Search PubMed⌕ Search

Biomedical subjects

H Yan

Publications and source records attributed to H Yan.

At least 217 records · Page 12Linked to original sources
PubMed1995

Homodimerization and intermolecular tyrosine phosphorylation of the Tyk-2 tyrosine kinase.

The Jak kinases and Stat transcription factors play a major role in signaling of various cytokines including IFN alpha. In this report we show a ligand-independent interaction between Tyk-2 and Jak-1 kinases. We also demonstrate that the Tyk-2 kinase forms a homodimer that has the ability to undergo intermolecular tyrosine phosphorylation. The formation of the Tyk-2 homodimer is independent of both tyrosine phosphorylation and the presence of the tyrosine kinase domain.

Cell Line↗
PubMed1995

Structural and electronic similarity but functional difference in methylmalonyl-CoA mutase between coenzyme B12 and the analog 2',5'-dideoxyadenosylcobalamin.

The cofactor analog 2',5'-dideoxyadenosylcobalamin (ddAdoCbl) differs from the natural cofactor coenzyme B12 [5'-deoxyadenosylcobalamin (dAdoCbl)] by lacking only one oxygen atom. The 1H and 13C NMR spectra of ddAdoCbl have been assigned unambiguously by homonuclear and heteronuclear 2D NMR techniques. The 1H, 13C, and 31P chemical shift values for ddAdoCbl were compared with those of another organocobalamin, namely dAdoCbl. This assessment shows that the analog is very similar both electronically and structurally to the natural cofactor. The effectiveness of ddAdoCbl as a cofactor for both the human and Propionibacterium shermanii methylmalonyl-CoA mutases was compared with that of the natural cofactor. ddAdoCbl was found to be a competitive inhibitor with respect to dAdoCbl. Similar binding affinities to both enzymes were found for both the ddAdoCbl analog and the natural cofactor. However, in the presence of ddAdoCbl, the rate of conversion of methylmalonyl-CoA to succinyl-CoA was only 1-2% of that seen with the natural cofactor. There were no changes with time in the visible absorption spectrum of the bound cofactor analog in the presence of substrate, suggesting that the Co-C bond was not cleaved. The CD (circular dichroism) spectra of dAdoCbl and ddAdoCbl are very similar, consistent with the NMR results. The CD spectral changes upon binding to P. shermanii methylmalonyl-CoA mutase are large compared to those reported on the binding of dAdoCbl to ethanolamine ammonia lyase. Furthermore, the CD spectra of both enzyme-bound cobalamins are very similar, suggesting that similar changes in the conformation or structure in these cobalamins occur on binding to the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Circular Dichroism↗
PubMed1995

FFA-1, a protein that promotes the formation of replication centers within nuclei.

In the nuclei of eukaryotic cells, initiation of DNA replication occurs at a discrete number of foci. One component of these foci is the DNA replication factor RP-A. Here, the process leading to the association of RP-A with foci was reconstituted with cytosolic fractions derived from Xenopus eggs. With the use of this fractionated system, a 170-kilodalton protein required for the assembly of RP-A into foci was identified and purified. The protein appears to be an integral component of the foci at which replication of DNA is initiated in eukaryotic nuclei.

Animals↗
PubMed1995

Expression of receptor protein tyrosine phosphatase-sigma (RPTP-sigma) in the nervous system of the developing and adult rat.

