全文获取类型
收费全文 | 118154篇 |
免费 | 10105篇 |
国内免费 | 185篇 |
专业分类
128444篇 |
出版年
2022年 | 665篇 |
2021年 | 1289篇 |
2020年 | 926篇 |
2019年 | 1109篇 |
2018年 | 1807篇 |
2017年 | 1723篇 |
2016年 | 2736篇 |
2015年 | 4442篇 |
2014年 | 4721篇 |
2013年 | 6133篇 |
2012年 | 7178篇 |
2011年 | 6612篇 |
2010年 | 4371篇 |
2009年 | 3668篇 |
2008年 | 5606篇 |
2007年 | 5490篇 |
2006年 | 5217篇 |
2005年 | 5044篇 |
2004年 | 4885篇 |
2003年 | 4361篇 |
2002年 | 4378篇 |
2001年 | 3290篇 |
2000年 | 3281篇 |
1999年 | 2817篇 |
1998年 | 1432篇 |
1997年 | 1199篇 |
1996年 | 1207篇 |
1995年 | 1135篇 |
1994年 | 1134篇 |
1993年 | 1094篇 |
1992年 | 2129篇 |
1991年 | 1801篇 |
1990年 | 1773篇 |
1989年 | 1720篇 |
1988年 | 1567篇 |
1987年 | 1406篇 |
1986年 | 1402篇 |
1985年 | 1479篇 |
1984年 | 1262篇 |
1983年 | 1041篇 |
1982年 | 931篇 |
1981年 | 848篇 |
1980年 | 706篇 |
1979年 | 1012篇 |
1978年 | 855篇 |
1977年 | 682篇 |
1975年 | 670篇 |
1974年 | 769篇 |
1973年 | 714篇 |
1972年 | 627篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
T Ishikawa M Müller C Klünemann T Schaub D Keppler 《The Journal of biological chemistry》1990,265(31):19279-19286
The liver is the major organ which eliminates leukotriene C4 (LTC4) and other cysteinyl leukotrienes from the blood circulation into bile. Transport of LTC4 was studied using inside-out vesicles enriched in canalicular and sinusoidal membranes from rat liver. The incubation of canalicular membrane vesicles with [3H]LTC4 in the presence of ATP resulted in an uptake of LTC4 into vesicles. The initial rate of ATP-stimulated LTC4 uptake was about 40-fold higher in canalicular than in sinusoidal membrane vesicles. When liver plasma membrane vesicles were incubated in the absence of ATP, an apparent transient uptake of LTC4 was observed which was temperature-dependent and not affected by the osmolarity. This indicates that LTC4 was bound to proteins on the surface of plasma membrane vesicles. Two proteins with relative molecular weights of 17,000 and 25,000 were detected by direct photoaffinity labeling as major LTC4-binding proteins. One protein (Mr 25,000) was ascribed to subunit 1 (Ya) of glutathione S-transferase which was associated with the membrane. LTD4, LTE4, N-acetyl-LTE4, and omega-carboxy-N-acetyl-LTE4 were also transported into liver plasma membrane vesicles in an ATP-dependent manner with initial rates relative to LTC4 (1.0) of 0.46, 0.11, 0.35, and 0.22, respectively. Mutual competition between the cysteinyl leukotrienes and S-(2,4-dinitrophenyl)-glutathione for uptake indicated that they are transported by a common carrier. Apparent Km values of the transport system for LTC4, LTD4, and N-acetyl-LTE4 were 0.25, 1.5, and 5.2 microM, respectively. The ATP-dependent transport of LTC4 into vesicles was not inhibited by doxorubicin, daunorubicin, or verapamil, or by the monoclonal antibody C219, suggesting that the transport system differs from P-glycoprotein. Liver plasma membrane vesicles prepared from mutant rats deficient in the hepatobiliary excretion of cysteinyl leukotrienes lacked the ATP-dependent transport of cysteinyl leukotrienes and S-(2,4-dinitrophenyl)-glutathione. These results demonstrate that the ATP-dependent carrier system is responsible for the transport of cysteinyl leukotrienes and glutathione S-conjugates from the hepatocytes into bile. 相似文献
972.
Thyroid antibodies as a risk factor for Down syndrome and other trisomies. 总被引:3,自引:1,他引:2 下载免费PDF全文
C P Torfs B J van den Berg F W Oechsli R E Christianson 《American journal of human genetics》1990,47(4):727-734
To test whether the presence of thyroid antibodies in a parent is a risk factor for meiotic nondisjunction, we measured the levels of thyroid antibodies in serum samples drawn during early pregnancy from 101 gravidas who delivered a child with a trisomy, from 11 gravidas who had had a trisomic child in a previous pregnancy, and from 44 of their husbands. For each case mother, three controls were randomly selected from the same population and matched for age, race, sex of the child, and hospital of birth. Cases and controls came from two longitudinal populations, the Child Health and Development Studies (CHDS) and the national Collaborative Perinatal Project (CPP), together comprising more than 70,000 live births. All cases with both a definite diagnosis of trisomy-Down syndrome (DS) or other-and available serum were included. Overall, there was no association between the presence of thyroid antibodies in a mother and a trisomy in her offspring (odds ratio [OR] = .98, confidence interval [CI] = .54-1.85). The lack of association was seen in all three subgroups (DS only, other trisomies, and DS in a previous pregnancy), in all ethnic groups, and in the age groups of white mothers either less than 30 years of age (OR = .80, CI = .40-1.6) or greater than or equal to 30 years of age (OR = 1.26, CI = .82-1.9). In the CHDS population, case fathers, as compared with control fathers, did not have a higher prevalence of thyroid antibodies.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
973.
