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Biomedical subjects

B D Michael

Publications and source records attributed to B D Michael.

At least 91 records · Page 5Linked to original sources
PubMed1986

Ultrafast chemical repair of DNA single and double strand break precursors in irradiated V79 cells.

The fast kinetics of reactions of free radical precursors of DNA single strand breaks (ssb) and double strand breaks (dsb) have been determined in Chinese hamster V79 cells by fast mixing and irradiation methods using the alkaline unwinding technique to assay breaks. Fast chemical repair of oxygen-dependent ssb and dsb precursors was observed and approached completion within 10-20 ms of irradiation. Treatment of cells with the glutathione synthesis blocking agent, buthionine sulphoximine, showed that approximately half of the chemical repair was attributable to intracellular non-protein thiols. The nature of the residual repair is obscure, but it is apparently not attributable to non-protein thiols. Similar repair rates and thiol dependences were also found for cell kill. With all three endopoints, oxygen competes with and blocks the chemical repair.

Animals↗
PubMed1985

Pulse radiolysis studies of the interactions of the sulfhydryl compound dithiothreitol and sugars.

Pulse radiolysis studies of the hydrogen atom transfer ("repair") reaction from the sulfhydryl-containing (RSH) compound dithiothreitol (DTT) to the DNA sugar deoxyribose and to several related sugars have been undertaken. The H transfer reaction is measured by monitoring the transient absorbance of the radical-anion RSSR-. The H atom transfer reactions for some sugars were fitted by a single time exponential function, but other sugars exhibited both a fast and a slow component (approximately 10-fold difference in rates) to the reaction. The reaction rates for the slow stage of the reaction between DTT and the sugars ranged from 0.5 X 10(7) dm3 mole-1 sec-1 for ribose-5-phosphate to 9 X 10(7) dm3 mole-1 sec-1 for 2-deoxyglucose. The maximum extent of total repair varied from 60% for ribose-5-phosphate to 100% for 2-deoxyglucose. The rate of repair, the extent of repair, and the appearance of more than one component of repair seem to depend on several factors: The occurrence of radical-radical reactions in the system is indicated by the demonstration of a dose dependence of the reaction kinetics, and this affects the observed rate of formation of RSSR-. Sugars with a deoxy group on the 2-carbon atom seem to have enhanced rates and extents of repair and to exhibit both fast and slow components to the reaction. The presence of a phosphate group on the sugar causes a decrease in the rate and extent of repair. The biological relevance of the reactions studied herein is discussed and the rates obtained are compared with rates for repair of damage in certain radiobiological systems.

DNA Repair↗
PubMed1984

Multifraction irradiation of mouse bladders.

The response of mouse bladders to multifraction irradiation was assessed from increases in urination frequency or the reduction in bladder capacity after irradiation. A range of electron doses were given as 1, 2, 5, 10 or 20 equal fractions in overall treatment times of 1-2 weeks. Dose-related increases in urination frequency were measured from 10 to 14 months after irradiation and a dose-related reduction in bladder capacity (at inflation pressures of 20 mm Hg) was apparent at the time of sacrifice. The extent of repair of sublethal and potentially lethal damage was estimated from a comparison of the isoeffective doses in fractionated regimes and single dose treatments. After small doses per fraction (2.5-6 Gy), the extent of repair in bladder was very similar to that in mouse skin. After larger doses per fraction (greater than 8 Gy) slightly more repair was seen in bladder than skin. Linear-quadratic analysis of the data suggests quite a high value for the ratio alpha/beta, in the region of 5 to 10 Gy. This is higher than the alpha/beta ratios which have been reported for most other slowly dividing normal tissues.

Animals↗
PubMed1984

Effects of oxygen and sulphydryl-containing compounds on irradiated transforming DNA. II. Glutathione, cysteine and cysteamine.

This paper extends our earlier observations on the effects of the sulphydryl (SH)-containing compound dithiothreitol (DTT) on the radiation response of Bacillus subtilis transforming DNA to three other SH-containing compounds-cysteamine, cysteine and glutathione (GSH). In general, all four compounds protect transforming DNA in a manner which is dependent on gassing conditions. In O2, the protection is consistent with the scavenging of OH radicals by the SH compounds, but in N2 there is additional protection which may be due to hydrogen atom donation from the SH compound to radiation-induced DNA lesions, a process which is blocked by O2. This additional protection in N2 results in an increase in the ratio of inactivation in the absence and presence of oxygen with increasing SH concentration to a maximum followed by a decrease at high SH concentrations. The maximum value of the ratio and the SH concentration at which it occurs depend on the SH compound. In particular, GSH appears to be significantly less efficient in the hydrogen-donation repair reaction with transforming DNA than are the other three SH compounds. Furthermore, on the basis of our results, we postulate the existence of a damage fixation process which occurs in the absence of O2, in competition with damage repair by SH compounds, and that this anoxic damage fixation occurs at a rate not less than 300 s-1. We also demonstrate here that the damage fixing reaction of O2 with transforming DNA radicals proceeds 200-fold faster than the competing repair reaction by hydrogen-donation from DTT.

Bacillus subtilis↗
PubMed1984

Effects of oxygen and sulphydryl-containing compounds on irradiated transforming DNA. III. Reaction rates.

The absolute rate for the repair reaction of radiation-induced, oxygen-dependent lesions in bacterial transforming DNA with the sulphydryl (SH)-containing compound dithiothreitol (DTT) has been determined using a fast response method, the gas explosion technique, to be 1.6 X 10(6) mol-1 s-1. Glutathione reacts ten times slower than DTT with the irradiated transforming DNA. It can also be calculated that transforming DNA radicals react with O2 in a damage-fixing reaction with a rate of about 3 X 10(8) dm3 mol-1 s-1. These rates are compared with values in the literature for reaction rates of SH, compounds and O2 with irradiated DNA constituents and with bacterial cells.

