Search PubMed⌕ Search

Biomedical subjects

L Zheng

Publications and source records attributed to L Zheng.

At least 307 records · Page 17Linked to original sources
PubMed1994

Optimization by trees on simple adaptive landscapes.

We evaluate the optimizing ability (rate of adaptation) of trees on simple adaptive landscapes. At points away from a peak, there is a strong negative relationship between rate of adaptation and tree precision P, a relationship that is independent of the size of the tree. P measures the variability among trial solutions generated by the tree: high precision trees have low variability, low precision trees have high variability. Near a peak, the situation reverses, with high precision trees showing higher rates of adaptation than low precision trees; however, for all trees, the absolute rate of adaptation is uniformly low. On multiple-peak landscapes, the probability of crossing an adaptive valley from a lower peak to a higher peak is also negatively correlated with tree precision. These results suggest that under a wide range of conditions, trees with low precision are, on average, the best optimizers.

Adaptation, Biological↗
PubMed1994

Comparison of the transport characteristics of D- and L-methionine in a human intestinal epithelial model (Caco-2) and in a perfused rat intestinal model.

Absorption mechanisms of L- and D-methionine (MET) in an in vitro cultured human intestinal epithelial cell model (Caco-2) and an in situ perfused rat intestinal model were investigated to determine if the kinetic characteristics of absorption are comparable in these two popular absorption models. The results indicate that the transport of L- and D-MET were concentration-dependent in both model systems, and displayed comparable Km values. The Km value for L-MET is 1.34 mM in the Caco-2 model and 3.6 mM in the perfused rat intestinal model, while the Km value for D-MET is 1.79 mM in the Caco-2 model and 2.87 mM in the perfused rat intestinal model. Although the Jmax values were not comparable because of significant methodology differences, the Jmax values for L-MET were always higher than that for D-MET. In addition, transport of L- and D-MET across the Caco-2 cell monolayers were also inhibited by 10 mM Phe and Lys while MeAIB, Pro and Glu were generally ineffective. Similar results were also observed with these inhibitors in the perfused rat intestinal model with the exception that a combination of Pro and Glu stimulated the uptake of L-MET. In conclusion, the transport characteristics of L- and D-MET are comparable in both model systems.

Animals↗
PubMed1994

Reduction of tomato polygalacturonase beta subunit expression affects pectin solubilization and degradation during fruit ripening.

The developmental changes that accompany tomato fruit ripening include increased solubilization and depolymerization of pectins due to the action of polygalacturonase (PG). Two PG isoenzymes can be extracted from ripe fruit: PG2, which is a single catalytic PG polypeptide, and PG1, which is composed of PG2 tightly associated with a second noncatalytic protein, the beta subunit. Previous studies have correlated ripening-associated increases in pectin solubilization and depolymerization with the presence of extractable PG1 activity, prior to the appearance of PG2, suggesting a functional role for the beta subunit and PG1 in pectin metabolism. To assess the function of the beta subunit, we produced and characterized transgenic tomatoes constitutively expressing a beta subunit antisense gene. Fruit from antisense lines had greatly reduced levels of beta subunit mRNA and protein and accumulated < 1% of their total extractable PG activity in ripe fruit as PG1, as compared with 25% for wild type. Inhibition of beta subunit expression resulted in significantly elevated levels of EDTA-soluble polyuronides at all stages of fruit ripening and a significantly higher degree of depolymerization at later ripening stages. Decreased beta subunit protein and extractable PG1 enzyme activity and increased pectin solubility and depolymerization all cosegregated with the beta subunit antisense transgene in T2 progeny. These results indicate (1) that PG2 is responsible for pectin solubilization and depolymerization in vivo and (2) that the beta subunit protein is not required for PG2 activity in vivo but (3) does play a significant role in regulating pectin metabolism in wild-type fruit by limiting the extent of pectin solubilization and depolymerization that can occur during ripening. Whether this occurs by direct interaction of the beta subunit with PG2 or indirectly by interaction of the beta subunit with the pectic substrate remains to be determined.

Amino Acid Sequence↗
PubMed1994

Protein tyrosine kinase activity is essential for Fc gamma receptor-mediated intracellular killing of Staphylococcus aureus by human monocytes.

