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PubMed2002

Fluence correction factors in plastic phantoms for clinical proton beams.

In recent codes of practice for reference dosimetry in clinical proton beams using ionization chambers, it is recommended to perform the measurement in a water phantom. However, in situations where the positioning accuracy is very critical, it could be more convenient to perform the measurement in a plastic phantom. In proton beams, a similar approach as in electron beams could be applied by introducing fluence correction factors in order to account for the differences in particle fluence distributions at equivalent depths in plastic and water. In this work, fluence correction factors as a function of depth were determined for proton beams with different energies using the Monte Carlo code PTRAN for PMMA and polystyrene with reference to water. The influence of non-elastic nuclear interaction cross sections was investigated. It was found that differences in proton fluence distributions are almost entirely due to differences in non-elastic nuclear interaction cross sections between the plastic materials and water. For proton beams with energies lower than 100 MeV, for which the contributions from non-elastic interactions become small compared to the total dose, the fluence corrections are smaller than 1%. For beams with energies above 200 MeV, depending on the cross sections dataset for non-elastic nuclear interactions, fluence corrections of 2-5% were found at the largest depths. The results could, with an acceptable accuracy, be represented as a correction per cm penetration of the beam, yielding values between 0.06% and 0.15% per cm for PMMA and 0.06% to 0.20% per cm for polystyrene. Experimental information on these correction factors was obtained from depth dose measurements in PMMA and water. The experiments were performed in 75 MeV and 191 MeV non-modulated and range-modulated proton beams. From the experiments, values ranging from 0.03% to 0.15% per cm were obtained. A decisive answer about which dataset for non-elastic nuclear interactions would result in a better representation of the measurements could not be given. We conclude that below 100 MeV, dosimetry could be performed in plastic phantoms without a dramatic loss of accuracy. On the other hand, in clinical high-energy proton beams, where accurate positioning in water is in general not an issue, substantial correction factors would be required for converting dose measurements in a plastic phantom to absorbed dose to water. It is therefore not advisable to perform absorbed dose measurements nor to measure depth dose distributions in a plastic phantom in high-energy proton beams.

Calibration↗

PubMed2002

Nuclear interactions in proton therapy: dose and relative biological effect distributions originating from primary and secondary particles.

The dose distribution delivered in charged particle therapy is due to both primary and secondary particles. The secondaries, originating from non-elastic nuclear interactions, are of interest for three reasons. First, if fast Monte Carlo treatment planning is envisaged, the question arises whether all nuclear interaction products deliver a significant contribution to the total dose and, hence, need to be tracked. Second, there could be an enhanced relative biological effectiveness (RBE) due to low energy and/or heavy secondaries. Third, neutrons originating from nuclear interactions may deliver dose outside the target volume. The particle yield from different nuclear interaction channels as a function of proton penetration depth was studied theoretically for different proton beam energies. Three-dimensional dose distributions from primary and secondary particles were simulated for an unmodulated 160 MeV proton beam with and without including a slice of bone material and for a spread-out Bragg peak (SOBP) of 3 x 3 x 3 cm3 in water. Secondary protons deliver up to 10% of the total dose proximal to the Bragg peak of an unmodulated proton beam and they affect the flatness of the SOBP. Furthermore, they cause a dose build-up due to forward emission of secondary particles from nuclear interactions. The dose deposited by d, t, 3He and alpha-particles was found to contribute less than 0.1% of the total dose. The dose distal to the target volume caused by liberated neutrons was studied for four proton beam energies in the range of 160-250 MeV and found to be below 0.05% (2 cm distal to SOBP) of the prescribed target dose for a 3 x 3 x 3 cm3 target. RBE values relative to 60Co were calculated proximal to and within the SOBP. The RBE proximal to the Bragg peak (100% dose) is influenced by secondary particles (mainly protons and a-particles) with a strong dose dependency resulting in RBE values up to 1.2 (2 Gy; inactivation of V79). Depending on the endpoint considered, secondary particles cause a shift in RBE by up to 8% at 2 Gy. In contrast, the RBE in the Bragg peak is almost entirely determined by primary protons due to a decreasing secondary particle fluence with depth. RBE values up to 1.3 (2 Gy; inactivation of V79) at 1 cm distal to the Bragg peak maximum were found. The inactivations of human skin fibroblasts and mouse lymphoma cells were also analysed and reveal a substantial tissue dependency of the total RBE. The outcome of this study shows that elevated RBE values occur not only at the distal edge of the SOBP. Although the variations are modest, and in most cases might have no observable clinical effect, they might have to be considered in certain treatment situations. The biological effect downstream of the target caused by neutrons was analysed using a radiation quality factor of 10. The biological dose was found to be below 0.5% of the prescribed target dose (for a 3 x 3 x 3 cm3 SOBP) but depends on the size of the SOBP. This dose should not be significant with respect to late effects, e.g. cancer induction.

