PubMed Health⌕ Search

Biomedical subjects

P Jursinic

Publications and source records attributed to P Jursinic.

10 recordsLinked to original sources
PubMed1995

Quality assurance tool for high dose rate brachytherapy.

An insert for a well ionization chamber is described for quality assurance tests for high dose rate remote afterloaders. The use of this insert over time has shown its applicability for efficient QA tests for the high dose rate afterloader. The measurement results are six times greater than the uncertainties involved for the measurements. Checks on position can be made better than 1 mm and dwell times better than 1 s. The overall measurements can be done to better than 1.5%. QA checks can be accomplished in a shorter time period with as great or better accuracy than prior film techniques.

Brachytherapy↗
PubMed1995

Intercomparison of normalized head-scatter factor measurement techniques.

Normalized head-scatter factors were measured with cylindrical beam coaxial miniphantoms and high purity graphite buildup caps for 4-, 6-, 10-, and 24-MV photon beams at field sizes from 4 x 4 to 40 x 40 cm2. The normalized head-scatter factors determined by the two methods matched well for 4- and 6-MV photon beams. The miniphantom technique produced normalized head-scatter factors 1.5% and 4.8% lower than the buildup caps for the 10- and 24-MV beams for large field sizes, respectively. At small field sizes, the miniphantom technique produced larger normalized head-scatter factors than the buildup caps. Measurements made with an electromagnet indicate that a significant portion of the ionization measured in the buildup cap at 24 MV arises from contamination electrons. Measurements made with the miniphantom and magnet found no contamination electron contribution. The miniphantom technique may exclude such contamination electrons, potentially leading to inaccuracies in tissue-maximum ratios and phantom scatter factors, as well as inaccuracies in monitor unit calculations.

Humans↗
PubMed1985

Reconciliation of the absorption change at 325 nm and other flash-yield determinations of concentrations of active photosystem II centers.

The concentration of photosystem II was determined in thylakoids of dwarf peas by the use of the following methods: absorption change at 325 nm; atrazine binding; and flash yields of oxygen evolution (Emerson-Arnold method), of protons from oxidation of water, and of reduction of DCIP. For the first time all of the flash-yield measurements have been done on the same sample and give equivalent values for the concentration of photosystem II. Agreement of the absorption change measurement at 325 nm with the other measurements was accomplished by the introduction of important improvements to the methods of Melis and co-workers [Proc. Natl. Acad. Sci. USA (1980) 77, 4712-4716]. The atrazine-binding method gave photosystem II values that were twice as large as any of the other photosystem II measurements. Possible reasons are discussed for this discrepancy in terms of the secondary acceptor (Q400) of Ikegami and Katoh [Plant Cell Physiol. (1973) 14, 829-836]. The concentration of photosystem I was measured by absorption change at 705 nm. From the concentration values of photosystem II and I the system II/I stoichiometry was calculated.

Atrazine↗
PubMed1983

The effects of carbonic anhydrase inhibitors formate, bicarbonate, acetazolamide, and imidazole on photosystem II in maize chloroplasts.

Inhibitors of carbonic anhydrase were tested for their effects on Photosystem II (PS II) activity in chloroplasts. We find that formate inhibition of PS II turnover rates increases as the pH of the reaction medium is lowered. Bicarbonate ions can inhibit PS II turnover rates. The relative potency of the anionic inhibitors N-3, I-, OA-c, and Cl- is the same for both carbonic anhydrase and PS II. The inhibitory effect of acetazolamide on PS II increases as light intensity decreases, indicating a lowering of quantum yields in the presence of the inhibitor. Imidazole inhibition of PS II increases with pH in a manner suggesting that the unprotonated form of the compound is inhibitory. Formate, bicarbonate, acetazolamide, and imidazole all inhibit DCMU-insensitive, silicomolybdate-supported oxygen evolution, indicating that the site(s) of action of the inhibitors is at, or before, the primary stable PS II electron acceptor, Q. This inhibitory effect of low levels of HCO-3 along with the known enhancement by HCO-3 of quinone-mediated electron flow suggests an antagonistic control effect on PS II photochemistry. We conclude that the responses of PS II to anions (formate, bicarbonate), acetazolamide, and imidazole are analogous to the responses shown by carbonic anhydrase. These findings suggest that the enzyme carbonic anhydrase may provide a model system to gain insight into the "bicarbonate-effect" associated with PS II in chloroplasts.

Acetazolamide↗
PubMed1983

Changes in [C]Atrazine Binding Associated with the Oxidation-Reduction State of the Secondary Quinone Acceptor of Photosystem II.

