全文获取类型
收费全文 | 3318篇 |
免费 | 275篇 |
专业分类
3593篇 |
出版年
2022年 | 16篇 |
2021年 | 33篇 |
2020年 | 19篇 |
2019年 | 18篇 |
2018年 | 40篇 |
2017年 | 32篇 |
2016年 | 46篇 |
2015年 | 92篇 |
2014年 | 96篇 |
2013年 | 181篇 |
2012年 | 137篇 |
2011年 | 136篇 |
2010年 | 71篇 |
2009年 | 86篇 |
2008年 | 123篇 |
2007年 | 153篇 |
2006年 | 168篇 |
2005年 | 163篇 |
2004年 | 148篇 |
2003年 | 163篇 |
2002年 | 154篇 |
2001年 | 136篇 |
2000年 | 150篇 |
1999年 | 106篇 |
1998年 | 44篇 |
1997年 | 55篇 |
1996年 | 42篇 |
1995年 | 52篇 |
1994年 | 36篇 |
1993年 | 26篇 |
1992年 | 85篇 |
1991年 | 81篇 |
1990年 | 69篇 |
1989年 | 75篇 |
1988年 | 73篇 |
1987年 | 55篇 |
1986年 | 45篇 |
1985年 | 43篇 |
1984年 | 34篇 |
1983年 | 28篇 |
1982年 | 18篇 |
1981年 | 18篇 |
1980年 | 15篇 |
1979年 | 28篇 |
1978年 | 19篇 |
1974年 | 26篇 |
1973年 | 19篇 |
1972年 | 18篇 |
1971年 | 13篇 |
1967年 | 13篇 |
排序方式: 共有3593条查询结果,搜索用时 0 毫秒
151.
152.
Production and characterization of N-acyl-D-glutamate amidohydrolase from Pseudomonas sp. strain 5f-1. 下载免费PDF全文
N-Acyl-D-glutamate amidohydrolase from Pseudomonas sp. strain 5f-1 was inducibly produced by D isomers of N-acetylglutamate, glutamate, aspartate, and asparagine. The enzyme has been purified to homogeneity by DEAE-cellulose, (NH4)2SO4 fractionation, and chromatofocusing followed by gel filtration on a Sephadex G-100 column. The enzyme was a monomer with molecular weight of 55,000. The enzyme activity was optimal at pH 6.5 to 7.5 and 45 degrees C. The isoelectric point and the pH stability were 8.8 and 9.0, respectively. N-Formyl, N-acetyl, N-butyryl, N-propionyl, N-chloroacetyl derivatives of D-glutamate and glycyl-D-glutamate were substrates for the enzyme. At pH 6.5 in 100 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer at 30 degrees C, a Km of 6.67 mM and a Vmax of 662 mumol/min/mg of protein for N-acetyl-D-glutamate were obtained. None of the metal ions stimulated the enzyme activity. Na+, K+, Mg2+, and Ba2+ acted as stabilizers. Hg2+, Cu2+, Zn2+, Fe3+, and EDTA were strongly inhibitory. 相似文献
153.
154.
Formaldehyde Fixation Contributes to Detoxification for Growth of a Nonmethylotroph, Burkholderia cepacia TM1, on Vanillic Acid 总被引:2,自引:0,他引:2 下载免费PDF全文
Ryoji Mitsui Yoko Kusano Hiroya Yurimoto Yasuyoshi Sakai Nobuo Kato Mitsuo Tanaka 《Applied microbiology》2003,69(10):6128-6132
During bacterial degradation of methoxylated lignin monomers, such as vanillin and vanillic acid, formaldehyde is released through the reaction catalyzed by vanillic acid demethylase. When Burkholderia cepacia TM1 was grown on vanillin or vanillic acid as the sole carbon source, the enzymes 3-hexulose-6-phosphate synthase (HPS) and 6-phospho-3-hexuloisomerase (PHI) were induced. These enzymes were also expressed during growth on Luria-Bertani medium containing formaldehyde. To understand the roles of these enzymes, the hps and phi genes from a methylotrophic bacterium, Methylomonas aminofaciens 77a, were introduced into B. cepacia TM1. The transformant strain constitutively expressed the genes for HPS and PHI, and these activities were two- or threefold higher than the activities in the wild strain. Incorporation of [14C]formaldehyde into the cell constituents was increased by overexpression of the genes. Furthermore, the degradation of vanillic acid and the growth yield were significantly improved at a high concentration of vanillic acid (60 mM) in the transformant strain. These results suggest that HPS and PHI play significant roles in the detoxification and assimilation of formaldehyde. This is the first report that enhancement of the HPS/PHI pathway could improve the degradation of vanillic acid in nonmethylotrophic bacteria. 相似文献
155.
