Two dinucleotide repeat polymorphisms at the DMD locus.
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Base Sequence↗
Biomedical subjects
S C King
Publications and source records attributed to S C King.
Prognostic importance of restaging gallium scans following induction chemotherapy for advanced Hodgkin's disease.
PURPOSE: This study was intended to assess the ability of restaging gallium scanning to distinguish between patients with residual radiographic abnormalities who still have active Hodgkin's disease (HD) and those who are truly complete responders. Early identification of the former patients might increase the success of secondary salvage therapy. MATERIALS AND METHODS: The charts of all patients with advanced HD treated at Duke University Medical Center during the years 1983 to 1991 who underwent gallium scanning were reviewed. Thirty-three patients were identified who had gallium scans performed as part of restaging following induction combination chemotherapy; no patient had other signs or symptoms of active or progressive HD. RESULTS: Thirteen of 33 patients had positive restaging gallium scans; 20 patients had negative scans. The 4-year actuarial relapse-free survival (RFS) rate was 75% for patients with negative restaging gallium scans compared with 8% for those with positive restaging scans (P < .001). The 4-year actuarial overall survival (OS) rate was 100% for those with negative scans compared with 51% for gallium-positive patients (P = .001). Twenty-four patients had residual chest x-ray or computed tomographic scan abnormalities. Calculated negative and positive predictive values for gallium scanning are 92% and 90%, respectively, compared with values of 48% and 83% for computed tomographic scanning. CONCLUSION: Restaging gallium scans separate complete responders from induction failures with a high degree of accuracy. Gallium scanning is clearly superior to computed tomography in this regard. Patients with advanced HD who have positive restaging gallium scans after induction chemotherapy should be classified as induction failures and are highly unlikely to be cured with involved-field low-dose radiotherapy.
Actuarial Analysis↗
Molecular cloning and structure of the human (GABATHG) GABA transporter gene.
A cDNA molecule encoding the human GABA transporter was synthesized by means of polymerase chain reaction (PCR) technique and used as probe for selecting a human genomic DNA fragment encoding GABA transporter. A positive clone harboring the whole gene was obtained from a human lymphocyte genomic library through utilizing the genomic 'walking' technique. The clone, designated as pHGAT, harbours a DNA fragment of about 39 kb in length inserted into the BamHI site in cosmid pWE15. The gene covers about 25 kb in length and is constituted by four EcoRI restricted fragments which are 13.7 kb, 3.1 kb, 4.2 kb and 7.2 kb long, respectively. The genomic clone contains 15 introns, including two introns prior to the initiator methionine (i.e., the translation start site is in exon 3). Eleven exons encode the twelve transmembrane regions in the transporter protein. Thus as in the case for a number of other membrane proteins, there appears to be a strong tendency for the putative transmembrane domains to be encoded by separate exons. It is noted that the structure of the human GABA transporter gene reported here differs from the mouse gene which is contains 12 introns.
Base Sequence↗
Differential regulation of the imipramine-sensitive serotonin transporter by cAMP in human JAr choriocarcinoma cells, rat PC12 pheochromocytoma cells, and C33-14-B1 transgenic mouse fibroblast cells.
The imipramine-sensitive serotonin transporter appears to be the receptor for clinically important antidepressants. Some studies suggest that this protein may fall under the influence of abnormal and as yet uncharacterized regulatory effects during depressive illness. Despite these putative disease-related effects, regulation has never been demonstrated in either the platelet or synaptosome model systems. Here we demonstrate for the first time that the imipramine-sensitive serotonin transport activity in either JAr choriocarcinoma or PC12 pheochromocytoma cell lines is subject to regulation by cAMP. Unexpectedly, the regulatory effect is opposite in the two cases, causing stimulation in JAr (increased Vmax) and inhibition in PC12 (kinetically complex). Appearance of these kinetic effects lagged 15 to 20 hours behind peak cAMP levels. The results are consistent with the interesting possibility that different tissues may express isoforms of the recently cloned serotonin transporter cDNAs. We suggest, therefore, that JAr and PC12 are attractive models in which to pursue detailed analysis of serotonin transport and its manner of regulation by cAMP.
1-Methyl-3-isobutylxanthine↗
Amino acid substitution in the lactose carrier protein with the use of amber suppressors.
