Reactivity of cyanate with valine-1 (alpha) of hemoglobin. A probe of conformational change and anion binding.
The 3-fold increase in the carbamylation rate of Val-1 (alpha) of hemoglobin upon deoxygenation described earlier is now shown to be a sensitive probe of conformational change. Thus, whereas this residue in methemoglobin A is carbamylated at the same rate as in liganded hemoglobin, upon addition of inositol hexaphosphate its carbamylation rate is enhanced 30% as much as the total change in the rate between the CO and deoxy states. For CO-hemoglobin Kansas in the presence of the organic phosphate, the relative increase in the carbamylation rate of this residue is about 50%. These results indicate that methemoglobin A and hemoglobin Kansas in the presence of inositol hexaphosphate do not assume a conformation identical with deoxyhemoglobin but rather form either a mixture of R and T states or an intermediate conformation in the region around Val-1 (alpha). Studies on the mechanism for the rate enhancement in deoxyhemoglobin suggest that the cyanate anion binds to groups in the vicinity of Val-1 (alpha) prior to proton transfer and carbamylation of this NH2-terminal residue. Thus, specific removal with carboxypeptidase B of Arg-141 (alpha), which is close to Val-1 (alpha) in deoxyhemoglobin, abolishes the enhancement in carbamylation. Chloride, which has the same valency as cyanate, is a better competitive inhibitor of the carbamylation of deoxyhemoglobin (Ki = 50 mM) compared with liganded hemoglobin. Nitrate and iodide are also effective inhibitors of the carbamylation of Val-1 (alpha) of deoxyhemoglobin (Ki = 35 mM); inorganic phosphate, sulfate, and fluoride are poor competitive inhibitors. The change in pKa of Val-1 (alpha) upon deoxygenation may be due to its differential interaction with chloride. (+info)
Role of glucagon on the control of hepatic protein synthesis and degradation in the rat in vivo.
The effect of glucagon on hepatic protein systhesis and proteolysis has been investigated. The intraperitoneal administration of 200 mug of glucagon produced an increase of the polypeptide chains completion time which was maximal 5 min after its administration and approached control values at 20 min. The increase of the polypeptides chains completion time observed at 5 min after the hormone administration represents a 38% inhibition of the hepatic protein synthetic rate. When glucagon was continuously supplied by intravascular infusion, maximal inhibition was attained throughout the experiment. This inhibition of protein synthesis brought about by glucagon was accompanied by an increase in the polyribosomal state of aggregation, indicating that the hormone acts mainly if not exclusively, on the elongation or termination step, or both. The administration of glucagon produced also a progressive increase in the hepatic valine concentration. This increase could not be accounted for the the decrease in plasma valine levels, suggesting that the rise in haptic valine concentration is an expression of hepatic proteolysis rather than the result of an accelerated transport of amino acids across the hepatocyte plasma membrane. The different time sequence in the glucagon-induced effects of protein synthesis and proteolysis suggests that both effects are independent and probably mediated by different mechanisms. (+info)
Structural determinants of the eosinophil: chemotactic activity of the acidic tetrapeptides of eosinophil chemotactic factor of anaphylaxis.
The acidic tetrapeptides of ECF-A, Ala/Val-Gly-Ser-Glu, exhibit peak in vitro chemotactic activity for human eosinophils at concentrations of 3 X 10(-8) M to 10(-6) M, and rapidly deactivate eosinophils to homologous and other stimuli at concentrations as low as 10(-10) M. The analogue Leu-Gly-Ser-Glu reaches peak activity at 10(-8)M-10(-7)M, while Phe-Gly-Ser-Glu requires 10(-4)M to elicit a peak response. Although inversion of the order of glycine and serine does not alter the eosinophil chemotactic activity of the tetrapeptides, deletion of glycine increases by 10-fold the concentration required for peak chemotactic activity, indicating the critical nature of the spacing between NH2- and COOH-terminal residues. The substituent COOH-terminal tripeptide, which is only marginally chemotactic, irreversibly suppresses eosinophil chemotactic responsiveness at a concentration 10,000-fold higher than concentrations necessary for deactivation by the intact tetrapeptide. The high concentration of tripeptide required for this cell directed effect, which is assumed to be analogous to deactivation, is attributed to the absence of the NH2-terminal residue which would facilitate effective interaction with the eosinophil. A substituent NH2-terminal tripeptide and amides of the NH2-terminal amino acids, which are devoid of chemotactic and deactivating activities, reversibly inhibit the tetrapeptide stimulus in a dose-response fashion. The additional finding that the NH2-terminal tripeptide protects the eosinophil from deactivation by the intact tetrapeptide confirms that the competitive interaction is stimulus specific. (+info)
Mechanisms of calcium influx into hippocampal spines: heterogeneity among spines, coincidence detection by NMDA receptors, and optical quantal analysis.
