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Biomedical subjects

C Guan

Publications and source records attributed to C Guan.

47 records · Page 3Linked to original sources
PubMed1995

Active site and oligosaccharide recognition residues of peptide-N4-(N-acetyl-beta-D-glucosaminyl)asparagine amidase F.

Crystallographic analysis and site-directed mutagenesis have been used to identify the catalytic and oligosaccharide recognition residues of peptide-N4-(N-acetyl-beta-D-glucosaminyl)asparagine amidase F (PNGase F), an amidohydrolase that removes intact asparagine-linked oligosaccharide chains from glycoproteins and glycopeptides. Mutagenesis has shown that three acidic residues, Asp-60, Glu-206, and Glu-118, that are located in a cleft at the interface between the two domains of the protein are essential for activity. The D60N mutant has no detectable activity, while E206Q and E118Q have less than 0.01 and 0.1% of the wild-type activity, respectively. Crystallographic analysis, at 2.0-A resolution, of the complex of the wild-type enzyme with the product, N,N'-diacetylchitobiose, shows that Asp-60 is in direct contact with the substrate at the cleavage site, while Glu-206 makes contact through a bridging water molecule. This indicates that Asp-60 is the primary catalytic residue, while Glu-206 probably is important for stabilization of reaction intermediates. Glu-118 forms a hydrogen bond with O6 of the second N-acetylglucosamine residue of the substrate and the low activity of the E118Q mutant results from its reduced ability to bind the oligosaccharide. This analysis also suggests that the mechanism of action of PNGase F differs from those of L-asparaginase and glycosylasparaginase, which involve a threonine residue as the nucleophile.

Amidohydrolases↗
PubMed1995

Crystal structure of endo-beta-N-acetylglucosaminidase H at 1.9 A resolution: active-site geometry and substrate recognition.

BACKGROUND: Endo-beta-N-acetylglucosaminidase H (Endo H), an endoglycosidase secreted by Streptomyces plicatus, hydrolyzes the glycosidic bond between the core N-acetyglucosamine residues of asparagine-linked high-mannose oligosaccharides. Endo H is a commonly used reagent in glycobiology research, including the characterization of oligosaccharides in glycoproteins. On-going crystallographic studies of Endo H and related endoglycosidases are aimed at identifying the molecular features that determine the different substrate specificities of these enzymes. RESULTS: The three-dimensional structure of Endo H has been determined to 1.9 A resolution. The overall fold of the enzyme is that of an irregular (alpha/beta)8-barrel comprising eight beta-strand/loop/alpha-helix units. Units 5 and 6 have very short loop sections at the top of the molecule and their alpha-helices are replaced by sections of extended geometry. The loop of unit 2 includes a small two-stranded antiparallel beta-sheet. A shallow curved cleft runs across the surface of the molecule from the area of units 5 and 6, over the core of the beta-barrel to the area of the beta-sheet of loop 2. This cleft contains the putative catalytic residues Asp130 and Glu132 above the core of the beta-barrel. These residues are surrounded by several aromatic residues. The loop 2 area of the cleft is formed by neutral polar residues, mostly asparagines. CONCLUSIONS: The structure of Endo H is very similar to that of Endo F1, a closely related endoglycosidase secreted by Flavobacterium meningosepticum. Detailed comparison of the structures of Endo H and Endo F1 supports the model previously proposed for substate binding and recognition, in which the area of loop 2 determines the substrate specificity and the alpha-helices of units 5 and 6 are missing to accommodate the protein moiety of the substrate.

Bacterial Proteins↗
PubMed1995

[Single high-dose and fractionated low dose cisplatin in the EP protocol for advanced NSCLC--a prospective randomized trial on 72 patients].

A prospective randomized trial on 72 patients (12 cases in stage III and 60 cases in stage IV) suffering from non-small cell lung cancer (NSCLC) was carried out from January 1993 to March 1994 to assess the immediate results of single high dose DDP and fractionated low doses DDP in the EP (Etoposide and Cisplatin, DDP) protocol. The response rate to the former regimen was 47.1% (16/34) as compared with 39.5% (15/38) of the latter. The difference between these two regimens were not statistically significant (P > 0.10). The former regimen had higher incedence of delayed vomiting (P < 0.01), but the latter had more severe bone marrow suppression (P < 0.05). Without significant difference in renal toxicity (P > 0.10). The authors suggest that, the EP protocol consisting of fractionated doses of DDP, may be more preferrable due to its mild gastro-intestinal toxic reaction and less expensive in the treatment of advanced NSCLC patients. Yet, it is necessary to guard against its renal toxicity and myelosuppression.

Antineoplastic Combined Chemotherapy Protocols↗
PubMed1995

[Analysis of 100 cases of precancerous laryngeal lesions].