The expression of receptor protein tyrosine phosphatase-sigma (RPTP-sigma) mRNA during rat development was examined by Northern blot and in situ hybridization analyses. Northern blot analysis revealed that the two transcripts (5.7 kb and 6.9 kb) had different spatial and temporal patterns of expression. The 6.9-kb transcript was more abundant during embryonic development, whereas the 5.7-kb transcript was more abundant during postnatal development and in the adult. In situ hybridization revealed that RPTP-sigma mRNA was widely expressed throughout the central and peripheral nervous system during embryonic development. Very high levels were seen in the ventricular zone, subventricular zone, cortex, dorsal root ganglia, cranial nerve ganglia, olfactory epithelium, and retina. During postnatal development the level of expression decreased in most brain regions. However, high levels continued to be seen in the hippocampus. Emulsion autoradiography revealed that the majority of RPTP-sigma mRNA is expressed in neurons. Northern analysis showed that cultured glial cells expressed the 6.9-kb transcript, but not the 5.7-kb. RPTP-sigma mRNA expression profiles were clearly distinct from those of leukocyte antigen-related protein (LAR), a closely related RPTP. The spatiotemporal pattern of RPTP-sigma mRNA expression indicates that RPTP-sigma may play a role in the development of the nervous system.

Age Factors↗
PubMed1995

Structure of the trigonal form of recombinant oxidized flavodoxin from Anabaena 7120 at 1.40 A resolution.

The oxidized recombinant flavodoxin from the cyanobacterium Anabaena 7120 has been crystallized in a trigonal form. The recombinant protein has an identical primary structure to that purified directly from Anabaena, which functions as a substitute for ferredoxin in an iron-deficient environment for electron transfer from photosystem I to ferredoxin-NADP(+) reductase. X-ray data to 1.40 A were collected on a Siemens area detector. Of the 311 379 reflections collected, 36069 reflections were unique in space group P3(1)21 (a = 55.36, c = 102.59 A) with an R(merge) of 3.8%. The structure was solved by molecular replacement using coordinates from the wild-type monoclinic structure previously solved in this laboratory [Rao, Shaffie, Yu, Satyshur, Stockman & Markley (1992). Protein Sci. 1, 1413-1427]. The structure was refined with X-PLOR and SHELXL93 to a crystallographic R-factor of 13.9% for 32963 reflections with I> 2sigma(I). The final structure contains 2767 atoms including 31 flavin mononucleotide (FMN) atoms, 299 water molecules, and one sulfate ion. The protein is comprised of a central five-stranded beta-sheet surrounded by five helices and binds a single molecule of FMN at the C-terminus of the sheet. The trigonal protein structure and the crystal packing are compared with the monoclinic wild-type protein. Helix alpha3 in this structure is less distorted than in the monoclinic structure and shows additional hydrogen bonds in the N-terminal portion of the helix. The trigonal structure is extensively hydrogen bonded in three major areas with neighboring molecules compared with five regions in the monoclinic structure, but using significantly fewer hydrogen bonds to stabilize the lattice. There are several hydrogen bonds to the amide groups from water molecules several of which stabilize and extend the ends of the beta-sheet.

Journal Article↗
PubMed1995

Proteoglycan biosynthesis is required in BC3H1 myogenic cells for modulation of vascular smooth muscle alpha-actin gene expression in response to microenvironmental signals.

Induction of vascular smooth muscle (VSM) alpha-actin mRNA expression during cytodifferentiation of mouse BC3H1 myogenic cells coincides with the accumulation of cell surface- and extracellular matrix-associated sulfated proteoglycans. Inhibition of proteoglycan biosynthesis in myogenic cells using an artificial beta-D-xyloside glycosaminoglycan acceptor was accompanied by a reduction in cell surface/extracellular matrix proteoglycans and VSM alpha-actin mRNA expression while enhancing the secretion of free chondroitin sulfate/dermatan sulfate and heparan sulfate glycosaminoglycans into the culture medium. Maximum inhibition of VSM alpha-actin mRNA expression required that proteoglycan biosynthesis be blocked during the early phase of cytodifferentiation when myoblasts were fully confluent and quiescent. The inhibitory effect of beta-D-xyloside on alpha-actin mRNA expression resulted from attenuation at both the transcriptional and post-transcriptional control points. Sustained proteoglycan biosynthesis was required for induction of VSM alpha-actin mRNA in quiescent myoblasts in response to cytodifferentiation-permissive, substrate-associated macromolecules (SAM) or upon exposure to soluble serum factors capable of transiently stimulating VSM alpha-actin gene transcription. The results suggested that efficient myoblast cytodifferentiation and modulation of VSM alpha-actin mRNA levels depended on intact cell surface proteoglycans to convey signals generated as a consequence of cellular interaction with substrate components and serum factors.

