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Biomedical subjects

R T Walker

Publications and source records attributed to R T Walker.

At least 73 records · Page 4Linked to original sources
PubMed1987

A comparison of the conformations adopted by some 5-bromovinyl-2'-deoxyuridines and a correlation with their antiviral properties: an X-ray study.

Crystal structures of (Z)-5-(2-bromovinyl)-2'-deoxyuridine, 3',5'-di-O-acetyl-(E)-5-(2-bromovinyl)-2'-deoxyuridine and 3',5'-di-O-p-chlorobenzoyl-5-(2-dibromovinyl)-2'-deoxyuridine are compared with each other and with that of the most potent antiviral agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (E-BVDU) reported earlier. A comparison of the conformation of 3',5'-di-O-acetyl-pyrimidine nucleoside structures in which intermolecular hydrogen bond network formation is minimized, with those of their parent compounds has shown that the greatest change in rotation about the glycosyl bond and in the sugar ring pucker is exhibited by E-BVDU. Upon acylation this molecule changes from C2'-endo/C3'-exo conformation to C3'-endo/C4'-exo conformation. The relevance of these structures upon the biological activity of the nucleosides and in particular to their ability to be a substrate for thymidine kinase is discussed.

Antiviral Agents↗
PubMed1987

The synthesis of some branched-chain-sugar nucleoside analogues.

1-(2,3-Epoxy-5-O-trityl-beta-D-lyxofuranosyl)uracil was treated with a number of carbon nucleophiles. Ethynyl lithium gave 3'-deoxy-3'-ethynyl-5'-O-trityl-ara-uridine, which was reduced to the corresponding 3'-ethenyl compound. Sodium cyanide gave 3'-cyano-3'-deoxy-5'-O-trityl-ara-uridine which upon alkaline hydrolysis gave the corresponding 3'-carboxamido compound. 1,3-Dithian-2-yl lithium gave 3'-deoxy-3'-(1,3-dithian-2-yl)-5'-O-trityl-ara-uridine. The trityl group was removed from each of these compounds by mild acidic hydrolysis. Treatment of 2 with 0.1M H2sO4 and mercury (II) acetate afforded 3'-acetyl-3'-deoxy-ara-uridine which upon reduction with NaBH4 gave 3'-deoxy-3'-(1-hydroxyethan-1-yl)-ara-uridine. Acetylation of 6 yielded 5'-O-acetyl-3'-acetyl-2',3'-didehydro-2',3'-dideoxyuridine which upon reduction with NaBH4 produced a mixture of 5'-O-acetyl-2',3'-didehydro-2',3'-dideoxy-3'-(1-hydroxyethan -1-yl)uridine and 1-(R)[5-(S)-acetoxymethyl-4-(1-hydroxyethan-1-yl)-tetrahydrofuran- 2-yl]- uracil. Reduction of 14 with Raney nickel followed by removal of the trityl group gave 3'-deoxy-3'-methyl-ara-uridine.

Indicators and Reagents↗
PubMed1987

Substrate specificity of DNA polymerases. I. Enzyme-catalysed incorporation of 5-'1-alkenyl)-2'-deoxyuridines into DNA.

A series of (E)-5-(1-alkenyl)-dUTPs as well as 5-vinyl-and (Z)-5-(1-propenyl)-dUTP have been synthesized to study steric requirements in DNA polymerase reactions. Experiments were carried out in E. coli DNA polymerase I Klenow fragment enzyme system. Substrates were characterized by KM and Vmax-values, initial incorporation rates as well as by total extent of incorporation of the analogues into poly(dA-dT) as a template-primer. Incorporation of the analogues could be best correlated with Vmax-values as well as the very similar initial incorporation rate values. Reactivity (Vmax/KM) showed no correlation with the extent of incorporation. 5-Vinyl-dUTP proved to be as good a substrate of the enzyme as dTTP, whereas (E)-5-(1-heptenyl)-and (E)-5-(1-octenyl)-dUTPs were very poor substrates, their incorporation was strongly limited and they also proved to be very efficient inhibitors of DNA replication, as shown by Ki-values. Substrate specificity of the Klenow enzyme can be explained by the steric hindrance of C-5 substituent, by the "orientational steric substituent effect" concept.

Alkenes↗
PubMed1987

Organization and structure of tRNA genes in Spiroplasma melliferum.

To investigate the organization of tRNA genes in the honeybee pathogen, Spiroplasma melliferum (previously referred to as Spiroplasma sp. BC3), total labeled tRNA from S. melliferum was used as a hybridization probe to an EcoRI digest of the genomic DNA. The results show two, or possibly three, strongly hybridizing bands. Comparing the pattern obtained to that of Bacillus subtilis DNA suggests either that the S. melliferum tRNA genes are more extensively clustered or, more likely, that the S. melliferum genome does not encode a full complement of tRNA genes, as has been suggested for other Mollicutes. We screened a library of EcoRI fragments of S. melliferum DNA cloned in pBR322 with total radioactive tRNA as a probe and selected one of the tRNA gene clusters. Subsequent sequence analysis of a portion of the clone showed 10 tRNA genes probably comprising a single operon. Comparison of sequence with a tRNA gene cluster from Mycoplasma mycoides and a portion of a cluster from B. subtilis showed an identical order of tRNA genes and the isoacceptors encoded. Such a striking comparison in gram-positive eubacteria suggests an important function for regulation and co-transcription of these tRNA genes.