Our previous study revealed that the intracellular killing of Staphylococcus aureus by human monocytes after cross-linking Fc gamma receptor I (Fc gamma RI) or Fc gamma RII is a phospholipase C (PLC)-dependent process. The aim of the present study was to investigate whether protein tyrosine kinase (PTK) activity plays a role in the Fc gamma R-mediated intracellular killing of bacteria and activation of PLC in these cells. The results showed that phagocytosis of bacteria by monocytes was not affected by the PTK inhibitors genistein and tyrphostin-47. The intracellular killing of S. aureus by monocytes after cross-linking Fc gamma RII or Fc gamma RII with anti-Fc gamma R monoclonal antibody and a bridging antibody or with human immunoglobulin G (IgG) was inhibited by these compounds in a dose-dependent fashion. The production of O2- by monocytes after stimulation with IgG or IgG-opsonized S. aureus was almost completely blocked by the PTK inhibitor. These results indicate that inhibition of PTK impairs the oxygen-dependent bactericidal mechanisms of monocytes. Genistein and tyrphostin-47, which do not affect the enzymatic activity of purified PLC, prevented activation of PLC after cross-linking Fc gamma RI or Fc gamma RII, measured as an increase in the intracellular inositol 1,4,5-trisphosphate concentration. Cross-linking Fc gamma RI or Fc gamma RII induced rapid tyrosine phosphorylation of several proteins in monocytes, one of which was identified as PLC-gamma 1, and the phosphorylation could be completely blocked by PTK inhibitors, leading to the conclusion that activation of PLC after cross-linking Fc gamma R in monocytes is regulated by PTK activity. Together, these results demonstrate that PTK activity is essential for the activation of PLC which is involved in the Fc gamma R-mediated intracellular killing of S. aureus by human monocytes.

Blood Bactericidal Activity↗
PubMed1994

Sequential activation of phospholipase-C and -D in agonist-stimulated gonadotrophs.

The contributions of phospholipase-C and -D to diacylglycerol (DG) formation during agonist-induced cell signaling were investigated in rat pituitary cells and alpha T3-1 gonadotrophs. In both cell types, GnRH caused a biphasic increase in DG formation, with an initial spike within 60 sec, followed by a larger and sustained rise to reach a second peak after 15 min of stimulation. Both phases of DG production were temporally correlated with inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] formation, consistent with the dependence of DG formation on phospholipase-C-mediated phosphoinositide hydrolysis. However, the ability of GnRH to stimulate phosphatidylethanol (PEt) in the presence of ethanol suggested that phospholipase-D may also participate in DG formation. Two inhibitors of phospholipase-C-dependent phosphoinositide hydrolysis, U73122 and neomycin sulfate, reduced the PEt as well as the Ins(1,4,5)P3 response to GnRH, indicating that phospholipase-D is activated during phospholipase-C-dependent signaling in pituitary gonadotrophs. The production of both DG and PEt was increased by treatment with the active phorbol ester phorbol 12-myristate 13-acetate (PMA), but not with inactive 4 alpha-phorbol 13-didecanoate, indicating that stimulation of protein kinase-C leads to activation of phospholipase-D. In accord with this, GnRH- and PMA-induced elevations of DG and PEt production were attenuated or abolished in protein kinase-C-depleted cells. In contrast, short and long term stimulation with PMA had no effect on basal inositol phosphate production. Also, GnRH-induced inositol phosphate production was not affected by protein kinase-C depletion. Finally, U73122 and neomycin sulfate did not inhibit PMA-induced PEt formation. These data indicate that GnRH activates a dual phospholipase pathway in a sequential and synchronized manner; phospholipase-C initiates the biphasic increase in Ins(1,4,5)P3 and DG formation, and protein kinase-C mediates the integration of phospholipase-D into the signaling response during the sustained phase of agonist stimulation.

Animals↗
PubMed1994

Effect of pituitary adenylate cyclase-activating polypeptide 38 on growth hormone and prolactin expression.

Time- and dose-dependent effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on prolactin (PRL) and growth hormone (GH) release were examined in static and dynamic rat pituitary cell incubations and on different pituitary cell (sub)populations separated according to their density on a discontinuous Percoll gradient. Quantitative in situ hybridization histochemistry allowed us to examine in parallel the effects of PACAP on PRL and GH gene expression. PACAP did not alter GH or PRL secretion in a dynamic superfusion system, in any cell population tested. Static incubations (30 min, 2-36 h) with PACAP 38 resulted in a significant increase in GH release and stimulated GH synthesis, as measured by the cytoplasmic accumulation of GH mRNA in the somatotrophs. These effects on synthesis and release were also observed after the enrichment of GH cells on Percoll gradients. PRL release was not altered by longer periods of incubation. Although no significant changes were observed in PRL secretion after 38 h, accumulation of cytoplasmic PRL mRNA was significantly stimulated in total pituitary cell suspension. After fractioning lactotrophs on Percoll gradients, the stimulatory effect of PACAP on PRL synthesis was lost. These results suggest that PACAP stimulates GH release and synthesis, and that it may act as a physiological regulator of this cell type. The PRL cell is not the most likely target cell type for PACAP. Effects observed on PRL synthesis in the total cell population may involve paracrine action of other hormone- or non-hormone-secreting cell types.

Animals↗
PubMed1994

Specific beta-adrenergic receptor binding of carazolol measured with PET.