Five lacY mutants with amber stop codons at known positions were each placed into 12 different suppressor strains. The 60 amino acid substitutions obtained in this manner were tested for growth on lactose-minimal medium plates and for transport of lactose, melibiose, and thiomethylgalactoside. Most of the amino acid substitutions in the regions of the putative loops (between transmembrane alpha helices) resulted in a reasonable growth rate on lactose with moderate-to-good transport activity. In one strain (glycine substituted for Trp-10), abnormal sugar recognition was found. The substitution of proline for Trp-33 (in the region of the first alpha helix) showed no activity, while four additional substitutions (lysine, leucine, cysteine, and glutamic acid) showed low activity. Altered sugar specificity was observed when Trp-33 was replaced by serine, glutamine, tyrosine, alanine, histidine, or phenylalanine. It is concluded that Trp-33 may be involved directly or indirectly in sugar recognition.
Amino Acid Sequence↗
The interaction between aspartic acid 237 and lysine 358 in the lactose carrier of Escherichia coli.
The lacY from Escherichia coli strains 020 and AE43 have been cloned on plasmids which were designated p020-K358T and pAE43-D237N. These lacY mutants contain amino acid substitutions changing Lys-358 to Thr or Asp-237 to Asn, respectively. The charge neutralizing effect of each mutation is associated with a functional defect in melibiose transport which we exploited in order to isolate second site revertants to the melibiose-positive phenotype. Eleven melibiose-positive revertants of p020-K358T were isolated. All contained a second-site mutation converting Asp-237 to a neutral amino acid (8 to Asn, 1 to Gly, and 2 to Tyr). Twelve melibiose-positive revertants of pAE43-D237N were isolated. Two were second-site revertants converting Lys-358 to a neutrally Gln residue, while the remainder directly reverted Asn-237 to the wild-type Asp-237. We conclude that the functional intimate relationship between Asp-237 and Lys-358 suggests that these residues may be closely juxtaposed in three-dimensional space, possibly forming a 'charge-neutralizing' salt bridge.
Amino Acid Sequence↗
Characterization of Escherichia coli lactose carrier mutants that transport protons without a cosubstrate. Probes for the energy barrier to uncoupled transport.
The Escherichia coli lactose carrier is an energy-transducing H+/galactoside cotransport protein which strictly couples sugar and proton transport in 1:1 stoichiometry. Here we describe five lactose carrier mutants which catalyze "uncoupled" sugar-independent H+ transport. Symptoms similar to uncoupling by a proton ionophore have been observed in cells expressing these mutant carriers. The mutations occur at two separate loci, encoding substitutions either for alanine 177 (valine) or tyrosine 236 (histidine, asparagine, phenylalanine, or serine). Compared to the parent, cells expressing the valine 177 carrier grew slowly on minimal media with glucose as carbon source. When washed cells were incubated in the absence of added sugars the mutant showed a reduced protonmotive force compared with the parent. Addition of either thiodigalactoside or alpha-p-nitrophenylgalactoside reduced the defect in protonmotive force. Sugar-independent H+ entry rate into cells expressing either the normal carrier or the Val-177 mutant were measured directly using the pH electrode. Following sudden acidification of the external medium (by either oxygen-pulse or acid-pulse) protons entered more rapidly into cells expressing the Val-177 carrier. This novel sugar-independent mode of H+ transport probably depends on an acquired capacity of the Val-177 carrier to bind the transported proton with higher than normal affinity in a transition state involving the binary carrier/H+ complex.
Biological Transport↗
Identification of valine 177 as a mutation altering specificity for transport of sugars by the Escherichia coli lactose carrier. Enhanced specificity for sucrose and maltose.
A mutant of the Escherichia coli lactose carrier has been selected (in an invertase-positive strain) based on its ability to grow on 6 mM sucrose in a manner dependent upon lactose carrier induction by isopropyl-1-thio-beta-D-galactopyranoside. The mutant was cloned, and DNA sequencing revealed a point mutation in lacY which changed alanine 177 to valine. The valine 177 mutation increased the transport rate for both [14C]sucrose and the maltose analog 4-nitrophenyl-alpha-maltoside. The potency for inhibition of beta-ONPG transport by several sugars containing the glucopyranosyl moiety (maltose, cellobiose, or palatinose) was increased significantly relative to the parental carrier. Similar experiments showed that the mutation did not affect the affinity for such commonly studied substrates as 4-nitrophenyl-alpha-D-galactopyranoside and beta-D-galactopyranosyl-1-thio-beta-D-galactopyranoside. These data indicate that gross structural alteration of the galactoside binding site cannot account for increased transport of sucrose and maltose by the valine 177 mutant. We conclude that effects of the valine 177 mutation are not limited strictly to changes in observed sugar affinity and that sugar-specific changes in turnover number may be an important determinant of the altered spectrum of sugar specificities exhibited by the Val-177 carrier. These phenomena may be related to the effect of this mutation on proton recognition (described in King, S.C., and Wilson, T.H. (1990) J. Biol. Chem. 265, 9645-9651).
