E.s.r. of free radicals in irradiated uracil-beta-D-arabinofuranoside.

Electron-spin-resonance measurements have been made on single crystals of uracil-beta-D-arabinofuranoside, which were irradiated by 4-0 MeV electrons at 77 K. At low temperatures, two radicals have been identified, one attributed to a hydrogen abstraction from 05' in the sugar moiety and the other to a radical anion located on the pyrimidine ring. The former is very unstable and seems to act as a precursor to other unidentified radical species stable at 77K. At room temperature, the main resonance is due to hydrogen addition to C5 and is probably produced by protonation of the anion. This same radical is also produced by X-irradiation at room temperature.     

Radical formation in salts of pyrimidines. II. Cytosine. HCl crystals.

Radicals produced by X-irradiation at 77 K and at 300 K of cytosine. HCl crystals have been analysed by electron spin resonance spectroscopy. Four radicals have been identified: the anion radical of the cytosine molecule, the radical resulting from H-addition at position C6, the radical resulting from H-addition at position O2, and finally a radical resulting from addition of a Cl- to nitrogen N3. Hückel molecular orbital calculations are presented, which support the hypothesis according to which in unsaturated pyrimidines the site of hydrogenation or protonation depends on the state of the molecule.     

Role of diffusion in nonaqueous enzymology. 1. Theory.

Using a set of standard equations, we have calculated the role of internal and external mass transfer in limiting the rate of enzyme-catalysed reactions in anhydrous organic solvents and supercritical fluids. We have shown that enzyme particles suspended in anhydrous organic solvents will be subject to increasing diffusional limitation as the enzyme activity and particle size increase. Using particle dimensions, as measured by scanning electron microscopy, we have prepared a series of graphs that will enable investigators to determine whether their combination of particle size and activity will result in internal or external diffusional limitations. We have shown that supercritical fluids can be expected to enhance the activity of enzymes in nonaqueous environments as a result of the high diffusivity of the bulk solvent. The plots also clearly indicate that enzyme particles in supercritical fluids require nearly two orders of magnitude less agitation than those suspended in conventional solvents in order to overcome any external mass transfer limitations.     

Aspects of audit. 2. Audit in British hospitals.

British hospitals have advantages over other areas of health care for developing and evaluating audit: more uniform and comprehensive medical records, statistical and diagnostic data, fewer patient contacts, more time to examine patients and record information. Although little has been published, a few studies show the extent to which formal audit has been developed in Britain.     

Changes in the ordering of lipids in the membrane of Dunaliella in response to osmotic-pressure changes. An e.s.r. study.

Changes in the ordering and motion of lipids in response to changes in the external solute concentration have been studied by using the 5-nitroxide stearate (5NS) and 16-nitroxide stearate (16NS) spin probes in the plasma membrane of the halotolerant unicellular alga Dunaliella salina. Increases in ordering of the 5NS probe and decreases in motion of the 16NS probe were observed in cells equilibrated over 18 h at increasing NaCl concentrations. These changes probably resulted from the influence of the high NaCl concentration on the charged phospholipid head groups of the membrane. A short-term (less than 100 min) decrease in the order parameter, S, of the 5NS probe was observed for cells swollen by exposure to a sudden decrease of NaCl concentration from 5.0 to 2.5 M. After 100 min the value of S for 5NS was close to the value obtained in cells that had been equilibrated in 2.5 M-NaCl for 18 h. Since the cells had regained their original size and shape by 100 min it was assumed that the short-term decrease in S was associated with the swelling. A similar result was obtained when the cells were suddenly changed from 3.0 M- to 1.5 M-sorbitol. Conversely, an increase in S was observed for cells shrunk when the external solute concentration was doubled from 1.5 M- to 3.0 M-NaCl. As the cells regained their original size and shape the value of S decreased to the value observed in cells that had been equilibrated in 3.0 M-NaCl for 18 h. It is suggested that the changes in S are related to the movement of lipid into or out of a reservoir of membrane material as the membrane shrinks or expands. This movement of lipid maintains the tension of the membrane below the value at which it is disrupted. Such changes in lipid ordering could provide a mechanism whereby information about external osmotic-pressure changes is transmitted across the cell wall.     

Bacteriorhodopsin in liposomes. II. Experimental evidence in support of a theoretical model.

In the preceding article equations describing relevant ion flows in illuminated suspensions of bacteriorhodopsin liposomes have been derived. Here these equations are subjected to experimental tests. Changes in permeability characteristics of the liposomal membrane are brought about by addition of specific ionophores and change of medium composition. Using light-driven proton uptake and electrochemical potential differences for protons across the membrane as observation parameters, ridig attempts to falsify the derived equations are unsuccessful. Agreement between equations and experimental results is established on the point of: (i) the antagonistic effect of valinomycin and nigericin on the two components of the proton-motive force, (ii) the time dependence of the changes in transmembrane electrical and chemical potential differences after the onset of illumination. In three independent experimental systems evidence was obtained for the correctness of the postulated dependence of the turnover rate of the photochemical cycle on back pressure by the transmembrane electrochemical potential difference for protons.     

Fenitrothion. I. Study of mutagenic activity in rats.

The mutagenic activity of fenitrothion was studied in rats given 0,10,40 or 80 ppm of fenitrothion in the diet. The study combined the dominant lethal test with cytogenetic analysis of chromosomal aberrations. Dominant lethal mutations were investigated: 1. by their so-called tentative determination in single mating in P-to F3 generation males and females following 200 days exposure; 2. by assessing the effect of the agent at individual stages of spermatogenesis, with F2 and F4 generation males having been exposed for 100 days and mated to unexposed females for 10 weeks. Chromosome aberrations were analyzed in the bone marrow of F2 generation males following 200-days exposure and F3 generation (males) following 500-day exposure to a dose of 80 ppm. Negative results were obtained in all experiments in relation both to dose and generation. Hence fenitrothion is not considered to be a substance with a mutagenic activity. The metodical advantages of the proposed combination of reproduction and mutagenic-activity studies of an agent for toxicological evaluation are discussed.