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排序方式: 共有498条查询结果,搜索用时 31 毫秒
121.
K. D. Altria N. W. Smith C. H. Turnbull 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1998,717(1-2)
Capillary electrochromatography, CEC, is a hybrid of CE and HPLC and is rapidly gaining interest as a potential complementary technique. This paper provides an overview of literature concerning the separation of acidic compounds by CEC which fall into three distinct groups. These groups are those performed using capillaries packed with novel or unique stationary phases designed for CEC, and a smaller group where standard HPLC stationary phases packings such as ODS has been used. The third group involves the use of surface coated capillaries. This paper reviews the separation of acidic compounds by CEC and also includes a number of novel applications to illustrate the separation approaches and the analytical performance possible. 相似文献
122.
Rhizobium leguminosarum biovar trifolii TA1 grows on 4-hydroxymandelate and enzymes involved in its catabolism are inducible. Strain TA1 does not grown on mandelate or cis, cis-muconate, but spontaneous mutants capable of growth on these substrates were isolated. Enzymes involved in mandelate degradation were also inducible. The presence of intermediates of the mandelate and hydroxymandelate pathways resulted in a significant decrease in some of the enzymes involved in their degradation. Succinate and acetate, end products of the pathways, and glucose caused reductions in the levels of enzymes in the mandelate and hydroxymandelate pathways. 相似文献
123.
Decarboxylation of phenylalanine by aromatic L-amino acid decarboxylase (AADC) is the rate-limiting step in the synthesis of 2-phenylethylamine (PE), a putative modulator of dopamine transmission. Because neuroleptics increase the rate of accumulation of striatal PE, these studies were performed to determine whether this effect may be mediated by a change in AADC activity. Administration of the D1 antagonist SCH 23390 at doses of 0.01-1 mg/kg significantly increased rat striatal AADC activity in an in vitro assay (by 16-33%). Pimozide, a D2-receptor antagonist, when given at doses of 0.01-3 mg/kg, also increased AADC activity in the rat striatum (by 25-41%). In addition, pimozide at doses of 0.3 and 1 mg/kg increased AADC activity in the nucleus accumbens (by 33% and 45%) and at doses of 0.1, 0.3, and 1 mg/kg increased AADC activity in the olfactory tubercles (by 23%, 30%, and 28%, respectively). Analysis of the enzyme kinetics indicated that the Vmax increased with little change in the Km with L-3,4-dihydroxyphenylalanine as substrate. The AADC activity in the striatum showed a time-dependent response after the administration of SCH 23390 and pimozide: the activity was increased within 30 min and the increases lasted 2-4 h. Inhibition of protein synthesis by cycloheximide (10 mg/kg, 0.5 h) had no effect on the striatal AADC activity or on the increases in striatal AADC activity produced by pimozide or SCH 23390. The results indicate that the increases in AADC activity induced by dopamine-receptor blockers are not due to de novo synthesis of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
124.
Yajie Fu Zeping Lu Ke Fang Xinyi He He Huang Yi Hu 《Bioorganic & medicinal chemistry letters》2019,29(10):1236-1240
Lipase RMIM was firstly used as a promiscuous biocatalyst to catalyze the Knoevenagel-Michael cascade reactions of 4-hydroxycoumarin with aromatic, heterocyclic or aliphatic aldehydes to synthesize dicoumarol derivatives in water. Results showed that the adopted methodology could offer many advantages, such as mild reaction conditions, pure aqueous reaction system, wide substrate applicability, recyclable catalyst, excellent yields (81–98%), operational simplicity, and environmentally friendly reactions. 相似文献
125.
Praphathip Eamsobhana Adisak Yoolek Wittaya Kongkaew Kriangkrai Lerdthusnee Nittaya Khlaimanee Anchana Parsartvit Nat Malainual Hoi-Sen Yong 《Experimental & applied acarology》2009,47(3):257-262
Scrub typhus, a rickettsial disease transmitted by several species of Leptotrombidium chiggers (larvae), is endemic in many areas of Asia. The disease is best prevented by the use of personal protective measures,
including repellents. In this study commercially produced aromatic, essential oils of 13 plant species and ethanol (control)
were tested in the laboratory for repellency against host-seeking chiggers of Leptotrombidium imphalum Vercammen-Grandjean and Langston (Acari: Trombiculidae). A rapid, simple and economic in vitro test method was used by exposing
the chigger for up to 5 min. Repellency was based on relative percentages of chiggers attracted to test and control substances.
Four of the 13 essential oils showed promise as effective repellent against L. imphalum chiggers. Syzygium aromaticum (clove) oil exhibited 100% repellency at 5% concentration (dilution with absolute ethanol), whereas Melaleuca alternifolia (tea tree) oil exhibited 100% repellency at 40% concentration. Undiluted oils of Zingiber cassamunar (plai) and Eucalyptus globules (blue gum) exhibited 100% repellency. Of the remaining nine essential oils, only 100% Pelargonium graveolens (geranium) exhibited >50% repellency (viz. 57%). Styrax torkinensis (benzoin) oil did not exhibit any repellency. These findings show that several aromatic, essential oils of plants may be
useful as chigger repellent for the prevention of scrub typhus. Syzygium aromaticum oil may be safer and more economical to prevent chigger attacks than commercially available synthetic chemicals, such as
DEET that may have harmful side effects. 相似文献
126.
The healthy sweetener isomaltulose is industrially produced from the conversion of sucrose by the sucrose isomerase SmuA from Protaminobacter rubrum. Crystal structures of SmuA in native and deoxynojirimycin complexed forms completed with modeling studies unravel the characteristics of the isomaltulose synthases catalytic pocket and their substrate binding mode. Comparison with the trehalulose synthase MutB highlights the role of Arg298 and Arg306 active site residues and surface charges in controlling product specificity of sucrose isomerases (isomaltulose versus trehalulose). The results provide a rationale for the specific design of optimized enzymes. 相似文献
127.
