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1.
In Escherichia coli, two enzymes catalyze the synthesis of methionine from homocysteine using methyltetrahydrofolate as the donor of the required methyl group: cobalamin-dependent and cobalamin-independent methionine synthases. Comparison of the mechanisms of these two enzymes offers the opportunity to examine two different solutions to the same chemical problem. We initiated the research described here to determine whether the two enzymes were evolutionarily related by comparing the deduced amino acid sequences of the two proteins. We have determined the nucleotide sequence for the metE gene, encoding the cobalamin-independent methionine synthase. Our results reveal an absence of similarity between the deduced amino acid sequences of the cobalamin-dependent and cobalamin-independent proteins and suggest that the two have arisen by convergent evolution. We have developed a rapid one-step purification of the recombinant cobalamin-independent methionine synthase (MetE) that yields homogeneous protein in high yield for mechanistic and structural studies. In the course of these studies, we identified a highly reactive thiol in MetE that is alkylated by chloromethyl ketones and by iodoacetamide. We demonstrated that alkylation of this residue, shown to be cysteine 726, results in complete loss of activity. While we are unable to deduce the role of cysteine 726 in catalysis at this time, the identification of this reactive residue suggests the possibility that this thiol functions as an intermediate methyl acceptor in catalysis, analogous to the role of cobalamin in the reaction catalyzed by the cobalamin-dependent enzyme. 相似文献
2.
A González-Fernández J Sans P Aller C de la Torre 《Cell biology international reports》1985,9(3):237-243
Bromosubstitution for most of the S period in synchronous populations of Allium cepa L. meristematic cells resulted in a delay in the late S-G2 transition point where protein synthesis is needed for later mitotic entrance to occur. This retardation in the position of the transition point was not accompanied by the expected delay in the entrance into mitosis, suggesting that such protein synthesis is a requisite, but not a timer for prophase triggering. 相似文献
3.
C González-Bosch V J Miralles J Hernández-Yago S Grisolía 《Biochemical and biophysical research communications》1987,146(3):1318-1323
Apocytochrome c, the cytosolic precursor of cytochrome c, competes with the precursor of ornithine carbamoyltransferase (OCT) for entry into isolated rat liver mitochondria. 相似文献
4.
Spermidine and spermine stimulate the transport of the precursor of ornithine carbamoyltransferase into rat liver mitochondria 总被引:1,自引:0,他引:1
C González-Bosch V J Miralles J Hernández-Yago S Grisolia 《Biochemical and biophysical research communications》1987,149(1):21-26
We have examined the effect of low molecular weight components of the transport mixture generally used for the import of rat liver pre-ornithine carbamoyltransferase by isolated rat liver mitochondria. These studies revealed that spermidine and spermine, at physiological concentrations, stimulate the transport of the precursor of ornithine carbamoyltransferase into mitochondria. This stimulatory effect of spermidine and spermine is concentration-dependent and is completely inhibited at higher than physiological concentrations (20 mM for spermidine and 4 mM for spermine). Magnesium ions, which also have a stimulatory effect, inhibit the stimulatory effect of spermidine. 相似文献
5.
Suárez-Cortez R Alvarez-Gallegos J González-Mora E 《Biotechnology and bioengineering》1989,33(4):377-385
The design of a sliding controller for a continuous fermentation process is presented. The results obtained by simulation have proved the control scheme to be very robust. Regulation of substrate concentration at its optimal value has been achieved even though process parameters change their nominal values. Chattering effects are decreased by introducing a minor modification of the control variable around the sliding surface. 相似文献
6.
J M Afonso M Hernández J M Larruga V M Cabrera A M González 《Human biology; an international record of research》1989,61(4):543-549
We analyzed the genetic polymorphism of eight red cell enzymes in samples from different geographical areas of Tenerife and the Iberian peninsula. The gene frequency heterogeneity found within the Tenerife samples was at the same level as that of Tenerife-mainland comparisons. The presence of the Negroid G6PD A+ allele in the Tenerife samples is evidence of an African admixture with a mean estimation of 4.5%. 相似文献
7.