Actins↗
PubMed1995

Susceptibility artifact reduction in fat suppression.

Strong fat signal in regions where a large susceptibility difference exists, for instance at the interface between air and tissue near the maxillary sinus, may not be eliminated by currently available fat suppression techniques without sacrificing the overall quality of the images. In this article, we show that this fat signal, which appears as a susceptibility artifact, can be significantly reduced by using an optimized presaturation pulse with sharp edges and a broad bandwidth, while causing minimal disturbance of the water signal. Several optimized presaturation pulses can be reproduced by the Fourier coefficients provided in the Appendix.

Adipose Tissue↗
PubMed1995

Alcohol and aldehyde dehydrogenase genotypes and drinking behavior of Chinese living in Shanghai.

Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH), the principal enzymes responsible for oxidative metabolism of ethanol, exist in multiple, genetically determined molecular forms. Widely different kinetic properties in some of these isozymes account for the individual differences in alcohol sensitivity. In this study we used the polymerase chain reaction/restriction fragment length polymorphism method to determine the genotypes of the ADH2 and ALDH2 loci of alcoholic and nonalcoholic Chinese living in Shanghai. We also investigated the subjects' drinking patterns by means of semistructured interviews. The alcoholics had significantly lower frequencies of the ADH2(2) and ALDH2(2) alleles than did the nonalcoholics, suggesting the inhibitory effects of these alleles for the development of alcoholism. In the nonalcoholic subjects, ADH2(2) had little, if any, effect, despite the significant effect of the ALDH2(2) allele in decreasing the alcohol consumption of the individual. Taken together, these results fit the proposed hypothesis for the development of alcoholism, i.e., drinking behavior is greatly influenced by the individual's genotypes of alcohol-metabolizing enzymes, and the risk of becoming alcoholic is proportionate with the ethanol consumption of the individual.

Adult↗
PubMed1995

Stat recruitment by tyrosine-phosphorylated cytokine receptors: an ordered reversible affinity-driven process.

Herein, we demonstrate that purified Stat1 binds to its tyrosine-phosphorylated docking site on the IFN gamma receptor alpha chain in a direct, specific, and reversible manner. Using surface plasmon resonance, we determine the affinity (KD = 137 nM) and specificity of the interaction and define the minimum affinity needed for receptor-mediated Stat1 activation. In addition, we quantitate the relative ability of purified Stat1 to interact with tyrosine-phosphorylated binding sites on other Stat proteins. Finally, we describe experiments that imply that the unidirectional release of activated Stat1 from the IFN gamma receptor reflects the preference of free tyrosine-phosphorylated Stat1 monomers to form high avidity reciprocal homodimers rather than reassociating with the receptor binding site. Our results demonstrate that IFN gamma-induced Stat1 activation is an ordered and affinity-driven process and we propose that this process may serve as a paradigm for Stat activation by other cytokine receptors.

Amino Acid Sequence↗
PubMed1995

An analysis of the regulation of DNA synthesis by cdk2, Cip1, and licensing factor.

