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1.
2.
(2')3',5'-Bisphosphate nucleotidase 总被引:2,自引:0,他引:2
(2')3',5'-Bisphosphate nucleotidase has been prepared in electrophoretically homogeneous form from guinea pig liver. The enzyme catalyzes the hydrolysis of the 2'- or 3'-phosphate from the appropriate nucleoside 2',5'- and 3',5'-bisphosphates and is active with 3'-phosphoadenosine 5'-phosphosulfate and with coenzyme A but not with ATP. The 40,000-dalton protein is a monomer that requires Mg2+ for activity. 相似文献
3.
Insulin has been reacted with five chromium(III) complexes that are capable of relatively facile substitution of aquo ligands. The new Cr(III) insulin derivatives have been characterized by means of electronic and infrared spectra, and evidence for major changes in the protein structure, including the state of aggregation, has been presented. Supporting evidence for the arguments favoring the beneficiary role of chromium(III) in glucose metabolism has been obtained using in vivo studies, and it has been shown that insulin derived with Cr(salen) (H2O)2+ is capable of reversing the blood sugar, serum cholesterol, and phospholipids levels to those of normal rats. The results emphasize the dependence of biopotency on the structure of Cr(III) complexes used for derivation of insulin and discount the postulates that Cr(III) serves to assemble insulin and receptor units through metal-sulphur bonding. The influence of Cr(III) on the structural stability and state of aggregation of insulin and their possible role in glucose metabolism is discussed. 相似文献
4.
We investigated the chymotrypsin-promoted hydrolysis of a series of chromium(III)-insulin complexes containing chelating or macrocyclic ligands. It has been shown that Cr(III) stabilizes insulin against the chymotrypsin-promoted hydrolysis of the protein. The molecular weights of Cr(III) containing peptides have been estimated to be of the order of 2,700-3,700 daltons. The Cr(III) containing peptides are richer in glutamic acid than the intact insulin and are devoid of any isoleucine. High molecular weights and the observed glutamic acid/histidine ratios in Cr(III) containing peptides have been rationalized in terms of Cr(III) being associated with insulin aggregates rather than the monomer of the protein. The chymotrypsin hydrolysis of Cr(III) insulin derivatives is influenced markedly by the nature, charge, and type of Cr(III) complex with which the protein has been reacted. Arguments have been advanced that chymotrypsin-promoted hydrolysis of insulin Cr(III) derivatives does not lead to cleavages at or near every tyrosine residue. 相似文献
5.
Characteristics of tripeptide transport in human jejunal brush-border membrane vesicles 总被引:3,自引:0,他引:3
These studies are aimed at characterizing the transport of the tripeptide, glycylglycyl-L-proline (GlyGlyPro) across human jejunal brush-border membrane vesicles. GlyGlyPro (0.65 mM) was hydrolyzed by brush-border membrane vesicles with the extent of hydrolysis per mg protein being 23% at 0.5 min, 57% at 1 min and complete hydrolysis at 60 min. Treatment of the membrane vesicles with gel-complexed papain (to remove membrane peptidases) resulted in minimal hydrolysis of GlyGlyPro up to 10 min of incubation. Measurement of GlyGlyPro influx with papain-treated vesicles in the presence of increasing medium osmolarity showed that uptake occurred into an osmotically reactive intravesicular space. Transport of GlyGlyPro with normal and papain-treated membrane vesicles was similar in the presence of an inward Na+ or K+ gradient. No overshoot phenomenon was observed in the presence of an inward proton gradient (extravesicular pH 5.5; intravesicular pH 7.5). An interior negative membrane potential induced by a K+ diffusion potential in the presence of valinomycin stimulated the uptake of the peptide. The effect of increasing concentrations on initial rates of GlyGlyPro uptake revealed the presence of a saturable component as well as a diffusional component. Preloading the membrane vesicles with 20 mM glycylsarcosylsarcosine stimulated uptake by 4-fold. Uptake of GlyGlyPro was inhibited greater than 50% by dipeptides and tripeptides and less than 15% by free amino acids. These results indicate that GlyGlyPro uptake in jejunal brush-border membrane vesicles is not energized by a Na+ or proton gradient and that transport occurs by carrier-mediated and diffusional processes. 相似文献
6.