Bacterial degradation of styrene involving a novel flavin adenine dinucleotide-dependent styrene monooxygenase. 总被引:17,自引:11,他引:6 下载免费PDF全文
By using styrene as the sole source of carbon and energy in concentrations of 10 to 500 microM, 14 strains of aerobic bacteria and two strains of fungi were isolated from various soil and water samples. In cell extracts of 11 of the bacterial isolates, a novel flavin adenine dinucleotide-requiring styrene monooxygenase activity that oxidized styrene to styrene oxide (phenyl oxirane) was detected. In one bacterial strain (S5), styrene metabolism was studied in more detail. In addition to styrene monooxygenase, cell extracts from strain S5 contained styrene oxide isomerase and phenylacetaldehyde dehydrogenase activities. A pathway for styrene degradation via styrene oxide and phenylacetaldehyde to phenylacetic acid is proposed. 相似文献
974.
The effect of interferon (IFN) on protein synthesis was studied in the Burkitt's lymphoma cell line P3HR-1 by [35S]methionine labelling of the cells, followed by two-dimensional gel electrophoresis of cell extracts. De novo synthesis of three proteins (mol. wts. 33 000, 62 000, and 98 000, respectively) and alterations in the rate of synthesis for a small number of additional proteins were observed during the first 12 h of treatment, while the rate of overall protein synthesis was unaffected. Treatment of P3HR-1 cells with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or hydrocortisone (HC), which induce similar changes in cell cycle distribution as does IFN, did not induce comparable changes in the rates of protein synthesis. Thus, the effects were specific for IFN and not induced by the change in cell cycle distribution per se, i.e., accumulation in G0. Treatment of cells with 2'-5' pA core did not mimic the effect of IFN at the translational level. A substrain of P3HR-1 cells, selected for resistance to the anti-proliferative effect of IFN, lacked six proteins found in the wild-type. The 62 000 mol. wt. protein was induced in this substrain as well as in native P3HR-1 cells on addition of IFN. The resistant substrain still developed an anti-viral effect in response to IFN. Thus, it seems as if the anti-proliferative and anti-viral effects of IFN, at least in some cells are mediated by different intracellular molecular mechanisms. 相似文献
975.
Multimers of anionic amphiphiles mimic calmodulin stimulation of cyclic nucleotide phosphodiesterase 下载免费PDF全文
Oleic acid, phosphatidylserine and pyrenedecanoic acid were found to activate calmodulin-deficient cyclic nucleotide phosphodiesterase at concentrations above their critical micellar concentration. In contrast with calmodulin these activators do not require the presence of Ca2+ for their action. It is shown that the size of phosphatidylserine vesicles is of crucial importance with respect to the activating potency of phosphatidylserine. Fluorescence measurements with the probe pyrenedecanoic acid revealed that micelles rather than monomers are the active species for stimulation of phosphodiesterase. There are indications that this result also may be applied to the other activators. 相似文献
976.
C. Lücke-Huhle W. Comper L. Hieber M. Pech 《Radiation and environmental biophysics》1982,20(3):171-185
Summary Survival and G2 delay following exposure to either60Cobalt--rays or241Americium--paticles were studied in eight mammalian cell lines of human and animal origin including human fibroblasts from normal individuals and from patients with Ataxia telangiectasia or Fanconi's anemia.For both endpoints the effectiveness of alpha particles was greater as compared to-rays. RBE values for G2 delay (4.6–9.2) were in general comparable to RBE values derived from initial slopes of survival curves (RBE
) but higher compared to the ratio of mean inactivation doses
.Ataxia cells were particularly sensitive to cell killing by-irradiation (D37 = 0.57 Gy), however, showed average sensitivity to-particles of high LET (D37 = 0.30 Gy).With the exception of Ataxia cells, cell killing and G2 delay seem to be related processes if individual cell cycle parameters are taken into account. 相似文献
977.
A general method for the synthesis of 5'-monophosphates of DNA fragments via phosphotriester intermediates. 总被引:10,自引:10,他引:0 下载免费PDF全文
A new and attractive phosphorylation procedure which allows the introduction, via phosphotriester intermediates, of 5'-phosphate functions of DNA fragments is described. The method is based on the activation of bifunctional phosphorylating agents with 1-hydroxybenzotriazole. The approach will be exemplified by the synthesis of pACGC using four different 5'-phosphotriester intermediates. 相似文献
978.
The nucleotide sequence of the rat epsilon-chain mRNA has been determined by sequencing cloned cDNA copies of the mRNA. The established sequence covers the coding region, the 3'-non coding region and most of the 5' non-coding region. A comparison with the nucleotide sequence of the human epsilon-chain constant region reveals that C3 and C4 are the most highly conserved domains. The rat epsilon-chain contains a C-terminal decapeptide which is not present in the human counterpart. 相似文献
979.
980.