Characterization of rat cellular retinol-binding protein II expressed in Escherichia coli

Rat cellular retinol-binding protein II (CRBP II) is a small (15.6 kDa) intracellular protein that binds all-trans-retinol. In the adult rat, expression of the CRBP II gene is essentially limited to the small intestinal lining cells (enterocytes), suggesting that CRBP II may be uniquely adapted for intestinal metabolism of newly absorbed retinol. Functional and structural analysis of this protein has been hampered by difficulties in freeing rat intestinal CRBP II from its ligand without denaturation. To circumvent this problem, we have obtained efficient expression of rat apoCRBP II in Escherichia coli. The purified E. coli-derived apoprotein, when complexed with all-trans-retinol, demonstrates fluorescence excitation-emission spectra and absorption spectra indistinguishable from that of CRBP II-retinol isolated from rat intestine. Quantitative ligand binding studies were performed by monitoring either the fluorescence of bound retinol or the quenching of protein fluorescence. They revealed that E. coli-derived CRBP II binds retinol tightly (the apparent dissociation constant is estimated to be 10(-7)-10(-8) M), with a stoichiometry of 1:1. Fluorescence quenching studies used acrylamide as a probe for the exposure of the 4 tryptophan residues to solvent. The results indicate that although there is heterogeneity in the exposure of these 4 tryptophan residues to solvent, they are situated in a relatively nonpolar environment. These studies suggest that E. coli-derived apoCRBP II will serve as a useful model for studying retinol-protein interactions.

Comparison of the tissue-specific expression and developmental regulation of two closely linked rodent genes encoding cytosolic retinol-binding proteins

Cellular retinol-binding protein (CRBP) and cellular retinol-binding protein II (CRBP II) are two highly homologous cytoplasmic proteins that bind all-trans-retinol. We have recently demonstrated that the mouse genes encoding CRBP and CRBP II are closely linked on chromosome 9 and that both human genes are located on chromosome 3 (Demmer, L.A., Birkenmeier, E.H., Sweetser, D.A., Levin, M.S., Zollman, S., Sparkes, R.S., Mohandas, T., Lusis, A.J., and Gordon, J.I. (1987) J. Biol. Chem. 262, 2458-2467). We have now used RNA blot hybridization analysis to assess the degree to which these genes are coordinately expressed in fetal, suckling, weaning, and adult rat tissues. Both genes exhibit different developmental patterns of expression in liver, intestine, lung, kidney, testes, and placenta. In the intestine, CRBP mRNA was detected during the 16th day of gestation--prior to the development of a well-differentiated absorptive epithelium--and remained essentially unchanged throughout the peri- and postpartum periods. By contrast, the pattern of intestinal CRBP II mRNA accumulation closely parallels the times of first appearance, and subsequent proliferation, of the intestinal absorptive columnar epithelium, supporting the hypothesis that CRBP II is involved in the intestinal uptake or intracellular trafficking of this hydrophobic vitamin. In the fetal liver, both genes were expressed by gestational day 16. Whereas the concentration of hepatic CRBP mRNA increased markedly during the suckling and early weaning periods, CRBP II mRNA levels fell abruptly immediately after birth. These peripartum changes were not paralleled by remarkable alterations in the steady state levels of hepatic retinol. Marked changes in the expression of CRBP in the liver and of CRBP II in the intestine were also documented in pregnant and lactating female rats. These differences in CRBP/CRBP II gene expression strongly suggest that their proteins serve different physiological functions. The peripartum liver may provide a useful model for dissecting the relative roles played by these homologous proteins in retinoid metabolism as well as the factors which modulate activation and repression their genes.