Partial purification and some properties of a latent CO2 reductase from green potato tuber chloroplasts 总被引:1,自引:0,他引:1
We have partially purified the CO2 reductase, present in green potato tuber chloroplasts, as a latent form. Illumination of the chloroplasts in the absence of substrate, bicarbonate, activated the enzyme, which could then be obtained in soluble forms. Purification of the enzyme was achieved by (NH4)2SO4 fractionation (0-30%) and adsorption and elution from a DEAE-Sephadex A-50 column. The final preparation showed 15-fold purification and 50% recovery of the activity. The pH optimum for CO2 reductase was 8.0. Hepes and Tricine buffers showed maximum activity whereas Tris/phosphate or borate failed to show any activity. The enzyme reaction was sensitive to the presence of metal ions like Fe3+, Hg2+, Cu2+, Mo6+ and Zn2+, however, a threefold activation was observed with Fe2+. The metal requirement for CO2 reductase was evident from the observed inhibition by metal chelators like o-phenanthroline, alpha, alpha'-dipyridyl, bathocuproine, 8-hydroxyquinoline etc. Out of these o-phenanthroline was the strongest inhibitor and its concentration for 50% inhibition was 40 microM. The presence of Fe2+ ions in the reaction mixture protected the enzyme from heat denaturation upto 50 degrees C. Maximum enzyme activity was observed at 15 degrees C. The enzyme activity showed a 30-s lag period and the maximum was reached in 90 s. Supplementation of sodium dithionite in the reaction activated enzyme activity threefold, suggesting involvement of dithiol groups in the catalytic activity. There was strong inhibition by -SH inhibitors like 5,5'-dithiobis(2-nitrobenzoic acid) and N-ethylmaleimide and -SH reagents like dithiothreitol, 2-mercaptoethanol and cysteine. Various nucleotide coenzyme tried inhibited the enzyme strongly. 相似文献
7.
8.
Clonidine, when administered for prolonged period showed no tolerance to its analgesic activity. Prior exposure to clonidine attenuated the tolerance development to morphine-induced analgesia and the supersensitivity to acetylcholine (ACh) in ileum during chronic morphine treatment. Further, acute administration of lower doses of clonidine (upto 1 mg) produced supersensitivity in ileum to ACh while the higher dose (10 mg) induced subsensitivity. In vas deferens, clonidine in all the concentrations tested induced dose and time dependent supersensitivity to norepinephrine (NE) similar to that produced by morphine. Chronic clonidine treatment failed to alter the ACh responses in ileum while it produced supersensitivity to NE in vas deferens. The results suggest that clonidine and morphine possess comparable properties and the antagonism of chronic morphine tolerance by clonidine may be the therapeutic basis for its clinical application in the treatment of opiate addicts. 相似文献
9.
4,5',8-Trimethylpsoralen (TMP) cross-links a 5' TpA or a 5' ApT site by photoreacting with one thymine moiety in each DNA strand. We are interested in whether psoralen interstrand cross-links all share one structure or whether there are significant differences. In this paper, we employed a rapid method for probing the structure of the cross-link by making a series of TMP cross-linked duplexes containing specific base-pair mismatches. The relative stability provided by a base pair can be correlated with neighboring base pairs by comparing the extents of gel retardation when base-pair mismatches happen in each position. From our studies, we infer that with respect to the furan-side strand, the 5'T.A base pair of the two T.A base pairs in the TpA site is not hydrogen bonded. Immediately on each side of the cross-linked TpA site is a highly stabilized base pair. Next, a region of decreased stability occurs in each arm of a cross-linked duplex and these base pairs of least stability are located farther away from the cross-linked thymines as the lengths of the arms of the cross-linked helix increase. Finally, even in 7 M urea at 49 degrees C the cross-linked helix is hydrogen bonded at both ends of a duplex of 22 base pairs. We propose that the structures of interstrand cross-links in DNA vary appreciably with the DNA sequence, the length of the DNA duplex, and the structures of the DNA cross-linking agents. 相似文献
10.
Crystal structure of cod liver class I alcohol dehydrogenase: substrate pocket and structurally variable segments.
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S. Ramaswamy M. el Ahmad O. Danielsson H. J?rnvall H. Eklund 《Protein science : a publication of the Protein Society》1996,5(4):663-671
The structural framework of cod liver alcohol dehydrogenase is similar to that of horse and human alcohol dehydrogenases. In contrast, the substrate pocket differs significantly, and main differences are located in three loops. Nevertheless, the substrate pocket is hydrophobic like that of the mammalian class I enzymes and has a similar topography in spite of many main-chain and side-chain differences. The structural framework of alcohol dehydrogenase is also present in a number of related enzymes like glucose dehydrogenase and quinone oxidoreductase. These enzymes have completely different substrate specificity, but also for these enzymes, the corresponding loops of the substrate pocket have significantly different structures. The domains of the two subunits in the crystals of the cod enzyme further differ by a rotation of the catalytic domains by about 6 degrees. In one subunit, they close around the coenzyme similarly as in coenzyme complexes of the horse enzyme, but form a more open cleft in the other subunit, similar to the situation in coenzyme-free structures of the horse enzyme. The proton relay system differs from the mammalian class I alcohol dehydrogenases. His 51, which has been implicated in mammalian enzymes to be important for proton transfer from the buried active site to the surface is not present in the cod enzyme. A tyrosine in the corresponding position is turned into the substrate pocket and a water molecule occupies the same position in space as the His side chain, forming a shorter proton relay system. 相似文献