One hypothesis of triazine-type herbicide action in photosynthetic material is that the herbicide molecule competes with a secondary quinone acceptor, B, for a binding site at the reaction center of photosystem II. The binding affinity of B has been suggested to change with its level of reduction, being most strongly bound in its semiquinone form. To test this hypothesis, [(14)C]atrazine binding studies have been carried out under different photochemically induced levels of B reduction in Pisum sativum. It is found that herbicide binding is reduced in continuously illuminated samples compared to dark-adapted samples. Decreased binding of atrazine corresponds to an increase in the semiquinone form of B. With flash excitation, the herbicide binding oscillates with a cycle of two, being low on odd-numbered flashes when the amount of semiquinone form of B is greatest. Treatment with NH(2)OH was found to significantly decrease the strength of herbicide binding in the dark as well as stop the ability of p-benzoquinone to oxidize the semiquinone form of B. It is suggested that the mode of action of NH(2)OH is disruption of quinones or their environment on both the oxidizing and reducing sides of photosystem II. Herbicide binding was found to be unaltered under conditions when p-benzosemiquinone oxidation of the reduced primary acceptor, Q(-), is herbicide insensitive; weak herbicide binding cannot explain this herbicide insensitivity. It is concluded that the quinone-herbicide competition theory of herbicide action is correct. Also, since quinones are lipophilic the importance of the lipid composition of the thylakoid membrane in herbicide interactions is stressed.

Journal Article↗
PubMed1981

Investigation of double turnovers in photosystem II charge separation and oxygen evolution with excitation flashes of different duration.

The characteristics of double hitting in Photosystem II charge separation and oxygen evolution in algae and chloroplasts were investigated with saturating excitation flashes of 3 microseconds, 300 ns and 5 ns duration. Two types of double hitting or advancement in S-states were found to occur in oxygen evolution: a non-photochemical type found even with 5 ns flashes and a photochemical type seen only with microsecond-long flashes, which have extensive tails. The non-photochemical type, occurring with a probability of about 3%, is sensitive to the physiological condition of the sample, and is only present in algae or chloroplast samples that have been freshly prepared. In chloroplasts incubated with ferricyanide, a 3-fold increase in double advancement of S-states is observed with xenon-flash illumination but not with 300 ns or 5 ns laser illumination. However, double turnovers in Photosystem II reaction center charge separation are large with xenon flash or 300 ns laser illumination but not with 5 ns laser illumination. This indicates that quite different kinetic processes are involved in double advancement in S-states for oxygen evolution and double turnovers in charge separation. Various models of the Photosystem II reaction center are discussed. Also, based on experiments with chloroplasts incubated with ferricyanide, an unique solution to the oxygen S-state distribution in the dark suggested by Thibault (Thibault, P. (1978) C.R. Acad. Sci. Paris 287, 725-728) can be rejected.

Chlorella↗
PubMed1977

The rise in chlorophyll a fluorescence yield and decay in delayed light emission in tris-washed chloroplasts in the 6-100 microseconds time range after an excitation flash.

Parallel measurements of the rise in chlorophyll a fluorescence yield and delayed light emission decay, after a 10 ns saturating excitation flash, have been made in tris (hydroxymethyl)aminomethane-washed chloroplasts. Various electron donor systems (Mn2+; ascorbate; reduced phenylenediamine and benzidine) were used in conjuction with different preillumination regimes to alter [P+-680], the oxidized form of the Photosystem II reaction center chlorophyll a. Conditions giving rise to high [p+ -680] resulted in only a small rise in fluorescence yield, an inhibition of a 6 microseconds component of delayed light emission. These results confirm the hypothesis that P+-680 acts as a quencher of fluorescence and that delayed light emission in the microsecond time range is due to the back reaction of P+-680 and Q-. (Q is the first "stable" electron acceptor of Photosystem II.) Two preillumination flashes are required before the full effect of Tris washing is observed in the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exists between the Tris block and P+-680. Tris washing has no direct effect on the movement of electrons from Z (the first electron donor to P+-680. Finally, Mn2+ donates electrons to P+-680 via Z.

Ascorbic Acid↗
PubMed1976

A major site of bicarbonate effect in system II reaction. Evidence from ESR signal IIvf, fast fluorescence yield changes and delayed light emission.

In order to determine the major site of bicarbonate action in the electron transport complex of Photosystem II, the following experimental techniques were used: electron spin resonance measurements of Signal IIvf, measurements of chlorophyll a fluorescence yield rise and decay kinetics, and delayed light emission decay. From data obtained using these experimental techniques the following conclusions were made: (1) absence of bicarbonate causes a reversible inactivation of up to 40% of Photosystem II reaction center activity; (2) there is no significant effect of bicarbonate on electron flow from the charge accumulating S state to Z; (3) there is no significant effect of bicarbonate on electron flow from Z to P-680+; (4) electron flow from Q-- to the intersystem electron transport pool is inhibited by from 4- to 6-fold under bicarbonate depletion conditions.

Bicarbonates↗