Animals↗

PubMed2005

Experimental characterization and physical modelling of the dose distribution of scanned proton pencil beams.

In this paper we present the pencil beam dose model used for treatment planning at the PSI proton gantry, the only system presently applying proton therapy with a beam scanning technique. The scope of the paper is to give a general overview on the various components of the dose model, on the related measurements and on the practical parametrization of the results. The physical model estimates from first physical principles absolute dose normalized to the number of incident protons. The proton beam flux is measured in practice by plane-parallel ionization chambers (ICs) normalized to protons via Faraday-cup measurements. It is therefore possible to predict and deliver absolute dose directly from this model without other means. The dose predicted in this way agrees very well with the results obtained with ICs calibrated in a cobalt beam. Emphasis is given in this paper to the characterization of nuclear interaction effects, which play a significant role in the model and are the major source of uncertainty in the direct estimation of the absolute dose. Nuclear interactions attenuate the primary proton flux, they modify the shape of the depth-dose curve and produce a faint beam halo of secondary dose around the primary proton pencil beam in water. A very simple beam halo model has been developed and used at PSI to eliminate the systematic dependences of the dose observed as a function of the size of the target volume. We show typical results for the relative (using a CCD system) and absolute (using calibrated ICs) dosimetry, routinely applied for the verification of patient plans. With the dose model including the nuclear beam halo we can predict quite precisely the dose directly from treatment planning without renormalization measurements, independently of the dose, shape and size of the dose fields. This applies also to the complex non-homogeneous dose distributions required for the delivery of range-intensity-modulated proton therapy, a novel therapy technique developed at PSI.

Calibration↗

PubMed2006

Experimental evaluation of a MOSFET dosimeter for proton dose measurements.

The metal oxide semiconductor field-effect transistor (MOSFET) dosimeter has been widely studied for use as a dosimeter for patient dose verification. The major advantage of this detector is its size, which acts as a point dosimeter, and also its ease of use. The commercially available TN502RD MOSFET dosimeter manufactured by Thomson and Nielsen has never been used for proton dosimetry. Therefore we used the MOSFET dosimeter for the first time in proton dose measurements. In this study, the MOSFET dosimeter was irradiated with 190 MeV therapeutic proton beams. We experimentally evaluated dose reproducibility, linearity, fading effect, beam intensity dependence and angular dependence for the proton beam. Furthermore, the Bragg curve and spread-out Bragg peak were also measured and the linear-energy transfer (LET) dependence of the MOSFET response was investigated. Many characteristics of the MOSFET response for proton beams were the same as those for photon beams reported in previous papers. However, the angular MOSFET responses at 45, 90, 135, 225, 270 and 315 degrees for proton beams were over-responses of about 15%, and moreover the MOSFET response depended strongly on the LET of the proton beam. This study showed that the angular dependence and LET dependence of the MOSFET response must be considered very carefully for quantitative proton dose evaluations.

Biophysical Phenomena↗

PubMed1992

Proton beams in radiation therapy.