The activation of DNA replication appears to involve at least four steps. These include origin recognition, origin unwinding, primer synthesis, and a switching step to a continuous elongation mode. Moreover, in higher eukaryotes a number of studies have shown that much of the DNA replication which occurs is restricted to specific sites within the nuclei. It has been proposed that these replication foci are composed of a large number of origin sites which are clustered together into an aggregate. The molecular basis for this aggregation is currently not well understood. Regulation of the activation of DNA replication is a complicated process. The G1-S kinase cdk2 is a positive regulator of replication. The p21 protein is a negative regulator of replication both by inhibiting cdk2 kinase and the replication protein PCNA. Moreover, it has been proposed that origin usage is restricted to a single firing per cell cycle by a "licensing factor." Using a cell-free replication system derived from Xenopus eggs we have investigated at what step in the replication process these regulators participate. We present evidence that the clustered organization of DNA into foci is not a transient arrangement, but rather, it persists following DNA replication. We also find that foci form on both sperm chromatin and bacteriophage lambda DNA incubated in extracts depleted of cdk2 kinase. Therefore, our data support the conclusion that organization of chromatin into foci is an early event in the replication pathway preceding activation of cdk2 kinase. With respect to the role of cdk2 during activation of DNA replication we find that in cdk2-depleted extracts primer synthesis does not occur and RP-A remains tightly associated with foci. This strongly suggests that cdk2 kinase is required for activating the origin unwinding step of the replication process. Consistent with this interpretation we find that addition of rate limiting quantities of the cdk2 inhibitor p21 protein to an extract delays primer synthesis. Interestingly, in the presence of p21 primer synthesis does occur after a delay and then replication arrests. This is consistent with the published demonstration that p21 can inhibit PCNA, a protein required for replication beyond the priming step. Therefore, our results provide additional support to the proposal that the post-priming switching step is a key regulatory step in replication. With respect to the role of licensing factor during DNA replication it has recently been shown that treatment of mitotic extracts with kinase inhibitor DMAP inactivates "licensing factor."(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1995

Comparison of multilayer neural network and Nearest Neighbor Classifiers for handwritten digit recognition.

The basic Nearest Neighbor Classifier (NNC) is often inefficient for classification in terms of memory space and computing time needed if all training samples are used as prototypes. These problems can be solved by reducing the number of prototypes using clustering algorithms and optimizing the prototypes using a special neural network model. In this paper, we compare the performance of the multilayer neural network and an Optimized Nearest Neighbor Classifier (ONNC) for handwritten digit recognition applications. We show that an ONNC can have the same recognition performance as an equivalent neural network classifier. The ONNC can be efficiently implemented using prototype and variable ranking, partial summation and distance triangular inequality based strategies. It requires the same memory space as, but less, training time and classification time than the neural network.

Computers↗
PubMed1995

Cooperativity between non-polar and ionic forces in the binding of bacterial cell wall analogues by vancomycin in aqueous solution.

The clinically important glycopeptide antibiotic vancomycin binds to bacterial cell wall peptides of Gram-positive bacteria which terminate in -Lys-D-Ala-D-Ala, thereby inhibiting cell wall synthesis resulting in cell death. We have removed the N-terminal leucine residue of vancomycin by an Edman degradation and acylated the exposed amino group of residue 2 with N-Me-Gly, N-Me-D-Ala, acetyl, butyl, and isohexyl groups to generate novel vancomycin analogues. The binding of vancomycin and these vancomycin analogues to the bacterial cell wall analogue di-N-Ac-L-Lys-D-Ala-D-Ala (DALAA) was studied by NMR techniques and UV spectroscopy. The effects that these structural modifications of the carboxylate binding pocket of vancomycin have on the antibiotic-DALAA recognition process show that a cooperative effect between non-polar and ionic forces appears to be partly responsible for the highly efficient sequestering of the DALAA C-terminal carboxylate from aqueous solution.

Amino Acid Sequence↗
PubMed1995

Clinical observation on 80 children with peptic ulcer treated primarily by traditional Chinese medicine.

Using the empirical recipe of Wang Pengfei (a very experienced TCM doctor) as the main treatment, the authors treated 80 children with peptic ulcer by means of promoting blood circulation, removing blood stasis as well as regulating the flow of qi, alleviating pain, tonifying the stomach and reinforcing the spleen. Measures for hemostasis, correction of anemia and other general treatment were also taken. After 8 weeks of treatment, a total effective rate of 92.5% was obtained.

