Search PubMed⌕ Search

Biomedical subjects

K Lee

Publications and source records attributed to K Lee.

At least 523 records · Page 29Linked to original sources
PubMed1995

A tactile sensory system of Myxococcus xanthus involves an extracellular NAD(P)(+)-containing protein.

CsgA is a cell surface protein that plays an essential role in tactile responses during Myxococcus xanthus fruiting body formation by producing the morphogenic C-signal. The primary amino acid sequence of CsgA exhibits homology with members of the short-chain alcohol dehydrogenase (SCAD) family and several lines of evidence suggest that NAD(P)+ binding is essential for biological activity. First, the predicted CsgA secondary structure based on the 3 alpha/20 beta-hydroxysteroid dehydrogenase crystal structure suggests that the amino-terminal portion of the protein contains an NAD(P)+ binding pocket. Second, strains with csgA alleles encoding amino acid substitutions T6A and R10A in the NAD(P)+ binding pocket failed to develop. Third, exogenous MalE-CsgA rescues csgA development, whereas MalE-CsgA with the amino acid substitution CsgA T6A does not. Finally, csgA spore yield increased approximately 20% when containing 100 nM of MalE-CsgA was supplemented with 10 microM of NAD+ or NADP+. Conversely, 10 microM of NADH or NADPH delayed development for approximately 24 hr and depressed spore levels approximately 10%. Together, these results argue that NAD(P)+ binding is critical for C-signaling. S135 and K155 are conserved amino acids in the catalytic domain of SCAD members. Strains with csgA alleles encoding the amino acid substitutions S135T or K155R failed to develop. Furthermore, a MalE-CsgA protein containing CsgA S135T was not able to restore development to csgA cells. In conclusion, amino acids conserved in the coenzyme binding pocket and catalytic site are essential for C-signaling.

Amino Acid Sequence↗
PubMed1995

Imidazoline compounds inhibit KATP channels in guinea pig ventricular myocytes.

Phentolamine and related imidazolines inhibit KATP channel activity in the pancreatic beta cell. In the present study, the effects of several imidazoline-based compounds were examined upon KATP channel activity in guinea pig ventricular myocytes. Phentolamine produced a potent inhibition of KATP channel activity when examined in either excised inside-out patches or in the whole-cell configuration. This effect was unrelated to phentolamine's ability to antagonise alpha-adrenoceptors since the nonselective alpha-adrenoceptor antagonists, benextramine and phenoxybenzamine, failed to affect channel activity. Furthermore, the alpha-adrenoceptor agonist clonidine together with several related imidazolines inhibited channel activity. This suggests that imidazoline compounds modulate KATP channel activity in guinea pig ventricular myocytes and this may have clinical implications for the use of such agents as hypoglycemic drugs.

Adenosine Triphosphate↗
PubMed1995

Characterization of an ATP-modulated large conductance Ca(2+)-activated K+ channel present in rat cortical neurones.

1. Single channel current recordings were used to study the characteristics of a large conductance Ca(2+)-activated K+ (BKCa) channel present in neurones acutely dissociated from the rat motor cortex. Application of ATP to the intracellular surface of excised inside-out patches produced a large, concentration-dependent increase in BKCa channel activity. 2. This ATP-mediated activation was dependent upon the presence of Mg2+ in the intracellular bathing solution and was diminished by the phosphatases 2,3-butanedione monoxime (BDM) or alkaline phosphatase and by the protein kinase inhibitors staurosporine, H-7 and PKI. 3. ADP stimulated BKCa channel activity in a Mg(2+)-dependent manner, an action also inhibited by the concomitant application of PKI or BDM. The effect of ADP was reduced by application of hexokinase and glucose or by application of the adenylate kinase inhibitor Ap5A. 4. Of other nucleotides tested, only CTP consistently activated BKCa channel activity. 5. Using the cell-attached configuration, bath application of forskolin or dibutyryl cAMP stimulated BKCa channel activity. 6. It is concluded that BKCa channel activity in the rat motor cortex is subject to modulation by the activity of a closely associated kinase. The ability of cAMP activators to stimulate BKCa channel activity in the intact cell suggests that this system may be of physiological importance.

Adenosine Triphosphate↗
PubMed1995

An in vivo model of intratumoural aromatase using aromatase-transfected MCF7 human breast cancer cells.

About two-thirds of human breast carcinomas contain detectable levels of aromatase, the enzyme which converts androgens to oestrogens. Assessment of the importance of this enzyme to breast cancer growth has been hampered by the absence of an adequate model system. We have previously reported that MCF7 human hormone-dependent breast cancer cells transfected with human aromatase cDNA (Arom1 cells) showed a growth response in vitro to exogenous androgens and this effect was blocked by aromatase inhibitors. We report here our use of these cells to develop a xenograft model in athymic nude mice. Neither MCF7 cells nor Arom1 cells formed tumours in oophorectomised (ovx) nude mice unless provided with oestradiol (E2) support. Once established, Arom1, but not MCF7, tumours could be grown in ovx females supplemented with androstenedione (delta 4A). The mean plasma level of delta 4A was 14 nmol/L in supplemented animals and < 0.5 nmol/L in unsupplemented animals. Similarly, unsupplemented male nude mice were able to support the growth of Arom1 tumours but not MCF7 tumours. The potent and highly specific aromatase inhibitor CGS20267 (letrozole) significantly decreased tumour growth at 2 mg/kg/day and completely inhibited growth at 20 mg/kg/day in delta 4A-supplemented but not E2-supplemented animals. Our results indicate that delta 4A-dependent growth of Arom1 tumours in vivo is mediated through the action of intratumoural aromatase. This model should allow an assessment of the critical levels of aromatase required for tumour growth support.