Biological Transport↗
Sensitivity of efflux-driven carrier turnover to external pH in mutants of the Escherichia coli lactose carrier that have tyrosine or phenylalanine substituted for histidine-322. A comparison of lactose and melibiose.
Two Escherichia coli lactose carrier mutants (tyrosine or phenylalanine substituted for histidine 322) were studied under conditions of net efflux or equilibrium exchange. Net lactose efflux by either mutant was 10-20-fold slower than by the parent and was sensitive to extracellular pH (5.6-8.0). The presence of extracellular lactose (equilibrium exchange) failed to accelerate loss of [14C]lactose, indicating that the step(s) rate limiting for exchange were also rate limiting for net lactose efflux. Net melibiose efflux by the Phe-322 mutant was comparable to the normal carrier, while that by the Tyr-322 mutant was 5-fold faster (pH 7.0). Melibiose efflux by either mutant was sensitive to pH (5.6-8.0). Melibiose in the extracellular medium significantly accelerated loss of [3H]melibiose from either mutant, showing that slow exchange is a sugar-specific phenomenon and not an intrinsic property of these mutants. The sugar-specific effect of these mutations could mean that the defect in these mutants is not on the path of the proton, although alternative explanations cannot as yet be eliminated. The modest effect of these mutations on the transport rate indicates that His-322 contributes a far smaller free energy increment to catalyzing of H+/galactoside cotransport than active site histidines contribute to catalyzing peptide bond hydrolysis in serine proteases. We interpret this to mean that in chemical terms the function of these catalytic histidine residues differ considerably.
Disaccharides↗
Mechanism of enhanced melibiose transport rate catalyzed by an Escherichia coli lactose carrier mutant with leucine substituted for serine-306. The pH-dependence of melibiose efflux.
The mechanism of melibiose transport was studied in cells containing plasmid pAA22 which expresses the mutant lactose carrier (serine-306 to leucine) cloned from Escherichia coli AA22. These studies were of interest because several lines of evidence suggested that the AA22 mutation conferred novel properties upon the lactose carrier, decreasing turnover with several beta-galactoside substrates, increasing turnover with melibiose, and abolishing active accumulation even though equilibration occurred via symport with H+. Although severely defective in active melibiose accumulation, the present study indicates that in cells poisoned with azide the AA22 carrier does in fact equilibrate melibiose across the membrane more rapidly than the normal lactose carrier. Similarly, melibiose efflux from cells preloaded with melibiose was more rapidly catalyzed by the AA22 carrier than by the normal carrier (pH 7.0). Furthermore, although external H+ did reduce net melibiose efflux to a rate slower than seen in equilibrium exchange, a lower than normal pH was required to achieve this effect. Therefore, at pH 7.0, the AA22 carrier (but not the normal carrier) catalyzed net efflux at a rate approaching that for the exchange process (which was pH-resistant in both the mutant and the parent). At pH 8.0 both the AA22 carrier and the normal carrier catalyzed net melibiose efflux at a rate identical to the equilibrium exchange rate. We suggest (i) that the sensitivity of melibiose efflux to external pH indicates that during efflux the AA22 carrier interacts with protons in a manner similar to the normal carrier (i.e., sugar is cotransported with H+) and hence the absence of accumulation is not explained by internal leak via a binary carrier-melibose complex; and (ii) that the modest increase in rate constants for melibiose exit reflect small changes in activation energy (1 kcal/mol) consistent with a steric mechanism possibly involving van der Waals contacts.
Biological Transport↗
Towards an understanding of the structural basis of 'forbidden' transport pathways in the Escherichia coli lactose carrier: mutations probing the energy barriers to uncoupled transport.
Recent progress in the analysis of mutants of the Escherichia coli lactose carrier function is reviewed, with special emphasis on the structural basis for energy barriers which prevent 'forbidden' conformational changes. Mutations which break down the barriers to forbidden isomerizations involving the binary carrier:sugar (CS) and carrier:proton (CH) complexes have been obtained in several laboratories. These mutants allow uncoupled transport of H+ or galactoside in the lactose carrier which normally couples cation and sugar movement in a 1:1 stoichiometry. These uncoupled mutants appear to be associated with changes in both sugar and cation recognition, suggesting that the physical interactions forming the basis for co-substrate recognition and uncoupling are not independently variable. By postulating that translocation involves transformation of the stable intermediate of the co-transport cycle to unstable transition state conformations of the carrier, it is possible to consider the consequences of mutagenesis in terms of transition state theory. Consistent with several experimental observations, the analysis predicts in each mutant the occurrence of more than one abnormality in the transport cycle (such as changes in sugar recognition, cation recognition or the coupling reaction). We have called the general phenomenon a 'mutational double-effect' because any mutation which alters the Gibbs free energy change of one reaction in the transport cycle must affect the free energy change of at least one other reaction in this cycle.
