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Biomedical subjects

T Li

Publications and source records attributed to T Li.

At least 469 records · Page 26Linked to original sources
PubMed1994

Antibody catalyzed cationic cyclization.

Two major goals for the design of new catalysts are the facilitation of chemical transformations and control of product outcome. An antibody has been induced that efficiently catalyzes a cationic cyclization in which an acyclic olefinic sulfonate ester substrate is converted almost exclusively (98 percent) to a cyclic alcohol. The key to the catalysis of the reaction and the restriction of the product complexity is the use of antibody binding energy to rigidly enforce a concerted mechanism in accord with the design of the hapten. Thus, the ability to direct binding energy allows the experimenter to dictate a reaction mechanism which is an otherwise difficult task in chemistry. New catalysts for cationic cyclization may be of general use in the formation of carbon-carbon and carbon-heteroatom bonds leading to multi-ring molecules including steroids and heterocyclic compounds.

Animals↗
PubMed1994

Chemical self-replication of palindromic duplex DNA.

Molecular replication, a fundamental process of life, has in recent years been the subject of laboratory investigations using simple chemical systems. Whereas the work of Rebek's group has focused on molecular architectures not known in living systems, self-replicating and template-based self-assembling systems based on nucleotides are regarded as potential models for exploring the evolution of replicating systems on the early Earth. Previous replicating oligonucleotides have been of the single-stranded, self-complementary type: small oligonucleotide fragments are assembled on a pre-existing template and linked to form an exact copy of the template. This process cannot easily be reiterated, however, because of the strong binding of the newly formed strand to the original template. Furthermore, DNA replication in living systems operates by complementarity rather than self-complementarity--each newly assembled strand is complementary to, rather than identical to, its template--and the replication process starts and finishes with double helices. Here we report the self-replication of palindromic (symmetrical) duplex DNA-like oligonucleotides, 24 monomers long, in the absence of enzymes by means of a cycle that transfers information from template to copy and is potentially capable of extension to include non-symmetrical sequences, selection and mutation. Replication proceeds by a chemical process involving the formation of an intermediate triplex structure, and is sequence-selective in the sense that mismatches impair its efficiency. These results indicate that DNA-like double-helical molecules can replicate without assistance from proteins, a finding that may be relevant both to the appearance of replicating systems on the early Earth and to the development of new approaches to DNA amplification.

Base Sequence↗
PubMed1994

Direct selection for a catalytic mechanism from combinatorial antibody libraries.

Semisynthetic combinatorial antibody library methodology in the phage-display format was used to select for a cysteine residue in complementarity-determining regions. Libraries were panned with an alpha-phenethyl pyridyl disulfide that undergoes disulfide interchange. Out of 10 randomly picked clones, two contained an unpaired cysteine, one of which was studied. The antibody catalyzed the hydrolysis of the corresponding thioester where the electrophilic carbonyl occupies the three-dimensional space that was defined by the reactive sulfur atom during selection. The reaction operates by covalent catalysis. Although the steady-state rate enhancement relative to the activated thiol ester substrate is modest, hydrolysis of the acylated cysteine intermediate is remarkably efficient with a catalytic advantage of about four orders of magnitude. The results suggest that iterative mechanism-based selection procedures can recapitulate the enzymatic mechanisms refined through evolution.

Amino Acid Sequence↗
PubMed1994

Structural determinants of the stretching frequency of CO bound to myoglobin.

In order to assess the relative importance of polar versus steric interactions, infrared spectra and overall CO binding properties were measured at room temperature for 41 different recombinant myoglobins containing mutations at His64(E7), Val68(E11), Phe43(CD1), Arg45(CD3), Phe46(CD4), and Leu29(B10). The results were compared to the crystal structures of wild-type, Phe29, Val46, Ala68, Phe68, Gln64, Leu64, and Gly64 sperm whale CO-myoglobin and that of Thr68 pig CO-myoglobin. As observed in several previous studies, replacement of the distal histidine (His64) with aliphatic amino acids results in the appearance of a single IR band in the 1960-1970-cm-1 region and in large increases in CO affinity (KCO). More complex behavior is observed for Gly, Ala, Gln, Met, and Trp substitutions at position 64, but in each case there is a net increase in the intensity of this high-frequency component. Replacement of Val68 with Ala, Leu, Ile, and Phe produces little effect on the IR spectrum, whereas these mutations cause 20-fold changes in KCO, presumably due to steric effects. Replacement of Val68 with Thr decreases KCO 4-5-fold, whereas the position of the major IR band increases from 1945 to 1961 cm-1. Replacement of Val68 with Asn also causes a large decrease in KCO, but in this case, the peak position of the major IR band decreases from 1945 to 1916 cm-1. Nine replacements were made in the CD corner at positions 43, 45, and 46. All of the resultant mutants show increased stretching frequencies that can be correlated with movement of the imidazole side chain of His64 away from the bound ligand. All five substitutions at position 29 cause changes in the IR spectra. The Leu29-->Phe mutation had the largest effect, producing a single band centered at 1932 cm-1. Together these data demonstrate that there is little direct correlation between affinity, vCO, and Fe-C-O geometry. The major factor governing vCO appears to be the electrostatic potential surrounding the bound ligand and not steric hindrance. The presence of positive charges from proton donors, such as N epsilon of His64 and N delta of Asn68, cause a decrease in the bond order and stretching frequency of bound CO. In contrast, the negative portion of the Thr68 dipole points directly toward the bound ligand and increases the C-O bond order and stretching frequency. Movement of His64 away from the bound ligand or replacement of this residue with aliphatic amino acids prevents hydrogen-bonding interactions, causing vCO to increase.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