UNLABELLED: Carazolol is a promising high-affinity beta-adrenergic receptor ligand for the noninvasive determination of beta receptor status using PET. Earlier investigations demonstrated specific receptor binding of carazolol in mice. These PET studies with S(-)-[2"-11C]carazolol in pigs were performed to explore the utility of the tracer for PET receptor studies. METHODS: Tracer uptake in the heart and lung was measured by PET as a function of time. Receptors were blocked with propranolol and different doses of ICI 118,551 to estimate specific binding. Fluorine-18-1"-Fluorocarazolol and the less active R-enantiomer of [11C]-carazolol were also studied. RESULTS: Specific receptor binding was 75% of the total uptake in the heart, preventable and displaceable by propranolol. Dose-dependent competition showed that carazolol binds in vivo to beta 1 and to beta 2 subtypes. Uptake of the labeled R(+) enantiomer of carazolol was not receptor-specific. CONCLUSIONS: Carazolol labeled with 11C or 18F is a strong candidate for use in receptor estimation with PET. The in vivo observations were consistent with its known high affinity and slow receptor dissociation rate. Its high specific receptor uptake and low metabolism allow existing kinetic models to be applied for receptor measurements. The 11C label is convenient for repeated administrations, though 18F allowed the long observation periods necessary for measurement of the receptor dissociation rate. If needed, nonspecific uptake can be estimated without pharmacologic intervention by using the labeled R enantiomer.

Adrenergic beta-Antagonists↗
PubMed1994

Analysis of MRP gene expression and function in HL60 cells isolated for resistance to adriamycin.

In an effort to define clearly the basis of non-P-glycoprotein multidrug resistance in HL60/ADR cells, we have analyzed expression of MRP mRNA levels and the MRP-encoded protein in resistant cells and also in resistant cells that have undergone a reversion to drug sensitivity. The results demonstrate that an MRP cDNA containing 5'-end coding sequences reacts with a 6-kb RNA, which is overexpressed in the resistant isolate. As resistant cells revert to drug sensitivity there is essentially a complete loss of the 6-kb RNA. Southern blot analysis indicates that the MRP gene is amplified compared to the copy number found in sensitive cells. Revertant cells no longer contain amplified MRP sequences. Western blot analysis has been conducted using an antibody prepared against the carboxyl terminus (15 amino acids) of the deduced sequence of the MRP-encoded protein. The antibody is reactive with a 190-kDa protein (P190) and with two closely migrating proteins of 65 and 70 kDa (P70), which are overexpressed in plasma membranes and endoplasmic reticulum of resistant cells. Both proteins are greatly reduced in revertant cells. Growth of cells in the presence of tunicamycin demonstrates that both P190 and P70 are glycosylated, with the deglycosylated forms migrating in polyacrylamide gels as proteins of 165 kDa and 45 kDa, respectively. Additional antisera have also been prepared against sequence domains contained in the C-terminal region of P190. These antisera are reactive with both P190 and P70. Antisera directed against sequences of the amino terminal region of P190 do not react with P70.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP-Binding Cassette Transporters↗
PubMed1993

Purification and immunological analysis of phospholipase D from castor bean endosperm.

Phospholipase D (EC 3.1.4.4) has been implicated in diverse cellular processes, but its physiological role is not well established in plants. In order to develop immunological and molecular biology approaches to address the problem, we report here the immunological analysis and N-terminal amino acid sequence of a cytosolic phospholipase D from castor bean (Ricinus communis L.). The enzyme was purified to apparent homogeneity from germinating castor bean endosperm. The specific activity of the purified enzyme was enhanced by approximately 670-fold with an overall yield of 4%. Its molecular mass was estimated at 92 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The N-terminal amino acid sequence of this enzyme was KLVENIEETVGFGKG. Polyclonal antibodies were raised against the purified enzyme. The antibodies inhibited the activity of transphosphatidylation more than that of hydrolysis of phospholipase D. The differential effect on the two activities of this enzyme implies that different active sites on this enzyme may be involved in the two reactions. Immunoblot analyses showed that the amounts of phospholipase D protein relative to the total endosperm proteins increased during the first 5 days of germination. The antibodies cross-reacted to proteins from several tested plant species, and those proteins had molecular masses similar to that of castor bean phospholipase D. These results indicate that the expression of phospholipase D in castor bean changes according to growth stages and that phospholipase D enzymes of different plant species are structurally related.

Amino Acid Sequence↗
PubMed1993

A detailed genetic map for the X chromosome of the malaria vector, Anopheles gambiae.

Anopheles gambiae, the primary vector of human malaria in Africa, is responsible for approximately a million deaths per year, mostly of children. Despite its significance in disease transmission, this mosquito has not been studied extensively by genetic or molecular techniques. To facilitate studies on this vector, a genetic map has been developed that covers the X chromosome at an average resolution of 2 centimorgans. This map has been integrated with the chromosome banding pattern and used to localize a recessive, sex-linked mutation (white eye) to within 1 centimorgan of flanking markers.