Bacillus subtilis↗
PubMed1986

Synthesis of two diastereoisomeric p-nitrophenyl phosphodiesters of 2',3'-secouridine and their affinity for phosphodiesterases.

The synthesis of the p-nitrophenyl esters of the 5'- and 3'-phosphates of the nucleoside analogue 2',3'-secouridine are described. Unlike the corresponding diesters of thymidine, these two compounds are diastereoisomers. Their affinity for phosphodiesterases types I and II were investigated. Both analogues were hydrolysed very slowly by snake venom phosphodiesterase but their affinity for the enzyme was similar to that of the p-nitrophenyl ester of thymidine 5'-monophosphate of which they were both competitive inhibitors with Ki approximately Km. Neither compound was hydrolysed by spleen phosphodiesterase but both competitively inhibited the p-nitrophenyl ester of thymidine 3'-monophosphate, with Ki's slightly higher than the Km. Although for each enzyme the Ki of the correct analogue phosphodiester (i.e. the 5'-derivative for snake venom and the 3'-derivative for spleen) was the lower, the absolute specificity seen for the normal substrates had been lost.

Affinity Labels↗
PubMed1986

Transcutaneous oxygen tension during exercise in patients with claudication.

Transcutaneous oxygen tension (TcPO2) was monitored during maximal exercise in 10 patients with stable moderate to severe claudication. The TcPO2 fell by 16% at the onset of claudication and 32% at the maximum walking distance. On resting this decrease reached a maximum of 66% roughly four minutes after exercise. This was followed by a steady recovery. The percentage changes were reproducible in each patient and were appreciably different from the TcPO2 exercise profiles of normal healthy volunteers. TcPO2 monitoring during exercise is a simple, reproducible, cheap, and useful technique for assessing claudication and compares favourably with other techniques used to quantify this condition.

Aged↗
PubMed1986

Synthesis and antiviral activity of (E)-5-(2-bromovinyl)uracil and (E)-5-(2-bromovinyl)uridine.

(E)-5-(2-Bromovinyl)uracil (BVU) and (E)-5-(2-bromovinyl)uridine (BVRU) were synthesized starting from 5-formyluracil via (E)-5-(2-carboxyvinyl)uracil or starting from 5-iodouridine via (E)-5-(2-carbomethoxyvinyl)uridine and (E)-5-(2-carboxyvinyl)uridine, respectively. Depending on the choice of the cell system, BVU and BVRU exhibited a marked activity against herpes simplex virus type 1 (HSV-1) in vitro. Although BVU and BVRU were less potent than the reference compound (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), their antiviral activity spectrum was remarkably similar to that of BVDU. The latter findings suggest that BVU and BVRU are metabolically converted to BVDU or a phosphorylated product thereof. In vivo, BVU protected mice against a lethal disseminated HSV-1 infection.

Animals↗
PubMed1985

Construction of the mycoplasma evolutionary tree from 5S rRNA sequence data.

The 5S rRNA sequences of eubacteria and mycoplasmas have been analyzed and a phylogenetic tree constructed. We determined the sequences of 5S rRNA from Clostridium innocuum, Acholeplasma laidlawii, Acholeplasma modicum, Anaeroplasma bactoclasticum, Anaeroplasma abactoclasticum, Ureaplasma urealyticum, Mycoplasma mycoides mycoides, Mycoplasma pneumoniae, and Mycoplasma gallisepticum. Analysis of these and published sequences shows that mycoplasmas form a coherent phylogenetic group that, with C. innocuum, arose as a branch of the low G+C Gram-positive tree, near the lactobacilli and streptococci. The initial event in mycoplasma phylogeny was formation of the Acholeplasma branch; hence, loss of cell wall probably occurred at the time of genome reduction to approximately to 1000 MDa. A subsequent branch produced the Spiroplasma. This branch appears to have been the origin of sterol-requiring mycoplasmas. During development of the Spiroplasma branch there were several independent genome reductions, each to approximately 500 MDa, resulting in Mycoplasma and Ureaplasma species. Mycoplasmas, particularly species with the smallest genomes, have high mutation rates, suggesting that they are in a state of rapid evolution.

Base Sequence↗
PubMed1985

Serotonin and the flow properties of blood.

The effects of serotonin and its pharmacological antagonists on the physical flow properties of the blood have been studied far less than their effects on blood vessels, although they may be equally important. Indirect evidence suggests that in pathological circumstances serotonin may locally increase whole blood viscosity, particularly at low shear rates, decrease red cell deformability and increase the adhesiveness of white cells. Although the viscosity of the plasma alone is not affected, the rheological effects of serotonin on blood cells is probably dependent on the presence of platelets. These mechanisms may have a systemic effect in some forms of hypertension as well as in situations of local ischaemia such as Raynaud's phenomenon, atherosclerotic pregangrene of the leg or acute myocardial infarction. Specific serotonergic-antagonists, administered either orally or intravenously, normalize the increased whole blood viscosity and decreased blood filterability found in essential hypertension, following myocardial infarction and in severe leg ischaemia. The effect on red cell deformability is usually greatest when the cells are resuspended in platelet rich plasma. Ketanserin given intravenously for seven days to patients with very severe leg ischaemia, significantly improves whole blood viscosity, increases red cell transit time and most dramatically decreases pore clogging. This last effect was at least partly due to a change in the physical properties, but not the number of the white cells. The reported beneficial clinical effects of such an antagonist in various forms of peripheral ischaemia and essential hypertension may well be due, at least partly, to the normalization of the rheological properties of the blood.

Blood Flow Velocity↗