H. Yanase M. Okuda K. Kita Y. Sato K. Shibata Y. Sakai N. Kato 《Applied microbiology and biotechnology》1995,43(2):228-234
Dihydroxyacetoone synthase (EC 2.2.1.3), which is a key enzyme of the C1-compound-assimilating pathway in yeasts, catalyzes transketolation between formaldehyde and hydroxypyruvate, leading to the formation of dihydroxyacetone and CO2. When [13C]formaldehyde was used as a substrate with dihydroxyacytone synthase from Candida boidinii 2201, 13C was confirmed to be incorporated to the C-1 and C-3 positions of dihydroxyacetone, and the 13C content of each carbon (atoms/100 atoms) was estimated to be 50%. [13C]Methanol was also useful for the enrichment of dihydroxyacetone with 13C, when alcohol oxidase from a methylotrophic yeast was added for the conversion of methanol to formaldehyde. A fed-batch reaction with periodic addition of the substrates was required for the accumalation of 13C-labelled dihydroxyacetone at a higher concentration, because the enzyme system was relatively susceptible to the C donor, formaldehyde or methanol. The optimum conditions for the production gave 160mM (14.4 mg/ml) dihydroxyacetone for 180 min; the molar yield relative to methanol added was 80%. Diyhdroxyacetone kinase (EC 2.7.1.29) from methanol-grown Hansenula polymorpha CBS 4732 was a suitable enzyme for the phosphorylation of dihydroxyacytone. The phosphorylation system, comprising of dihydroxyacetone kinase, adenylate kinase, and ATP, could be coupled with the system for dihydroxyacetone production. A fed-batch reaction afforded 185 mM [1, 3-13C]dihydroxyacetone phosphate from [13C]methanol; the molar yield of the ester relative to methanol added was 92.5% 相似文献
156.
Daisuke Suzuki Keiichi Inoue Makoto Sakai Hideki Kandori Yuki Sudo 《Journal of molecular biology》2009,392(1):48-4922
Microbial organisms utilize light not only as energy sources but also as signals by which rhodopsins (containing retinal as a chromophore) work as photoreceptors. Sensory rhodopsin I (SRI) is a dual photoreceptor that regulates both negative and positive phototaxis in microbial organisms, such as the archaeon Halobacterium salinarum and the eubacterium Salinibacter ruber. These organisms live in highly halophilic environments, suggesting the possibility of the effects of salts on the function of SRI. However, such effects remain unclear because SRI proteins from H. salinarum (HsSRI) are unstable in dilute salt solutions. Recently, we characterized a new SRI protein (SrSRI) that is stable even in the absence of salts, thus allowing us to investigate the effects of salts on the photochemical properties of SRI. In this study, we report that the absorption maximum of SrSRI is shifted from 542 to 556 nm in a Cl−-dependent manner with a Km of 307 ± 56 mM, showing that Cl−-binding sites exist in SRI. The bathochromic shift was caused not only by NaCl but also by other salts (NaI, NaBr, and NaNO3), implying that I−, Br−, and NO3− can also bind to SrSRI. In addition, the photochemical properties during the photocycle are also affected by chloride ion binding. Mutagenesis studies strongly suggested that a conserved residue, His131, is involved in the Cl−-binding site. In light of these results, we discuss the effects of the Cl− binding to SRI and the roles of Cl− binding in its function. 相似文献
157.
