Characterization of the lipid-carrier involved in the synthesis of enterobacterial common antigen (ECA) and identification of a novel phosphoglyceride in a mutant of Salmonella typhimurium defective in ECA synthesis

The polysaccharide chains of enterobacterial common antigen (ECA) consist of linear trisaccharide repeat units with the structure -->3)- alpha-d-Fuc4NAc-(1-->4)-beta-d-ManNAcA-(1--> 4)-alpha-d-GlcNAc-(1-->, where Fuc4NAc is 4-acetamido-4, 6-dideoxy-d-galactose, ManNAcA is N - acetyl-d- mannosaminuronic acid, and GlcNAc is N -acetyl-d-glucosamine. The major form of ECA (ECAPG) consists of polysaccharide chains that are believed to be covalently linked to diacylglycerol through phosphodiester linkage; the phospholipid moiety functions to anchor molecules in the outer membrane. The ECA trisaccharide repeat unit is assembled as a polyisoprenyl-linked intermediate which has been tentatively identified as Fuc4NAc-ManNAcA-GlcNAc- pyrophosphorylundecaprenol (lipid III). Subsequent chain-elongation presumably occurs by a block-polymerization mechanism. However, the identity of the polyisoprenoid carrier-lipid has not been established. Accordingly, the current studies were conducted in an effort to structurally characterize the polyisoprenyl lipid-carrier involved in ECA synthesis. Isolation and characterization of the lipid carrier was facilitated by the accumulation of a ManNAcA-GlcNAc- pyrophosphorylpolyisoprenyl lipid (lipid II) in mutants of Salmonella typhimurium defective in the synthesis of TDP-Fuc4NAc, the donor of Fuc4NAc residues for ECA synthesis. Analyses of lipid II preparations by fast atom bombardment tandem mass spectroscopy (FAB-MS/MS) resulted in the identification of the lipid-carrier as the 55-carbon polyisoprenyl alcohol, undecaprenol. These analyses also resulted in the identification of a novel glycolipid which copurified with lipid II. FAB-MS/MS analyses of this glycolipid revealed its structure to be 1,2-diacyl- sn -glycero-3-pryophosphoryl-GlcNAc-ManNAcA (DGP- disaccharide). An examination of purified ECAPGby phosphorus-31 nuclear magnetic resonance spectroscopy confirmed that the polysaccharide chains are linked to diacylglycerol through phosphodiester linkage. Thus, DGP-disaccharide does not appear to be an intermediate in ECAPGsynthesis. Nevertheless, although the available evidence clearly indicate that lipid II is a precursor of DGP-disaccharide, the function of this novel glycolipid is not yet known, and it may be an intermediate in the biosynthesis of a molecule other than ECAPG.      

角质形成细胞生长因子(KGF)在喉粘膜良性、癌前及恶性病变中的mRNA水平分析

利用原位杂交的方法检测KGFmRNA在正常喉粘膜上皮（N）、慢性非特异性炎症（IF）、不典型增生（DYS）及鳞癌（SCC）中的转录水平,探讨KGF在喉粘膜良性及恶性病变中的分布和可能的作用。结果表明,KGFmRNA不仅在间质中的成纤维细胞中表达,少量的炎细胞及血管内皮细胞中亦表达,而且从N、IF、DYS到SCC、KGFmRNA转录水平逐渐增强;上皮细胞及肿瘤性上皮细胞不表达KGFmRNA,KGFmRNA在分化差的SCC周围间质中表达较分化好的SCC周围间质增多。结论：KGF在上皮与间充质细胞的交互作用中发挥着重要的作用,对维持喉粘膜正常结构、代谢及喉癌的发生发展具有重要意义。     

Stereospecific ring opening of conduritol-B-epoxide by an active site asparatate residue of sucrase-isomaltase

Conduritol-B-epoxide inactivates sucrase-isomaltase (sucrose alpha-glucohydrolase, EC 3.2.1.48-dextrin 6-alpha-glucohydrolase, EC 3.2.1.10) irreversibly with incorporation of 1 mol inhibitor/mol subunit, the affinity label being bound in both subunits to a beta-carboxyl group of an aspartic acid (Quaroni, A. and Semnza; G. (1976) J. Biol. Chem. 251, 3250-3253). Conduritol-B-epoxide is a racemic mixture of 1-L-1,2-anhydro-myo-inositol and 1-D-1,2-anhydro-myo-inositol, but only the latter one is the reactive component, since 1-L-1,2-anhydro-myo-inositol alone did not inactivate the enzyme. After inactivation by 1-D-1,2-anhydro-myo-inositol the label was released by hydroxylamine and identified as scyllo-inositol. One can decide now which C atom of the epoxide ring has been attacked by the enzyme's aspartate residue. This explains why only the D-enantiomer is the reactive species and provides further information about the role of the carboxylate residue during enzymic hydrolysis.     

Carboxyl groups at the two active centers of sucrase-isomaltase from rabbit small intestine.

1. Seveal selective reagents were employed to identify the amino acid residues essential for the catalytic activity of sucrase-isomaltase. 2. Modification of histidine, lysine and carboxyl residues resulted in a partial inactivation of the enzyme. Substrates or competitive inhibitors provided protection against inactivation only in the reaction of carboxyl groups with carbodiimide (+lycine ethyl ester) or with diazoacetic ethyl ester. This indicated the occurrence of carboxyl groups at the two active centers of the enzyme complex. 3. Protection against inactivation of the enzyme by carbodiimide was provided also by the presence of alkali and alkaline earth metal ions, which are non-essential activators of sucrase-isomaltase. The presence of Na+ and Ba2+ protected approximately one carboxyl group per active center from reacting with carbodiimide plus glycine ethyl ester. 4. The carbodiimide-reactive groups were not identical with the two carboxylate groups recently found to react with conduritol-B-epoxide, an active-site-directed inhibitor of sucrase-isomaltase (Quaroni, A. and Semenza, G., 1976, J. Biol. Chem 251,3250--3253). A possible role for the carbodiimide-reactive carboxyl groups at the active centers of sucrase-isomaltase is discussed.     

Distribution of sulfhydryl groups in intestinal brush border membranes. Localization of side-chains essential for glucose transport and phlorizin binding

1. Brush border membrane vesicles from rabbit small intestine were found to contain 46 nmol SH groups/mg protein, 52% of which could react with 4,4'-dithiodipyridine, a membrane permeating probe. Only 18% of the total SH-groups reacted with the impermeant probe 5,5'-dithiobis(2-nitrobenzoic acid), indicating that only this fraction is externally located. 2. Brush border membrane vesicles could be disrupted by a gentle treatment with deoxycholate, releasing most of their electron-dense core material. In deoxycholate-treated vesicles most of the SH groups that reacted with 4,4'-dithiodipyridine react with 5,5'-dibiobis(2-nitrobenzoic acid), suggesting that both membrane surfaces became exposed to the extravesicular medium. 3. In intact vesicles (1.2 mg protein/ml), the binding of phlorizin (a competitive inhibitor of the monosaccharide transport system) was 50% inhibited by 67 microM of the penetrating organomercurial p-chloromercuribenzoate, but was about ten times less sensitive to the poorly permeating p-chloromercuriphenylsulfonate. In contrast, binding of phlorizin to leaky (deoxycholate-treated) membranes was equally susceptible to either reagent. 4. Mercurial inhibition of phlorizin binding could be reversed by dithioerythritol in both sealed and leaky membranes, whereas the less permeant thiol L-glutathione (reduced form) could only revert the inhibition in leaky membranes.