Dendritic spines receive most excitatory inputs in the vertebrate brain, but their function is still poorly understood. Using two-photon calcium imaging of CA1 pyramidal neurons in rat hippocampal slices, we investigated the mechanisms by which calcium enters into individual spines in the stratum radiatum. We find three different pathways for calcium influx: high-threshold voltage-sensitive calcium channels, NMDA receptors, and an APV-resistant influx consistent with calcium-permeable AMPA or kainate receptors. These pathways vary among different populations of spines and are engaged under different stimulation conditions, with peak calcium concentrations reaching >10 microM. Furthermore, as a result of the biophysical properties of the NMDA receptor, the calcium dynamics of spines are exquisitely sensitive to the temporal coincidence of the input and output of the neuron. Our results confirm that individual spines are chemical compartments that can perform coincidence detection. Finally, we demonstrate that functional studies and optical quantal analysis of single, identified synapses is feasible in mammalian CNS neurons in brain slices. (+info)
Identification and functional analysis of novel human melanocortin-4 receptor variants.
Inactivation of the melanocortin-4 receptor (MC4-R) by gene-targeting results in mice that develop maturity-onset obesity, hyperinsulinemia, and hyperglycemia. These phenotypes resemble common forms of human obesity, which are late-onset and frequently accompanied by NIDDM. It is not clear whether sequence variation of the MC4-R gene contributes to obesity in humans. Therefore, we examined the human MC4-R gene polymorphism in 190 individuals ascertained on obesity status. Three allelic variants were identified, including two novel ones, Thr112Met and Ile137Thr. To analyze possible functional alterations, the variants were cloned and expressed in vitro and compared with the wild-type receptor. One of the novel variants, Ile137Thr, identified in an extremely obese proband (BMI 57), was found to be severely impaired in ligand binding and signaling, raising the possibility that it may contribute to development of obesity. Furthermore, our results also suggest that sequence polymorphism in the MC4-R coding region is unlikely to be a common cause of obesity in the population studied, given the low frequency of functionally significant mutations. (+info)
Angiotensin II receptor blockade in normotensive subjects: A direct comparison of three AT1 receptor antagonists.
Use of angiotensin (Ang) II AT1 receptor antagonists for treatment of hypertension is rapidly increasing, yet direct comparisons of the relative efficacy of antagonists to block the renin-angiotensin system in humans are lacking. In this study, the Ang II receptor blockade induced by the recommended starting dose of 3 antagonists was evaluated in normotensive subjects in a double-blind, placebo-controlled, randomized, 4-way crossover study. At 1-week intervals, 12 subjects received a single dose of losartan (50 mg), valsartan (80 mg), irbesartan (150 mg), or placebo. Blockade of the renin-angiotensin system was assessed before and 4, 24, and 30 hours after drug intake by 3 independent methods: inhibition of the blood pressure response to exogenous Ang II, in vitro Ang II receptor assay, and reactive changes in plasma Ang II levels. At 4 hours, losartan blocked 43% of the Ang II-induced systolic blood pressure increase; valsartan, 51%; and irbesartan, 88% (P<0.01 between drugs). The effect of each drug declined with time. At 24 hours, a residual effect was found with all 3 drugs, but at 30 hours, only irbesartan induced a marked, significant blockade versus placebo. Similar results were obtained when Ang II receptor blockade was assessed with an in vitro receptor assay and by the reactive rise in plasma Ang II levels. This study thus demonstrates that the first administration of the recommended starting dose of irbesartan induces a greater and longer lasting Ang II receptor blockade than that of valsartan and losartan in normotensive subjects. (+info)
Interactions of a nonpeptidic drug, valacyclovir, with the human intestinal peptide transporter (hPEPT1) expressed in a mammalian cell line.