Rat cellular retinol-binding protein II: use of a cloned cDNA to define its primary structure, tissue-specific expression, and developmental regulation

The primary structure of rat cellular retinol-binding protein (CRBP) II has been determined from a cloned cDNA. Alignment of this 134-amino acid, 15,580-Da polypeptide with rat CRBP revealed that 75 of 133 comparable residues are identical. Both proteins contain four tryptophan residues, which occupy identical relative positions in the two primary structures, providing a structural explanation for their similar fluorescence spectra when complexed to retinol. Two of the three cysteines in each single-chain protein are comparably positioned. Both polypeptides contain reactive thiol groups, but the rate of disruption of CRBP II-retinol complexes by p-chloromercuribenzoate is greater than that of CRBP-retinol. The small intestine contains the highest concentrations of CRBP II mRNA in adult rats. CRBP II mRNA is first detectable in intestinal RNA during the 19th day of gestation, a time that corresponds to the appearance of an absorptive columnar epithelium. Unlike in intestine, a dramatic fall in liver CRBP II mRNA concentration occurs immediately after birth. The CRBP II gene remains quiescent in the liver during subsequent postnatal development. These data suggest that ligand-protein interactions may be somewhat different for the two rat CRBPs. They also support the concept that CRBP II plays a role in the intestinal absorption or esterification of retinol and suggest that changes in hepatic metabolism of vitamin A occur during development.

Iohexol: a new, nonionic agent in adult peripheral arteriography

The safety and efficacy of iohexol, a new nonionic contrast agent, were compared with a diatrizoate (Renografin-76) in a double-blind, parallel study of peripheral arteriography by femoral puncture in 60 patients. Extra-large field serial peripheral arteriography was used and a posterior tibial nerve block was applied to all patients in the study. Similar changes in blood chemistry were observed following the injection of iohexol and diatrizoate but these changes did not require corrective measures. Significantly more patients complained of a sensation of severe heat after receiving diatrizoate (38%) than after the injection of iohexol (10%) (p = 0.001). Four patients in the diatrizoate group experienced one or more adverse reactions, including mild urticaria. Only mild nausea was reported by a single patient in the iohexol group. Overall, 100% of the studies were diagnostic but more of the radiographs taken after the injection of iohexol were rated excellent than after the injection of diatrizoate.

Line shapes of atomic hydrogen in hollow-cathode discharges

Emission lines of atomic hydrogen produced in the hollow cathode of an electric glow discharge are found to have articulated line shapes indicating three quite different excitation mechanisms for the emitting atoms. Particularly striking is the extensive development of the far wings, which have Gaussian line shapes with FWHM values attaining 10 cm(-1) or more. This corresponds to an atomic kinetic energy greater than 100 eV and suggests an origin for such energy in the cathode-fall region.

Biosynthesis of lipid-linked oligosaccharides. Isolation and structure of a second lipid-linked oligosaccharide in Chinese hamster ovary cells

Previous work has shown that vesicular stomatitis virus-infected Chinese hamster ovary cells contain a major high molecular weight lipid-linked oligosaccharide which is transferred en bloc to protein during the formation of the asparagine-linked complex-type oligosaccharides of the vesicular stomatitis virus G protein (Tabas, I., Schlesinger, S., and Kornfeld, S. (1978) J. Biol. Chem. 253, 716-722). We now report the characterization of a second, lower molecular weight lipid-linked oligosaccharide. The oligosaccharide portion of this molecule was isolated and its structure was determined by methylation analysis, digestion with exoglycosidases, acetolysis and Smith periodate degradation to be: (formula: see text). Several lines of evidence are presented which indicate that this lipid-linked oligosaccharide is primarily involved in the assembly of the major lipid-linked oligosaccharide rather than in the direct glycosylation of proteins.