The rationale for study of proton radiation therapy is that, for some anatomic sites and tumors, the treatment volume is smaller; i.e., there is less irradiation of nontarget tissue while the target is included in three dimensions at each treatment session. As a result, the dose to the target can be raised. The consequence is that the tumor control probability improves and the frequency and severity of treatment-related morbidity decrease. These results come about from the physical fact that the proton range in tissue is finite; in comparison, absorption of photons is an exponential function and, hence, some dose is received for the full-beam path through the body. Accordingly, the dose deep to the target for proton treatments can be zero for each beam path. This situation provides a virtually certain means of improving the treatment outcome for selected categories of patients. Experience to date with proton radiation therapy has been quite limited. As of June 1991, the total number of proton radiation-treated patients was 11,763 from the various centers. Of that number, approximately 46% and 32% have been treated for small benign intracranial lesions (principally pituitary adenomas and arteriovenous malformations) and for tumors of the eye, respectively. Thus, only some 2500 patients have been treated for all other tumor types. The results from three centers and approximately 2800 patients with uveal melanoma are that the local control rate was 96% (for failures in-field, marginal, and in other parts of the eye). The local control results for chondrosarcomas and chordomas of the skull base are 91% and 65%, respectively. These percentages compare with some 35% achieved with conventional treatment. Experience with arteriovenous malformations indicates that control of bleeding and disappearance of the lesion are comparable to those achieved by other procedures. The developments from the proton therapy programs have contributed greatly to radiation treatment planning, e.g., the first three-dimensional treatment planning system put into regular clinical use (uveal melanoma), beam's eye view, digital-reconstructed radiograph, dose-volume histograms, and definitions of the uncertainty in dose around any defined point. The potential for clinical gains is high. In May 1991, the Proton Radiation Oncology Group was formed to design, supervise, and coordinate clinical trials and to assist in data analysis. The efficacy of proton radiation therapy will be compared with that of photon therapy of the very highest technology.

Arteriovenous Malformations↗

PubMed1996

Acid-induced exchange of the imino proton in G.C pairs.

Acid-induced catalysis of imino proton exchange in G.C pairs of DNA duplexes is surprisingly fast, being nearly as fast as for the isolated nucleoside, despite base-pair dissociation constants in the range of 10(-5) at neutral or basic pH. It is also observed in terminal G.C pairs of duplexes and in base pairs of drug-DNA complexes. We have measured imino proton exchange in deoxyguanosine and in the duplex (ATATAGATCTATAT) as a function of pH. We show that acid-induced exchange can be assigned to proton transfer from N7-protonated guanosine to cytidine in the open state of the pair. This is faster than transfer from neutral guanosine (the process of intrinsic catalysis previously characterized at neutral ph) due to the lower imino proton pK of the protonated form, 7.2 instead of 9.4. Other interpretations are excluded by a study of exchange catalysis by formiate and cytidine as exchange catalysts. The cross-over pH between the regimes of pH-independent and acid-induced exchange rates is more basic in the case of base pairs than in the mononucleoside, suggestive of an increase by one to two decades in the dissociation constant of the base pair upon N7 protonation of G. Acid-induced catalysis is much weaker in A.T base pairs, as expected in view of the low pK for protonation of thymidine.

Base Sequence↗

PubMed1988

Proton release associated with respiratory burst of polymorphonuclear leukocytes.

The stimulation of polymorphonuclear leukocytes (PMNs) by phorbol-12-myristate-13-acetate in the presence of sodium fluoride caused the release of protons into the reaction medium concomitant with the generation of superoxide anions. The rates of oxygen consumption and proton release due to the metabolic burst were 16.3 +/- 3.5 and 10.2 +/- 1.1 nmol/min/10(7) cells respectively. When the superoxide anions were trapped with cytochrome c, the proton release was increased (35.8 +/- 0.5 nmol/min/10(7) cells) until the cytochrome c was reduced. Since the protons released from the activated cells would be consumed by the generated superoxide anions in the extracellular medium, the net amount of the protons released was 3-4-fold greater than that observed in the absence of extracellular cytochrome c. The increased proton release may be coupled to increased cellular respiration, since the inhibition of the respiratory burst with deoxyglucose, p-chloromercuribenzoic acid or chlorpromazine decreased the proton release. Amiloride (2 mM) inhibited the proton release by up to 40%. These observations suggest that some mechanisms other than a Na+/H+ antiport and carbon dioxide diffusion could be transporting the H+ generated in the cytosol of the activated PMNs.