Adolescent↗
PubMed1994

p135tyk2, an interferon-alpha-activated tyrosine kinase, is physically associated with an interferon-alpha receptor.

Recent genetic studies have linked the tyk2 gene, which encodes a novel type of non-receptor tyrosine kinase, to the interferon-alpha intracellular signaling pathway. In this report, biochemical evidence is presented which supports this proposed function for the tyk2 tyrosine kinase and further defines its role in the interferon-alpha signaling cascade. Specifically, the tyk2 gene is shown to encode a 135-kDa protein which is rapidly phosphorylated on tyrosine in response to interferon-alpha treatment. Indirect evidence suggests that the tyrosine phosphorylation of p135tyk2 is the result of autokinase activity, implying that the Tyk2 tyrosine kinase is activated by interferon-alpha treatment. Two complementary methods demonstrate a physical association between p135tyk2 and the alpha-subunit of the interferon-alpha receptor. First, immunoblots show that monoclonal antibodies against the alpha-subunit of the interferon-alpha receptor can co-immunoprecipitate p135tyk2. Second, interferon-alpha receptor proteins which have been labeled by affinity cross-linking with 125I-interferon-alpha 2 can be co-immunoprecipitated using anti-tyk2 antisera. Taken together, these data suggest that an interferon-alpha receptor-p135tyk2 complex functions, in a manner analogous to the CD4-lck tyrosine kinase complex, to initiate the interferon-alpha signaling cascade.

Animals↗
PubMed1994

A computer simulation study on the input function sampling schedules in tracer kinetic modeling with positron emission tomography (PET).

Tracer kinetic modeling with positron emission tomography (PET) requires measurements of the time-activity curves in both plasma (PTAC) and tissue (TTAC) to estimate physiological parameters, i.e. to fit the parameters of certain compartmental models using PTAC and TTAC as the model input and output functions, respectively. In this paper, we first explored the optimal blood sampling schedule (OBSS) for the input function, based on the tracer [18F]2-fluoro-2-deoxy-D-glucose (FDG) blood sample experimental data. Then using a 5-parameter FDG model we investigated the effects of the plasma sampling schedule, as well as PTAC measurement noise, on the estimation accuracy and reliability of FDG model macro- and micro-parameters and the physiological parameter local cerebral metabolic rates of glucose (LCMRGlc), using computer simulation. Three different methods were used: (a) estimation of the FDG model parameters ignoring PTAC noise using the traditional PTAC schedule (non-OBSS); (b) estimation of the PTAC model parameters and FDG model parameters simultaneously using both non-OBSS and OBSS; (c) estimation of the PTAC model parameters first, then the FDG model parameters using both non-OBSS and OBSS. The results show that OBSS can provide more reliable estimates and largely simplifies the experiment operations.

Blood Specimen Collection↗
PubMed1994

Functional consequences of iron supplementation in iron-deficient female cotton mill workers in Beijing, China.

Eighty iron-deficient, nonpregnant female workers were randomly assigned to ferrous sulphate (60 or 120 mg Fe/d) or placebo treatment for 12 wk. Energy expenditure was estimated during 3 d by heart rate (HR) recording. Production efficiency (PE) was calculated as the ratio of productivity to energy expenditure. In the iron-treated group mean hemoglobin (Hb) increased from 114 to 127 g/L (P < 0.001), mean serum ferritin increased from 9.7 to 30.0 micrograms/L (P < 0.001), and mean free erythrocyte protoporphyrin decreased from 1.01 to 0.49 mumol/L (P < 0.001). Mean HR at work decreased from 95.5 to 91.1 beats/min (P < 0.001), which was inversely correlated with the change in Hb (r = -0.60, P < 0.001). PE increased significantly in the iron-treated group (P < 0.001) and its change paralleled the change in Hb (r = 0.58, P < 0.001). The results show that iron supplementation enabled these women to do the same work at a lower energy cost.

Adult↗