Structural studies of the major high mannose oligosaccharide units from Chinese hamster ovary cell glycoproteins

The major high mannose-type glycopeptides present in Chinese hamster ovary cells have the compositions (Man)9(GlcNAc)2-Asn, (Man)8(GlcNAc)2-Asn, and (Man)6(GlcNAc)2-Asn. The structures of these glycopeptides were determined by the combination of methylation analysis, acetolysis, Smith periodate degradation, and alpha- and beta-mannosidase digestion. Their complete structures are: manalpha1 leads to 2Manaalpha1 leads to 6(Manalpha1 leads to 2Manalpha1 leads to 3)Manalpha1 leads to 6(Manalpha1 leads to 2Manalpha1 leads to 2Manalpha1 leads to 3)Manbeta1 leads to 4GlcNAcbeta1 leads to 4GlcNAc-Asn, Manalpha1 leads to 2Manalpha1 leads to 6(Manalpha1 leads to 3)Manalpha1 leads to 6(Manalpha1 leads to 2Manalpha1 leads to 2Manalpha1 leads to 3)Manbeta1 leads to 4GlcNAcbeta1 leads to 4GlcNAc-Asn, and Manalpha1 leads to 6(Manalpha1 leads to 3)Manalpha1 leads to 6(Manalpha1 leads to 2Manalpha1 leads to 3)Manbeta1 leads to 4GlcNAcbeta1 leads to 4GlcNAc-Asn. These structures are compared with the structures of the peptide-bound oligosaccharide intermediates that are processed to form complex-type oligosaccharides. From these results, it is proposed that the high mannose-type oligosaccharides are a product of "incomplete" processing of the protein-bound oligosaccharide along the same pathway which leads ultimately to the formation of a complex-type oligosaccharide.

The synthesis of complex-type oligosaccharides. I. Structure of the lipid-linked oligosaccharide precursor of the complex-type oligosaccharides of the vesicular stomatitis virus G protein

The synthesis of the complex-type oligosaccharide unit of the vesicular stomatitis virus G protein is initiated by the en bloc transfer of a high molecular weight oligosaccharide from a lipid carrier to the nascent polypeptide. Following transfer the oligosaccharide is "processed" by removal of glucose and mannose residues and the sugars that constitute the outer branches of the complex-type oligosaccharide are added. The structure of the oligosaccharide moiety of the lipid-linked precursor has been elucidated in order to further define the steps involved in processing. Since it was not feasible to obtain adequate amounts of material for standard structural studies, most of the structural studies were performed on radiolabeled material, with radioactivity incorporated differentially into glucose, mannose, and N-acetylglucosamine. Based on endo-beta-N-acetylglucosaminidase CII digestion, alpha-mannosidase digestion, acetolysis, Smith periodate degradation, methylation analysis, and periodate oxidation, we propose the following structure for the oligosaccharide moiety of the lipid-linked oligosaccharide.

Structure of the altered oligosaccharide present in glycoproteins from a clone of Chinese hamster ovary cells deficient in N-acetylglucosaminyltransferase activity

Clone 15B cells, derived from Chinese hamster ovary cells and deficient in a specific UDP-N-acetylglucosamine:glycoprotein N-acetylglucosaminyltransferase activity, synthesize glycoproteins with altered oligosaccharide units. Glycopeptides prepared from these glycoproteins contain large quantities of a glycopeptide with the composition (Man)5(GlcNAc)2-Asn whereas parent cells have only small amounts of this glycopeptide. The structure of the glycopeptide was determined by the combination of methylation analysis, acetolysis, Smith periodate degradation, and alpha- and beta-mannosidase digestion. Its complete structure is Manalpha 1 leads to 6[Manalpha1 leads to 3]-Manalpha1 leads to 6[Manalpha 1 leads to 3]-Manbeta1 leads to 4 GlcNAcbeta1 leads to 4 GlcNAc leads to Asn-peptide. The structures of two other glycopeptides found in smaller quantities in clone 15B but not detected in the parent cells were determined and are Manalpha 1 leads to 6 [Manalpha 1 leads to 3]-Manalpha1 leads to 6Manbeta 1 leads to 4 GlcNAcbeta 1 leads to 4GlcNAc-Asn-peptide and Manalpha 1 leads to 3 Manalpha 1 leads to 6[Manalpha 1 leads to 3] Manbeta 1 leads to 4GlcNAcbeta 1 leads to 4GlcNAc-Asn-peptide. It is proposed that the (Man)5(GlcNAc)2-Asn unit is the physiologic acceptor for the particular N-acetylglucosaminyltransferase which is deficient in clone 15B cells and that this reaction is necessary for complex oligosacchari-e biosynthesis.