The results of previous work performed in our laboratory using an in situ perfusion technique in rats and rabbit apical brush border membrane vesicles have suggested that the intestinal uptake of valacyclovir (VACV) appears to be mediated by multiple membrane transporters. Using these techniques, it is difficult to characterize the transport kinetics of VACV with each individual transporter in the presence of multiple known or unknown transporters. The purpose of this study was to characterize the interaction of VACV and the human intestinal peptide transporter using Chinese hamster ovary (CHO) cells that overexpress the human intestinal peptide transporter (hPEPT1) gene. VACV uptake was significantly greater in CHO cells transfected with hPEPT1 than in cells transfected with only the vector, pcDNA3. The optimum pH for VACV uptake was determined to occur at pH 7.5. Proton cotransport was not observed in hPEPT1/CHO cells, consistent with previously observed results in tissues and Caco-2 cells. VACV uptake was concentration dependent and saturable with a Michaelis-Menten constant and maximum velocity of 1.64 +/- 0.06 mM and 23.34 +/- 0.36 nmol/mg protein/5 min, respectively. A very similar Km value was obtained in hPEPT1/CHO cells and in rat and rabbit tissues and Caco-2 cells, suggesting that hPEPT1 dominates the intestinal transport properties of VACV in vitro. VACV uptake was markedly inhibited by various dipeptides and beta-lactam antibiotics, and Ki values of 12.8 +/- 2.7 and 9.1 +/- 1.2 mM were obtained for Gly-Sar and cefadroxil at pH 7.5, respectively. The present results demonstrate that VACV is a substrate for the human intestinal peptide transporter in hPEPT1/CHO cells and that although transport is pH dependent, proton cotransport is not apparent. Also, the results demonstrate that the hPEPT1/CHO cell system has use in investigating the transport kinetics of drugs with the human intestinal peptide transporter hPEPT1; however, the extrapolation of these transport properties to the in vivo situation requires further investigation. (+info)
Role of bkdR, a transcriptional activator of the sigL-dependent isoleucine and valine degradation pathway in Bacillus subtilis.
A new gene, bkdR (formerly called yqiR), encoding a regulator with a central (catalytic) domain was found in Bacillus subtilis. This gene controls the utilization of isoleucine and valine as sole nitrogen sources. Seven genes, previously called yqiS, yqiT, yqiU, yqiV, bfmBAA, bfmBAB, and bfmBB and now referred to as ptb, bcd, buk, lpd, bkdA1, bkdA2, and bkdB, are located downstream from the bkdR gene in B. subtilis. The products of these genes are similar to phosphate butyryl coenzyme A transferase, leucine dehydrogenase, butyrate kinase, and four components of the branched-chain keto acid dehydrogenase complex: E3 (dihydrolipoamide dehydrogenase), E1alpha (dehydrogenase), E1beta (decarboxylase), and E2 (dihydrolipoamide acyltransferase). Isoleucine and valine utilization was abolished in bcd and bkdR null mutants of B. subtilis. The seven genes appear to be organized as an operon, bkd, transcribed from a -12, -24 promoter. The expression of the bkd operon was induced by the presence of isoleucine or valine in the growth medium and depended upon the presence of the sigma factor SigL, a member of the sigma 54 family. Transcription of this operon was abolished in strains containing a null mutation in the regulatory gene bkdR. Deletion analysis showed that upstream activating sequences are involved in the expression of the bkd operon and are probably the target of bkdR. Transcription of the bkd operon is also negatively controlled by CodY, a global regulator of gene expression in response to nutritional conditions. (+info)