Animals↗
PubMed1995

Stereospecific sulfoxidation by toluene and naphthalene dioxygenases.

Studies on the sulfoxidation of aryl alkyl sulfides by purified toluene dioxygenase and naphthalene dioxygenase showed that naphthalene dioxygenase produces sulfoxides of (S) absolute configuration in high enantiomeric purity while those formed by toluene dioxygenase are of variable enantiomeric purity depending on the p-substituents on the benzene ring. Oxygen uptake experiments with naphthalene dioxygenase showed that the reaction rate and degree of oxygen incorporation are affected by both aryl and alkyl substituents. 18O2-Incorporation experiments showed that the oxygen atom of methyl phenyl sulfoxide formed by toluene dioxygenase and naphthalene dioxygenase is derived exclusively from O2. Accompanying studies showed that chloroperoxidase produces single (R)-sulfoxides (> 98% enantiomeric excess) from the aryl alkyl sulfides examined in the present study.

Chloride Peroxidase↗
PubMed1995

NS 1619 activates BKCa channel activity in rat cortical neurones.

Single channel recordings of large conductance Ca2(+)-activated K+ (BKCa) channels were made from neurones isolated from rat motor cortex. Application of levcromakalim, pinacidil or diazoxide had no effect on BKCa channel activity in excised patches. In contrast, NS 1619 (1-(2'-hydroxy-5'-trifluoromethylphenyl)-5-trifluoromethyl- 2(3H)benzimidazalone) induced concentration-dependent activation of BKCa channels with a calculated EC50 of 32 microM. The NS 1619-induced activity was dependent on the presence of free Ca2+ at the intracellular surface, but was not associated with a change in channel voltage sensitivity. Niflumic acid had no effect on BKCa activity per se but prevented NS 1619-mediated activation.

Adenosine Triphosphate↗
PubMed1995

Cloning and functional expression of the cDNA encoding an inwardly-rectifying potassium channel expressed in pancreatic beta-cells and in the brain.

A cDNA clone encoding an inwardly-rectifying K-channel (BIR1) was isolated from insulinoma cells. The predicted amino acid sequence shares 72% identity with the cardiac ATP-sensitive K-channel rcKATP (KATP-1;[6]). The mRNA is expressed in the brain and insulinoma cells. Heterologous expression in Xenopus oocytes produced currents which were K(+)-selective, time-independent and showed inward rectification. The currents were blocked by external barium and caesium, but insensitive to tolbutamide and diazoxide. In inside-out patches, channel activity was not blocked by 1 mM internal ATP. The sequence homology with KATP-1 suggests that BIR1 is a subunit of a brain and beta-cell KATP channel. However, pharmacological differences and the lack of ATP-sensitivity, suggest that if, this is the case, heterologous subunits must exert strong modulatory influences on the native channel.

Amino Acid Sequence↗
PubMed1995

Topographic distribution of fixation-related units in the dorsomedial frontal cortex of the rhesus monkey.

Most cells in the dorsomedial frontal cortex of the rhesus monkey had activity related to saccadic eye movement and/or visual fixation. This activity changed depending upon the position of a fixation target, which suggested coding for the target location in spatial coordinates. Further analysis of such activity revealed a topographical distribution of neurons: neurons in the rostral part of the area were more active with eyes to the contralateral position, while those in the caudal part were more active with eyes to the ipsilateral position; also, cells in the medial part of the area had higher activity with a downward fixation position, whereas those in the lateral part had higher activity with an upward fixation position. This distribution of units was in agreement with the map of termination zones of saccadic eye movements evoked by electrical stimulation of the same area. These observations provide evidence for the hypothesis that the dorsomedial frontal cortex is organized in spatial coordinates and is involved in specifying the position of visual fixation.

Animals↗
PubMed1995

The CONSTANS gene of Arabidopsis promotes flowering and encodes a protein showing similarities to zinc finger transcription factors.

The vegetative and reproductive (flowering) phases of Arabidopsis development are clearly separated. The onset of flowering is promoted by long photoperiods, but the constans (co) mutant flowers later than wild type under these conditions. The CO gene was isolated, and two zinc fingers that show a similar spacing of cysteines, but little direct homology, to members of the GATA1 family were identified in the amino acid sequence. co mutations were shown to affect amino acids that are conserved in both fingers. Some transgenic plants containing extra copies of CO flowered earlier than wild type, suggesting that CO activity is limiting on flowering time. Double mutants were constructed containing co and mutations affecting gibberellic acid responses, meristem identity, or phytochrome function, and their phenotypes suggested a model for the role of CO in promoting flowering.

Alleles↗