Biological Transport↗
Galactoside-dependent proton transport by mutants of the Escherichia coli lactose carrier: substitution of tyrosine for histidine-322 and of leucine for serine-306.
The lac Y genes from two Escherichia coli mutants, MAB20 and AA22, have been cloned in a multicopy plasmid by a novel 'sucrose marker exchange' method. Characterization showed that the plasmids express a lactose carrier with poor affinity for lactose. Neither mutant carried out concentrative uptake with methyl beta-D-galactopyranoside, lactose, or melibiose as the substrate. Nor did the mutants catalyze counterflow or exchange with methyl beta-D-galactopyranoside. Both mutants did, however, retain the capacity to carry out facilitated diffusion with lactose or melibiose. DNA sequencing revealed that MAB20 (histidine-322 to tyrosine) and AA22 (serine-306 to leucine) have amino acid substitutions within the putative 'charge-relay' domain thought to be responsible for proton transport. Galactoside-dependent H+ transport was readily measured in both mutants. We conclude, therefore, that the presence of a histidine residue at position 322 of the lactose carrier is not obligatory for H+ transport per se.
Binding Sites↗
Galactoside-dependent proton transport by mutants of the Escherichia coli lactose carrier. Replacement of histidine 322 by tyrosine or phenylalanine.
Mutations have been introduced into the Escherichia coli lac Y gene by oligonucleotide-directed mutagenesis such that the lactose carrier contains either tyrosine or phenylalanine in place of histidine 322. These mutants did not carry out active accumulation of lactose, melibiose, or methyl-beta-D-galactopyranoside, but facilitated diffusion was still catalyzed. Galactoside-dependent H+ transport, measured with the pH electrode, was retained in both mutants. We conclude that although histidine 322 is important for energy transduction, neither an electronegative atom nor a dissociable proton is essential for proton cotransport with lactose or melibiose.
Biological Transport↗
Transport-specific fractionation for purification of ATP-dependent Ca2+ pumps.
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Animals↗
Total respiratory resistance in chronic obstructive pulmonary disease.
Using a recently described automated system, we measured total respiratory resistance (Rrs) of 59 patients with obstructive lung disease during spontaneous tidal breathing. We also measured airway resistance (Raw) and various other indices of pulmonary function. The results demonstrate that this system gives values of Rrs which correlate well with other objective measures of the severity of obstructive lung disease. It can also be used to detect changes in resistance after inhalation of a bronchodilator. In 10 patients we measured Raw during tidal breathing and computed the total non-airway resistance (Rnaw) as the difference between Rrs and Raw. Rnaw made up a larger portion of Rrs than would be expected in these patients. These results could not be accounted for by differences in breathing frequency, and may instead have been caused by differences between the times in the ventilatory cycle at which Rrs and Raw were measured. We cannot exclude the possibility that Rnaw was abnormally elevated in these patients.
Adult↗
Measurements of components of resistance to breathing.
Total respiratory resistance (Rrs), pulmonary resistance (RL), and airway resistance (Raw) of 12 male patients with chronic obstructive lung disease were measured by the partial occlusion method, intraesophageal balloon technique, and body plethysmography, respectively. Chest wall resistance (Rew) and lung tissue resistance (Rti) were computed. Percentages of Rew/Rrs, RL/Rrs, Raw/Rrs, Rti/Rrs, Raw/RL, and Rti/RL were calculated. The magnitude of the components of resistance to breathing of this study and the data appearing in the literature are compared. Wide variation between the data reported by various authors was observed. The possible causes of these variations are discussed.
Aged↗
Biochemical and immunological evidence for a calcium pump in chromaffin granules.
ATP stimulated the accumulation of 45Ca2+ by chromaffin granule ghosts which contained 5 mM oxalate to trap transported calcium within the lumen. Inasmuch as the ATP-dependent 45Ca2+ transport was resistant to 25 mM ammonium acetate as well as the proton ionophore, carbonylcyanide-m-chlorophenylhydrazone, the chromaffin granule proton translocating ATPase does not provide the energy for this process. Instead, we suggest that chromaffin granules contain a calcium translocating ATPase which catalyzes the 45Ca2+ uptake directly. The observation that chromaffin granules bind to a monoclonal antibody raised against a calcium pump from bovine brain supports this hypothesis.
Adenosine Triphosphate↗