158.
Independent differentiation of mammotropes and somatotropes in the chicken embryonic pituitary gland
Zheng J Nakamura K Maseki Y Geelissen SM Berghman LR Sakai T 《Histochemistry and cell biology》2006,125(4):429-439
It has been reported that mammotropes in a rodent pituitary gland are derived from somatotropes via somatomammotropes (SMTs),
cells that produce both growth hormone (GH) and prolactin (Prl). However, no studies have been done on the transdifferentiation
of somatotropes in the chicken pituitary gland. In this study, in order to determine the origin of mammotropes, we studied
detail property of appearance of chicken somatotropes, mammotropes and pit-1 cells and then evaluated the existence of SMTs
in the chicken embryonic pituitary gland. Immunohistochemical analysis revealed that GH-immunopositive (GH-ip) cells appeared
on embryonic day (E) 14 and were mainly distributed in the caudal lobe, while Prl-immunopositive (Prl-ip) cells appeared in
the cephalic lobe of the pituitary gland on E16. In situ hybridization (ISH) and RT-PCR analysis showed that expression of
GH and Prl mRNA starts at E12 in the caudal lobe and at E14 in the cephalic lobe respectively. Pit-1 mRNA was first detected
on E5 by RT-PCR, and pit-1 mRNA-expressing cells were found in the cephalic lobe on E8. Then with the ontogeny of the chicken,
these cells spread into both lobes. Using a double staining method with ISH and immunohistochemistry, we could not detect
the existence of SMTs in the chicken embryonic pituitary gland even in the marginal region of either lobe. These results suggest
that chicken somatotropes and mammotropes independently appear in different lobes of pituitary gland and that transdifferentiation
from somatotropes to mammotropes is not the central route for differentiation of mammotropes in the embryonic chicken pituitary
gland. 相似文献
159.
Hakariya T Shida Y Sakai H Kanetake H Igawa T 《Biochemical and biophysical research communications》2006,342(1):92-100
Epidermal growth factor (EGF) and its receptor (EGFR) are involved in hormone-refractory growth and poor prognosis of a subgroup of human prostate cancer. In this communication, we investigated the regulation of PSA by the EGFR signaling pathway using LNCaP C-81 prostate cancer cells. Administration of EGF stimulated the growth of LNCaP C-81 cells, however, PSA expression and secretion were suppressed. An EGFR inhibitor, AG1478, abrogated the PSA suppression effect by EGF, in concurrence with the suppression of tyro-phosphorylation levels of EGFR. Interestingly, the AR level was also decreased in EGF-treated LNCaP C-81 cells. Moreover, LY294002, but not PD98059, inhibited the PSA and AR suppression effect by EGF in concurrence with the suppression of phosphorylation levels of Akt. In conclusion, our results strongly suggest the existence of a novel androgen-independent PSA regulatory mechanism, i.e., the EGFR signaling pathway negatively regulates PSA expression which may be induced by the alteration of AR expression via the PI3K-Akt pathway in LNCaP C-81 cells. 相似文献
160.
We investigated clonal traits in the dioecious herb Rumex acetosella to characterize sexual dimorphism in clonal forms and to correlate below-ground clonal patterns and above-ground ramet distributions.
We recorded creeping root length, branching patterns, ramet and clump (caespitose ramets from the same position on the root)
sprouting patterns, and biomass allocations in three females and males. We also estimated the patch size of flowering ramets
within a quadrat. No sexual dimorphism was detected in the frequencies of branches and flowering ramets per root length. Male
plants allocated proportionally more biomass to below-ground organs. Total root length did not differ between the sexes. Females
sprouted more clumps with fewer flowering ramets per root length than males, which sprouted fewer clumps with more flowering
ramets, which meant that clump sprouting patterns were phalanx-like in females and guerrilla-like in males. Flowering ramets
were aggregately distributed in both females and males and patch sizes were similar between sexes, indicating that the spreader
propagations were not found in the guerrilla-like males. We assumed that sexual dimorphism occurred in response to physiological
integration for higher reproductive effort in females. 相似文献