No association between alleles or genotypes at the dopamine transporter gene and schizophrenia.

The dopamine transporter gene (DAT1) is an important candidate gene for schizophrenia. A 40-bp VNTR (variable number of tandem repeats) polymorphism of DAT1 has been typed in 105 schizophrenic patients and 98 normal control subjects from Sichuan (China). Compared with allele frequencies for Caucasians reported in the literature, the Chinese population investigated showed a reduced frequency of the 9-copy allele and an increased frequency of the 10-copy allele. The observed frequency of genotypes was in agreement with the expected values according to Hardy-Weinberg equilibrium. No significant difference was found between patients and control subjects with regard to allele frequency, allele prevalence, and genotype counts. The results of the association study presented here are in agreement with the negative results of linkage analyses in schizophrenia pedigrees from Iceland (Kristbjarnarson et al., submitted) and from Utah (Byerley et al., 1993). Taken together, these studies suggest that variation in the dopamine transporter gene (DAT1) is unlikely to be a factor in the etiology of schizophrenia. The observed differences in allele frequencies between Chinese and Caucasian groups suggest that the human transporter gene might be useful for the construction of evolutionary trees in humans and primates as illustrated by Cavalli-Sforza's work (Mountain et al., 1992).

Adolescent↗
PubMed1994

Influence of viral quasispecies on effectiveness of interferon therapy in chronic hepatitis C patients.

The quasispecies nature of hepatitis C virus genome distribution is most evident in hypervariable regions of the putative envelope 2 domain. Eight patients with chronic hepatitis C treated with interferon-alpha were studied as to heterogeneity of the hypervariable regions to clarify the implications of quasispecies. More than 10 recombinant clones generated from polymerase chain reaction-amplified products of the hypervariable regions were sequenced. The sets of clones derived from long-term responders before interferon therapy showed a significantly lower (p < 0.05) degree of sequence complexity of the hypervariable region 1 quasispecies than those from short-term ones or non-responders. The values of nucleotide diversity (the average number of nucleotide differences per site between two randomly chosen sequences) in hypervariable region 1 before interferon therapy were also significantly lower (p < 0.05) for long-term responders (mean, 2.31 x 10(-2)). In some cases, nucleotide diversity decreased remarkably during interferon therapy, whereas the values remained unchanged in other cases. In one interesting case, a short-term response was first noted with the nucleotide diversity decreasing from 13.98 x 10(-2) to 0.21 x 10(-2); namely, the diversity of the quasispecies was significantly reduced, and then a long-term response was observed after an additional course of interferon therapy. Thus, the degree of quasispecies' complexity and diversity of hypervariable region 1 was closely correlated with the responsiveness to interferon therapy in chronic hepatitis C patients, and thus may have some influence on interferon efficacy.

Adult↗
PubMed1994

[Changes in lymphocyte membrane fluidity after burn and its significance].

In the present experiment, the change in lymphocyte membrane fluidity after burn was observed by measurement of fluorescent depolarization with DPH as a probe. 11%-12% TBSA of full-thickness skin burn was made in Balb/c mice. Six days later the animals were killed and their spleen were obtained. Then membrane fluidity and IL-2 production of splenic lymphocytes were measured, and lipid peroxide level of the spleen was assayed. The results showed that the fluidity decreased significantly with suppression of lymphocyte proliferation and IL-2 production, and that the lipid peroxide level increased. There was a negative correlation between the decrease in membrane fluidity and the suppression of lymphocyte functions. The same correlation also existed between the changes in membrane fluidity and the lipid peroxide level. The data indicated that the decreased membrane fluidity might be one of the causes for lymphocyte dysfunction and related to the enhancement of lipid peroxidation after burn.

Animals↗
PubMed1994

[Protective effect of somatostatin against stress injury of gastric mucosa may be related to the scavenge of free radicals].