Animals↗

PubMed1985

Dynamic behavior of the imino protons of the gamma OR3 17mer in H2O solution studied by high-resolution NMR.

The imino proton resonances of gamma OR3 17mer in water were observed at 500 MHz with the time-shared Redfield pulse train. All of the 17 imino proton resonances could be assigned specifically to individual base pairs by utilizing the trace of NOE connectivities between the imino and adenine C2H protons and between imino protons themselves. AT1 and 17 showed abnormally high chemical shifts in comparison with the other AT pairs. On raising the temperature, broadening of the signal occurred in a sequential manner from the terminals except for AT10 and AT11, which were broadened at a lower temperature than GC12. The relaxation rates of the imino protons were measured by the inversion recovery method. The rates at higher temperatures represent the exchange rates of the imino protons. From the temperature dependences, activation energies of about 15 kcal/mol for the AT imino protons and 23-26 kcal/mol for the GC imino protons were obtained.

Bacteriophage lambda↗

PubMed2002

Electrostatics and proton transfer in photosynthetic water oxidation.

Photosystem II (PSII) oxidizes two water molecules to yield dioxygen plus four protons. Dioxygen is released during the last out of four sequential oxidation steps of the catalytic centre (S(0) --> S(1), S(1) --> S(2), S(2) --> S(3), S(3) --> S(4) --> S(0)). The release of the chemically produced protons is blurred by transient, highly variable and electrostatically triggered proton transfer at the periphery (Bohr effect). The extent of the latter transiently amounts to more than one H(+)/e(-) under certain conditions and this is understood in terms of electrostatics. By kinetic analyses of electron-proton transfer and electrochromism, we discriminated between Bohr-effect and chemically produced protons and arrived at a distribution of the latter over the oxidation steps of 1 : 0 : 1 : 2. During the oxidation of tyr-161 on subunit D1 (Y(Z)), its phenolic proton is not normally released into the bulk. Instead, it is shared with and confined in a hydrogen-bonded cluster. This notion is difficult to reconcile with proposed mechanisms where Y(Z) acts as a hydrogen acceptor for bound water. Only in manganese (Mn) depleted PSII is the proton released into the bulk and this changes the rate of electron transfer between Y(Z) and the primary donor of PSII P(+)(680) from electron to proton controlled. D1-His190, the proposed centre of the hydrogen-bonded cluster around Y(Z), is probably further remote from Y(Z) than previously thought, because substitution of D1-Glu189, its direct neighbour, by Gln, Arg or Lys is without effect on the electron transfer from Y(Z) to P(+)(680) (in nanoseconds) and from the Mn cluster to Y(ox)(Z).

Binding Sites↗

PubMed2005

Electron and proton transport by NADPH oxidases.

The NADPH oxidase is the main weapon of phagocytic white blood cells that are the first line of defence of our body against invading pathogens, and patients lacking a functional oxidase suffer from severe and recurrent infections. The oxidase is a multisubunit enzyme complex that transports electrons from cytoplasmic NADPH to molecular oxygen in order to generate superoxide free radicals. Electron transport across the plasma membrane is electrogenic and is associated with the flux of protons through voltage-activated proton channels. Both proton and electron currents can be recorded with the patch-clamp technique, but whether the oxidase is a proton channel or a proton channel modulator remains controversial. Recently, we have used the inside-out configuration of the patch-clamp technique to record proton and electron currents in excised patches. This approach allows us to measure the oxidase activity under very controlled conditions, and has provided new information about the enzymatic activity of the oxidase and its coupling to proton channels. In this chapter I will discuss how the unique characteristics of the electron and proton currents associated with the redox activity of the NADPH oxidase have extended our knowledge about the thermodynamics and the physiological regulation of this remarkable enzyme.

