measured by radioimmunoassay. Basal secretion exceeded apical secretion by a ratio of 2.3 ± 0.2:1.0 (P < 0.05). We also identified 35 kDa IGF-I binding protein activity that is preferentially secreted on the basal surface of endothelial cells. We conclude that both IGF-I and IGF-I binding protein activity secretion by endothelial cells is polarized towards the basal surface of the endothelium. A polarized secretion mechanism for IGF-I may be of importance in the normal growth and differentiation of the vasculature as well as in the development of vascular pathology. © 1993 Wiley-Liss, Inc. 相似文献
210.