Alleles↗
PubMed1993

Cysteine desulfurase activity indicates a role for NIFS in metallocluster biosynthesis.

Biological nitrogen fixation is catalyzed by nitrogenase, a complex metalloenzyme composed of two separately purifiable component proteins encoded by the structural genes nifH, nifD, and nifK. Deletion of the Azotobacter vinelandii nifS gene lowers the activities of both nitrogenase component proteins. Because both nitrogenase component proteins have metallocluster prosthetic groups that are composed of iron- and sulfur-containing cores, this result indicated that the nifS gene product could be involved in the mobilization of the iron or sulfur required for metallocluster formation. In the present work, it is shown that NIFS is a pyridoxal phosphate-containing homodimer that catalyzes the formation of L-alanine and elemental sulfur by using L-cysteine as substrate. NIFS activity is extremely sensitive to thiol-specific alkylating reagents, which indicates the participation of a cysteinyl thiolate at the active site. Based on these results we propose that an enzyme-bound cysteinyl persulfide that requires the release of the sulfur from the substrate L-cysteine for its formation ultimately provides the inorganic sulfide required for nitrogenase metallocluster formation. The recent discovery of nifS-like genes in non-nitrogen-fixing organisms also raises the possibility that the reaction catalyzed by NIFS represents a universal mechanism that involves pyridoxal phosphate chemistry, in the mobilization of the sulfur required for metallocluster formation.

Amino Acid Sequence↗
PubMed1993

DNA splicing by an active site mutant of Flp recombinase. Possible catalytic cooperativity between the inactive protein and its DNA substrate.

Each strand transfer catalyzed by the Flp recombinase is the composite of two transesterification reactions. The active nucleophilic species in the two reactions are the catalytic site tyrosine (Tyr-343) of Flp and the 5'-hydroxyl from the Flp-nicked DNA substrate, respectively. A "half recombination site" is capable of undergoing this pair of transesterifications in the presence of Flp. When the substrate is a half-site containing a chiral phosphorothioate at the exchange point, the Flp reaction yields a product in which the phosphate chirality is retained. A mutant of Flp that lacks the active site tyrosine, Flp(Y343F), is incapable of mediating strand transfer in a full-recombination site but can execute strand transfer in a half-site. The efficiency of this reaction is about 2% of that of the wild type reaction. The activity of Flp(Y343F) is critically dependent on the length of the half-site spacer. Furthermore, in this reaction, the strand cleavage and strand exchange steps cannot be uncoupled. These results strongly suggest a direct attack by the 5'-hydroxyl of the half-site spacer on the phosphodiester at the normal strand transfer point.

Base Sequence↗
PubMed1993

Stimulation of the intracellular killing of Staphylococcus aureus by human monocytes mediated by Fc gamma receptors I and II.

Previous studies have shown that intracellular killing of bacteria by monocytes is stimulated by interaction between IgG and Fc gamma receptors (Fc gamma R) in the membrane of these cells. In the present study anti-Fc gamma R monoclonal antibodies (mAb) were used to investigate the relative contributions of the various classes of Fc gamma R to the intracellular killing of Staphylococcus aureus by human monocytes and the biochemical pathways involved. Anti-Fc gamma RI or anti-Fc gamma RII mAb, but not anti-Fc gamma RIII mAb, efficiently stimulated the intracellular killing of bacteria by monocytes. Cross-linking Fc gamma RI or Fc gamma RII, but not Fc gamma RIII, on monocytes with mouse anti-Fc gamma R mAb followed by bridging with F(ab')2 fragments of goat anti-mouse IgG enhanced this process. Since the NADPH oxidase inhibitor diphenyleneiodonium blocked the Fc gamma R-mediated intracellular killing of S. aureus, oxygen-dependent bactericidal mechanisms are most probably involved. Cross-linking Fc gamma RI or Fc gamma RII but not binding of the mAb to the Fc gamma R on monocytes activated phospholipase C, as demonstrated by the increase in the intracellular concentration of inositol-(1,4,5)-triphosphate. The enhanced intracellular killing stimulated by cross-linking Fc gamma R on monocytes was completely blocked by U-73122, an inhibitor of phospholipase C-dependent processes. Protein kinase C activity, but not the rise in the cytosolic free Ca++ concentration or pertussis toxin-sensitive G proteins, is essential for the Fc gamma R-mediated intracellular killing of bacteria by monocytes. Together, these results demonstrate that cross-linking Fc gamma RI or Fc gamma RII is equally effective in stimulating the intracellular killing of bacteria by monocytes and that this stimulation is a phospholipase C-dependent process.

Antibodies, Monoclonal↗