Effects of wheat germ agglutinin on membrane transport

(1) Low concentrations of wheat germ agglutinin are cytotoxic toward several tissue culture lines, including Chinese hamster ovary cells, Swiss 3T3 cells, mouse L cells and baby hamster kidney cells. The LD50 ranged from 1 to 5 microgram wheat germ agglutinin per ml. Similar concentrations of the lectin inhibited the transport of the non-utilizable amino acids alpha-aminoisobutyric acid and cycloleucine and inhibited the uptake of thymidine. In contrast, 2-deoxy-D-glucose uptake was not altered and colchicine uptake was enhanced. (2) The inhibition of alpha-aminoisobutyric acid uptake occurred within minutes after lectin addition and was maximal by 1 h. Maximal inhibition ranged from 50 to 70% of control values. Studies of the kinetics of the uptake demonstrated that wheat germ agglutinin decreased the V of the uptake by 70% without affecting the apparent Km. Ovomucoid, a haptene inhibitor of wheat germ agglutinin-binding to cell surface receptors, prevented the wheat germ agglutinin-induced inhibition of alpha-aminoisobutyric acid transport. Three other lectins (Concanavalin A, Phaseolus vulgaris E-phytohemagglutinin and L-phytohemagglutinin) inhibited the uptake by 20% or less at doses up to 50 microgram/ml. (3) We propose that the cytotoxicity of wheat germ agglutinin probably results in part, if not totally, from membrane alterations which impair multiple membrane transport systems.

Isolation of wheat germ agglutinin-resistant clones of Chinese hamster ovary cells deficient in membrane sialic acid and galactose

Several clones of Chinese hamster ovary cells have been selected for their resistance to the toxic effects of wheat germ agglutinin. The clones do not bind wheat germ agglutinin as well as parent cells and are 5- to 250-fold more resistant to the toxic effects of the lectin. Of three clones studied in detail, all exhibit a decrease in wheat germ agglutinin binding affinity. Two have normal numbers of wheat germ agglutinin binding sites, while one (Clone 13) has a 65% decrease in binding sites. Crude membrane preparations of the clones have a decrease in sialic acid content relative to parent cells, and Clone 13 membranes are also deficient in galactose, while the mannose and hexosamine contents of all three clones are normal. The membrane sugar deficiencies affect both glycoproteins and glycolipids. Sialyl-lactosylceramide is the major glycolipid in parent cells, while Clones 1 and 1021 have lactosylceramide and Clone 13 has glucosylceramide as the predominant glycolipid. Labeling experiments with N-[G-3H]acetylmannosamine suggest that Clone 1021 cells have a block in the transfer of sialic acid from CMP-sialic acid to glycoprotein and glycolipid acceptors. Yet CMP-sialic acid:glycoprotein sialyl-transferase activity in cell lysates of Clone 1021 cells is 80% of normal. While CMP-sialic acid:lactosylceramide sialyl-transferase activity is only 25% of normal, it can be restored to normal or elevated levels by sodium butyrate induction without an associated increase in cellular sialyl-lactosylceramide content. Similarly, the galactose-deficient Clone 13 can synthesize UDP-galactose and has normal levels of UDP-galactose:glycoprotein galactosyltransferase and UDP-galactose:glucosylceramide galactosyltransferase when assayed in vitro. The glycosyltransferases of both these clones can utilize their own glycoproteins as sugar acceptors in in vitro assays. These data suggest that the variant cells fail to carry out specific glycosyltransferase reactions in vivo despite the fact that they possess the appropriate nucleotide sugars, glycoprotein and glycolipid acceptors, and glycosyltransferases.