Dorjee G. Tamang Ralf Rabus Ravi D. Barabote Milton H. SaierJr. 《The Journal of membrane biology》2009,229(2):53-90
The denitrifying bacterium “Aromatoleum aromaticum” strain EbN1 is specialized for the aerobic utilization of aromatic compounds including crude oil constituents. We here report
whole-genome analyses for potential transport proteins in A. aromaticum strain EbN1. This organism encodes very few transporters for simple sugars and most other common carbon sources. However,
up to 28% of its putative transporters may act on fairly hydrophobic aromatic and aliphatic compounds. We categorize the putative
transporters encoded within the genome, assign them to recognized families, and propose their preferred substrates. The bioinformatic
data are correlated with available metabolic information to obtain an integrated view of the metabolic network of A. aromaticum strain EbN1. The results thus indicate that this organism possesses a disproportionately large percentage of transporters
for the uptake and efflux of hydrophobic and amphipathic aromatic and aliphatic compounds compared with previously analyzed
organisms. We predict that these findings will have important implications for our ecophysiological understanding of bioremediation. 相似文献
128.
Sierin Lim James R. Springstead Marcella Yu Wojciech Bartkowski Imke Schröder Harold G. Monbouquette 《Extremophiles : life under extreme conditions》2009,13(1):191-198
In the aromatic amino acid biosynthesis pathway, chorismate presents a branch point intermediate that is converted to tryptophan,
phenylalanine (Phe), and tyrosine (Tyr). In bacteria, three enzymes catalyze the conversion of chorismate to hydroxyphenylpyruvate
or pyruvate. The enzymes, chorismate mutase (CM), prephenate dehydratase (PDT), and prephenate dehydrogenase (PDHG) are either
present as distinct proteins or fusions combining two activities. Gene locus AF0227 of Archaeoglobus fulgidus is predicted to encode a fusion protein, AroQ, containing all three enzymatic domains. This work describes the first characterization
of a trifunctional AroQ. The A. fulgidus
aroQ gene was cloned and overexpressed in Escherichia coli. The recombinant protein purified as a homohexamer with specific activities of 10, 0.51, and 50 U/mg for CM, PDT, and PDHG,
respectively. Tyr at 0.5 mM concentration inhibited PDHG activity by 50%, while at 1 mM PDT was activated by 70%. Phe at 5 μM
inhibited PDT activity by 66% without affecting the activity of PDHG. A fusion of CM, PDT, and PDHG domains is evident in
the genome of only one other organism sequenced to date, that of the hyperthermophilic archaeon, Nanoarchaeum equitans. Such fusions of contiguous activities in a biosynthetic pathway may constitute a primitive strategy for the efficient processing
of labile metabolites. 相似文献
129.
Proton 2D NMR was used to confirm in solution a highly conserved portion of the molecular structure upon substrate loss for the heme oxygenase from the pathogenic bacterium Corynebacterium diphtheriae, HmuO. The chemical shifts for the conserved portion of the structure are assessed as references for the dipolar shifts needed to determine the orientation of the paramagnetic susceptibility tensor, χ, in paramagnetic substrate complexes of HmuO. It is shown that the chemical shifts for the structurally conserved portion of substrate-free HmuO serve as excellent references for residues with only small to moderate sized dipolar shifts in the cyanide-inhibited substrate complex of HmuO, yielding an orientation of χ that is essentially the same as conventionally obtained from large dipolar shifts based on empirical estimates of the diamagnetic reference. The implications of these diamagnetic chemical shifts for characterizing the hydrogen bonding in the physiologically relevant, resting-state, high-spin aquo complex are discussed. The pattern of labile proton exchange in the distal H-bond network of substrate-free HmuO allowed comparison of changes in dynamic stability of tertiary contacts in the substrate-free and substrate-bound HmuO and with the same complexes of human heme oxygenase. 相似文献
130.
Amit K. Singh Ramasamy P. Kumar Nisha Pandey Nagendra Singh Mau Sinha Asha Bhushan Punit Kaur Sujata Sharma Tej P. Singh 《The Journal of biological chemistry》2010,285(2):1569-1576
Isoniazid (INH) is an anti-tuberculosis prodrug that is activated by mammalian lactoperoxidase and Mycobacterium tuberculosis catalase peroxidase (MtCP). We report here binding studies, an enzyme assay involving INH, and the crystal structure of the complex of bovine lactoperoxidase (LPO) with INH to illuminate binding properties and INH activation as well as the mode of diffusion and interactions together with a detailed structural and functional comparison with MtCP. The structure determination shows that isoniazid binds to LPO at the substrate binding site on the distal heme side. The substrate binding site is connected to the protein surface through a long hydrophobic channel. The acyl hydrazide moiety of isoniazid interacts with Phe422 O, Gln423 Oϵ1, and Phe254 O. In this arrangement, pyridinyl nitrogen forms a hydrogen bond with a water molecule, W-1, which in turn forms three hydrogen bonds with Fe3+, His109 Nϵ2, and Gln105 Nϵ2. The remaining two sides of isoniazid form hydrophobic interactions with the atoms of heme pyrrole ring A, Cβ and Cγ atoms of Glu258, and Cγ and Cδ atoms of Arg255. The binding studies indicate that INH binds to LPO with a value of 0.9 × 10−6 m for the dissociation constant. The nitro blue tetrazolium reduction assay shows that INH is activated by the reaction of LPO-H2O2 with INH. This suggests that LPO can be used for INH activation. It also indicates that the conversion of INH into isonicotinoyl radical by LPO may be the cause of INH toxicity. 相似文献