Pollution indicators in Do?ana National Park show a cyclic seasonal pattern with maximum level in the autumn and minimum in the spring. While faecal streptococci proved to be the best indicator micro-organisms in this type of salty, alkaline environment, total coliforms gave an overestimation of pollution because of the presence of hydroteluric coliforms. 相似文献
8.
Enrique Palacián Pedro J. González Manuel Piñeiro Francisco Hernández 《Molecular and cellular biochemistry》1990,97(2):101-111
Dissociation of protein-containing structures by modification of protein amino groups with dicarboxylic acid anhydrides is a mild procedure which, in some cases, offers advantages over treatment with alternative dissociating agents, such as urea, guanidine hydrochloride, detergents, high ionic strength, and extremes of pH: In addition to dissociating multimeric proteins and protein aggregates, dicarboxylic acid anhydrides are effective dissociating agents for membrane-bound proteins and nucleoprotein particles. With most dicarboxylic acid anhydrides reviewed, the introduced reagent residues can be eliminated under moderate acid conditions, which allows the purification of unmodified individual components, and the use of disassembly-reconstitution systems valuable for investigating the structural and functional roles played by the individual components of complex particles:Each reagent can be suitable for a particular purpose, depending on the required specificity of the modification and stability of the modified groups: The stability of the acylated amino groups ranges from the very stable succinylated amino groups to the very labile acylation obtained with dimethylmaleic anhydride: Between these extremes, the stability of the modified amino groups decreases stepwise in the following order: maleic, exo-cis-3,6-endoxo-4-tetrahydrophthalic, citraconic, and 3,4,5,6-tetrahydrophthalic anhydride. With respect to the selectivity of the produced modification, little or no modification of hydroxyamino acid and cysteine residues has been observed with dimethylmaleic, exo-cis-3,6-endoxo-4-tetrahydrophthalic, and 3,4,5,6-tetrahydrophthalic anhydrides: With the other reagents, the extent of modification of hydroxyamino acid residues increases in the order citraconic, maleic and succinic anhydride: Citraconic and maleic anhydrides can produce irreversible modification of cysteine residues, the reactivity of sulfhydryl groups being higher with maleic anhydride: 相似文献
9.
E González A L Gimeno M A Gimeno 《Prostaglandins, leukotrienes, and essential fatty acids》1990,40(1):79-84
The effects of platelet activating factor (PAF) on glucose oxidation in uterine strips isolated from rats in the 4 th and 5 th day of pregnancy, were explored. PAF, at a concentration of 10(-10) and 10(-8) M, augmented significantly the generation of 14CO2 from labelled glucose in uteri from pregnant rats in the 4 th day of pregnancy. When the tissue was obtained from 5 days pregnant rats, the addition of PAF at 10(-8) increased significantly more than PAF at 10(-10) M the metabolism of glucose. On the other hand, PAF at 10(-8) M failed to alter the uterine basal production of 14CO2 from labelled glucose in animals at estrus. BN52021, a specific PAF antagonist employed at 10(-5) M, blocked completely the action of PAF in the pregnant rat uterus. PGE1, PGE2 and PGF2 alpha enhanced significantly the formation of 14CO2 from labelled glucose in uteri from 5 days pregnant rats. Indomethacin, a well known inhibitor of prostaglandin synthesis, did not alter the basal glucose metabolism in uteri from 5 days pregnant rats, but antagonized completely the stimulating action of PAF on 14CO2 production from labelled glucose an effect that was partially reverted by the addition of PGE1, PGE2 or PGF2 alpha (10(-7) M). Furthermore, nordihydroguaiaretic acid (NDHGA), a specific inhibitor of 5-lipoxygenase at 10(-5) M, as well as FPL-55712, an antagonist of leukotrienes (LTs), at the same concentration, blocked the action of PAF on the metabolism of glucose. The action of NDHGA was partially counteracted by the addition of LTC4 at 10(-7) M.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
10.
Nitroso-aldicarb was tested for its ability to induce sister-chromatid exchanges (SCE) and cell-cycle delay in human peripheral blood lymphocytes in vitro. This derivative of aldicarb induced a dose-dependent increase in SCE values per cell. In addition, a slight decrease in the successive mitotic progression of cells in culture was observed. 相似文献