Bacillus subtilis↗
PubMed1982

Comparative studies of hypoxic-cell radiosensitization using artificially hypoxic skin in vivo.

The survival of epidermal cells in vivo has been used to assess potential radiosensitizers. Mouse skin was made acutely hypoxic for the irradiations, to give radioprotection by a factor of 2.7-3.0. Several concentrations of each drug were used to determine whether any of them were more effective sensitizers than misonidazole. The SER at each concentration was determined from radiobiological dose-response curves. The blood concentration and toxicity of the compounds were also determined. The sensitizing efficiency, assessed in several ways, indicated that only Ro 03-8799 gave significantly greater sensitization than misonidazole, and then only when assessed by comparing the compounds on the basis of equimolar blood concentrations. If the comparison of efficiency was made in terms of LD50 the ranking order change. The need for a more clinically relevant test of peripheral neurotoxicity is stressed.

Animals↗
PubMed1981

Effects of oxygen and sulphydryl-containing compounds on irradiated transforming DNA. Part I. Actions of dithiothreitol.

The actions and interactions of oxygen and the sulphydryl-containing compound dithiothreitol (DTT) upon the radiation sensitivity of the biological activity of purified Bacillus subtilis transforming DNA have been examined. It has previously been shown that the sensitivity of transforming DNA irradiated in dilute solution is less when irradiation is performed in 100 per cent O2 than when in 100 per cent N2, i.e. O2 protects transforming DNA with a dose-modifying factor of about 0.7. DTT protects transforming DNA in a manner that is dependent on DTT concentration and on gassing conditions. In O2 the DTT protection can largely be attributed to the scavenging of .OH radicals by the DTT, but in anoxia DTT exerts a further protective effect which results in an increasing oxygen enhancement ratio (o.e.r.) with increasing DDT concentration to a maximum o.e.r. of about 14 at 2-5 mM DTT. This additional protective effect of DTT is attributable to hydrogen atom donation from DTT to DNA radicals, thus chemically repairing the DNA. Oxygen appears to block this chemical repair reaction.

Bacteria↗
PubMed1981

Repair during fractionated irradiation of the mouse bladder.

The capacity for repair of sublethal damage during fractionated irradiation of the mouse bladder has been measured. Graded doses of electrons were given as 1, 2, or 5 equal daily fractions. Two functional end points were used to assess bladder damage: (a) increased urination frequency and b) decreased bladder capacity. Repair of sublethal injury within 24 hours was found to be similar using both assays for bladder damage and was greater than the repair observed in mouse skin for a given dose per fraction. The possibility of a slower repair process occurring in bladder was investigated by giving two fractions in increasing overall times (from 24 hours to one month). No increased repair was observed with the longer time intervals; hence there was no evidence for slow repair in the bladder.

Animals↗
PubMed1978

A fast kinetics study of the modes of action of some different radiosensitizers in bacteria.

Using a fast mixing a irradiation technique, the gas explosion method, with Serratia marcescents, the decay of oxygen-dependent damage is found to consist of a fast and a slow stage, each of which is associated with a sub-component of this damage. In the present work, the interactions of these components with radiosensitizers are examined. At low concentrations O2, TAN (a nitroxyl) and misonidazole all preferentially sensitize the slow-stage damage. At higher concentrations, O2 and TAN sensitize the fast-stage damage by a fixation reaction that competes with its repair; in contrast, misonidazole appears mainly to operate by reaction with an earlier, ever shorter form of oxygen-dependent damage.

Kinetics↗
PubMed1978

Effects of some hypoxic cell radiosensitizers on the decay of potentially lethal oxygen-dependent damage in fully hydrated spores.

Using a stopped-flow mixing and pulsed irradiation apparatus, a study has been made of the decay, to a harmless form, of radiation-induced species that would otherwise be lethal to spores on contact with oxygen. Aqueous suspensions of Bacillus megaterium spores were irradiated with electrons for approximately 1 s; at various times after irradiation oxygen in solution was added. As the interval between anoxic irradiation and introduction of oxygen increased, the fraction of spores surviving increased. This change in survival reflects the decay of potentially lethal species. The presence of electron-affinic radiosensitizers during irradiation enhanced the decay rate of this damage, the greatest enhancement being seen with sensitizers of the highest electron affinity. In contrast, the nitroxyl-free radical sensitizer TAN fixed the radiation-induced damage so that no increase in survival, and hence no decay, was seen.

Bacillus megaterium↗
PubMed1977

The decay of potentially lethal oxygen-dependent damage in fully hydrated Bacillus megaterium spores exposed to pulsed electron irradiation.

Using a stopped-flow mixing and pulsed irradiation apparatus, a study has been made of the decay, to a harmless form, of radiation-induced species which would otherwise be lethal to spores on contact with oxygen. Aqueous suspensions of Bacillus megaterium spores were irradiated with 600 krad of electrons given over approximately 1 s; at various times after irradiation oxygen in solution was added. As the interval between irradiation and introduction of oxygen increased, the fraction of spores surviving increased. For spores irradiated in a deoxygenated condition the decay of the potentially lethal species, reflected by this change in survival, proceeded as if two parallel first-order reactions with half-lives of 9 and 120 s operate. In contrast, for spores equilibrated with oxygen and then irradiated, the decay is described by a single first-order expression with an associated half-life similar to that of the faster of the two reactions operating in anoxia.

Bacillus megaterium↗