Hundred cases of precancerous laryngeal lesions were analysed on the basis of clinical pathology. The results showed that there was close relationship between the precancerous laryngeal lesions and voice abuse and long-term heavy smoking. The pathology showed epithelial hyperplasis and mild to severe atypical hyperplasia (heterotype). Scanning electronmicroscopy revealed that the more severe the degree of the epithelial heterotype, the more pleomorphic the shape of cell morphology, margin. Twenty cases had malignant changes later, and the rate of malignant change was high in those with atypical hyperplasia. Some lesions which were taken for polyps clinically and pathologycally could also have malignant changes, so more attention should be paid to these lesions during clinical management.

Adolescent↗
PubMed1994

Crystallization and preliminary crystallographic analysis of two endo-beta-N-acetylglucosaminidases, endo H and endo F1.

Endo H and F1 are endoglycosidases that cleave the oligosaccharide moiety of asparagine-linked glycoproteins by hydrolysis of the glycosidic bond in the N,N'-diacetylchitobiose core. The two enzymes are specific for high-mannose oligosaccharides. Here, we report the crystallization and preliminary crystallographic analysis of Endo H and Endo F1. Crystals were grown by hanging drop vapor diffusion methods. Both proteins crystallize from crystallization buffers containing polyethyleneglycol 8000 and zinc acetate as precipitating agents in cacodylate buffer. The crystals of Endo H belong to the tetragonal space group P4(1)2(1)2 (or P4(3)2(1)2) with cell dimensions: a = 85.22 A, c = 89.41 A. The crystals of Endo F1 belong to the hexagonal space group P6(1) (or P6(5)) with cell dimensions: a = 70.61 A, c = 100.32 A. Crystals of both proteins diffract to at least 1.8 A resolution.

Carbohydrate Sequence↗
PubMed1994

[Characterization of receptor-mediated binding of 99mTc-NGA in vitro].

This paper reports results of an assay of the binding of 99mTc-galactosylneoglycoalbumin (NGA) and plasma membranes of the rat liver in vitro. The data showed that 99mTc-NGA could be bound to the membrane receptor with high affinity. Specificity and saturability, and that the amount of membrane-bound 99mTc-NGA varied linearly with the amount of membrane within each reaction tube. The biphasic curve of scatchard plot revealed that 99mTc-NGA was bound to two sites of the receptor with different affinity and concentration (K1 8.4089 x 10(9) L/mol, R1 1.7350 x 10(-12) mol/mg); K2 2.0827 x 10(8) L/mol, R2 6.9530 x 10(-12) mol/mg). Therefore, the authors consider that 99mTc-NGA is a promising hepatic receptor agent for imaging.

Albumins↗
PubMed1993

[The preparation of 99mTc-NGA instant lyophilized kit for hepatic imaging].

The preparation of one-step kit for the 99mTc labelled galactosyl-neo-glycoalbumin (NGA) using SnCl2 reduced method was reported in this paper. We developed a method of quality control and established the optimum scheme of technetium labelling on the basis of exploring the effects of varied labelled conditions on radiochemical purity. Our data indicates : (a) the kit is sterile, pyrogen-free and no toxic effects; (b) the operation is simple and fast; (c) its period of validity lasts no less than 3 months; (d) the radiochemical purity > 95%; (e) the stability of 99mTc-NGA in vitro > 4 hours; (f) the results of animal imaging and clinical probation are satisfactory.

Albumins↗
PubMed1993

[Pharmacokinetic studies on 99mTc-NGA in human].

Two normal subjects and three patients with hepatic disease were intravenously injected each with a single dose of 99mTc-NGA (galactosyl-neoglycoalbumin), a liver receptor imaging agent. The results showed that pharmacokinetics of 99mTc-NGA accorded with the two compartment open model. In the normal subjects, T1/2 alpha and T1/2 beta averaged 1.65 and 19.39 min respectively, indicating a rapid distribution process and a slow elimination process. There were significant differences in pharmacokinetics of 99mTc-NGA between the normal subjects and the patients with hepatic disease, and therefore the relevant parameters are of importance for the evaluation of hepatic function.

Adolescent↗
PubMed1991

[Galactosyl neoglycoalbumin:preparation and studies of its binding with hepatic special receptor].

Galactosyl-neoglycoalbumin (NGA) can bind with the asialoglycoproteins receptor specifically on the surface of mammal's liver. We prepared NGA in different carbohydrate density and made some animal and clinical tests with technetium-99m labeled NGA. The maximum absorption of 99mTc-NGA-21 was 82.6% in liver. We also got the satisfactory radiographs of liver. NGA was synthesized by the covalent coupling of a carbohydrate bifunctional reagent, 2-imino-2-methoxyethyl-1-thio-galactose (4), to human serum albumin. The cyanomethyl-1-thio-glycoside of beta-D-galactose (3) was prepared from the pseudothiourea derivative (2) which was synthesized by acetylation, bromination and substitution of galactose (3) could be converted to (4) by treatment with sodium methoxide.

Albumins↗