In the present study, it was observed that somatostatin could significantly protect rat gastric mucosa from injury induced by cold-restraint stress and inhibit the stress induced increase of malonaldehyde (MDA) content. In the gastric mucosa of stress rats, the xanthine oxidase (XO) activity were increased and the glutathione peroxidase (GSH-Px) activity were decreased respectively, while the superoxide dismutase (SOD) activity showed no change. After pretreatment with somatostatin, the decrease of GSH-Px activity was significantly reversed, whereas XO and SOD activities were not significantly affected. The above results show that the protective effect of somatostatin against the stress-induced injury of gastric mucosa may be related to an enhancement of the ability of gastric mucosa to scavenge oxygen-derived free radicals.

Animals↗
PubMed1994

In vivo transfer of a reporter gene to the retina mediated by an adenoviral vector.

PURPOSE: The ability of replication-deficient adenovirus to mediate gene transfer to retinal cells was evaluated. METHODS: A replication-deficient adenoviral vector, AdCMV beta A.ntlacZ, which contains the bacterial beta-galactosidase (lacZ) reporter gene, was injected into the subretinal space of normal, rd, and rds strains of mice at various ages. The efficiency and duration of transgene expression were assessed by histochemical examination and transmission electron microscopy. RESULTS: AdCMV beta A.ntlacZ was effective in mediating gene transfer to the retinal pigment epithelial cells, rod and cone photoreceptor cells, and cells in the inner nuclear layer of the retina for periods of up to 1 month. Gene transfer to retinal pigment epithelial cells occurred at much lower viral titers than was required for gene transfer to photoreceptor cells. The extent to which photoreceptor cells could be transduced varied with the age of the animals and the conditions of the photoreceptor cells: greater numbers of photoreceptor cells were transduced in 5- to 7-day-old pups and in mice at the initial stages of photoreceptor degeneration than in normal adult mice. No evidence of gross pathogenic effects or viremia in recipient mice was observed. CONCLUSIONS: Replication-deficient adenovirus mediates transfer and expression of a foreign gene in retinal pigment epithelial and photoreceptor cells. Gene transfer to photoreceptor cells is enhanced in developing retinas or at the predegenerate stage of photoreceptors in genetically programmed retinal degeneration.

Adenoviruses, Human↗
PubMed1993

Epibatidine is a nicotinic analgesic.

Epibatidine, an alkaloid isolated from skin of the poison frog, Epipedobates tricolor, has been shown to be a very potent analgesic with a non-opioid mechanism of action. We found that epibatidine was about 120 times more potent and has longer duration than nicotine in analgesia, which could be antagonized by pretreatment with mecamylamine. Furthermore, epibatidine competed with high affinity (IC50 = 70 pM, Ki = 43 pM) for [3H]cytisine binding in rat brain preparations. These results indicated that the analgesic activity of epibatidine is attributed to its unique property as the most potent nicotinic acetylcholine receptor agonist.

Alkaloids↗
PubMed1993

Distal pocket polarity in ligand binding to myoglobin: deoxy and carbonmonoxy forms of a threonine68(E11) mutant investigated by X-ray crystallography and infrared spectroscopy.

The crystal structures of the deoxy and carbonmonoxy forms of a distal pocket myoglobin mutant in which valine68(E11) is replaced by threonine have been solved to 2.1- and 2.2-A resolution, respectively. This substitution has been shown previously to cause large decreases in the rate of oxygen binding and to lower the equilibrium association constants for O2 and CO. The synchrotron Laue method was used for the rapid acquisition of X-ray diffraction data to overcome problems caused by the very rapid rate of autooxidation of the mutant protein. The refined deoxy structure shows that the noncoordinated water molecule in the distal pocket is in a position to form strong hydrogen bonds with both the N epsilon-H of the distal histidine64 and O gamma of threonine68 with no other unexpected alterations in the protein structure. In the carbonmonoxy form, the bound ligand is well-defined and inclined away from the two hydrogen-bonding groups, refining to a position in which the Fe-C-O angle is 162 degrees. This value is very close to that previously observed in recombinant wild-type and position-64 (E7) mutants of sperm whale myoglobin (160-170 degrees). The similarity of the CO conformations contrasts with the 150-fold range in equilibrium binding constants (KCO) among the distal pocket myoglobin mutants and indicates that CO affinities cannot be predicted from the coordination geometry of the bound ligand. Furthermore, a comparison of the infrared stretching frequencies of CO in wild-type, valine64 and threonine68 single mutant, and valine64-threonine68 double mutant pig carbonmonoxymyoglobins shows a lack of correlation between KCO and vCO. These effects can be understood in terms of the stability of noncovalently bound water in deoxymyoglobin and electrostatic interactions between bound ligands and the distal pocket residues.

Animals↗