Biological Transport, Active↗

PubMed2004

Oesophageal and gastric pH profiles in patients with gastro-oesophageal reflux disease and Barrett's oesophagus treated with proton pump inhibitors.

BACKGROUND: Acid plays a significant role in the development of gastro-oesophageal reflux symptoms and tissue damage. It is generally assumed that acid suppressive therapy with proton pump inhibitors improves or eliminates symptoms of gastro-oesophageal reflux disease by normalizing intra-oesophageal pH. However, the degree of acid suppression induced by proton pump inhibitor therapy in patients with gastro-oesophageal reflux disease and/or Barrett's oesophagus has not been adequately studied. AIM: To assess the efficacy of proton pump inhibitors in normalizing intra-oesophageal and intra-gastric pH in patients with gastro-oesophageal reflux disease with and without Barrett's oesophagus who have been rendered symptom-free by acid-suppressive therapy. METHODS: Patients with gastro-oesophageal reflux disease and Barrett's oesophagus were prospectively evaluated by dual sensor 24-h pH monitoring while receiving proton pump inhibitor therapy for complete control of gastro-oesophageal reflux disease symptoms. Analyses and comparisons of intra-oesophageal and intra-gastric pH profiles on therapy were then made. RESULTS: One hundred and ten patients, 98 men and 12 women, with gastro-oesophageal reflux disease (n = 62) and/or Barrett's oesophagus (n = 48), were studied. All tolerated proton pump inhibitors well and were asymptomatic at the time of the study. Thirty-six (58%) patients with gastro-oesophageal reflux disease and 24 (50%) patients with Barrett's oesophagus (P = 0.4) normalized their intra-oesophageal pH profiles on proton pump inhibitors. Compared with patients with gastro-oesophageal reflux disease, patients with Barrett's oesophagus were more likely to have higher degree of pathologic acid reflux despite proton pump inhibitor therapy (DeMeester score 50.5 +/- 8.2 vs. 31.4 +/- 4.6, P = 0.03) and exhibited less intra-gastric acid suppression (% total pH < 4.0: 53.9 +/- 2.7 vs. 39.9 +/- 2.6, P = 0.0004), particularly supine (% pH < 4.0: 62.1 +/- 3.4 vs. 44.8 +/- 3.4, P = 0.0006). CONCLUSIONS: Gastro-oesophageal reflux disease patients with or without Barrett's oesophagus continue to exhibit pathologic gastro-oesophageal reflux disease and low intra-gastric pH despite proton pump inhibitor therapy that accomplishes complete reflux symptom control. Further, intra-oesophageal and intra-gastric pH control is significantly more difficult to achieve in patients with Barrett's oesophagus. These findings may have significant therapeutic implications.

Adult↗

PubMed2005

Proton-pump inhibitor use by coronary artery disease patients is associated with fewer chest pain episodes, emergency department visits and hospitalizations.

BACKGROUND: Patients with coronary artery disease presenting to an emergency department with chest pain are likely to undergo hospitalization in an attempt to elucidate its aetiology. AIM: To examine whether coronary artery disease patients receiving proton-pump inhibitor therapy are associated with fewer chest pain events and evaluations. METHODS: A veteran patient population with documented coronary artery disease was identified, and chest pain episodes, emergency department visits and hospitalizations for chest pain were followed over 2 years. Patient outcomes between proton-pump inhibitor use and non-use of proton-pump inhibitor therapy were compared. RESULTS: In 415 male patients, 23% utilized a proton-pump inhibitor and 77% did not. Proton-pump inhibitor therapy was associated with fewer chest pain episodes (12% vs. 26%, P = 0.002), emergency department visits, (12% vs. 24%, P = 0.044) and hospitalizations (13% vs. 24%, P = 0.086). The incidence of adverse events was decreased in the proton-pump inhibitor group: 70% fewer chest pain episodes (P = 0.002, RR = 3.3), 55% fewer emergency department visits (P = 0.049, RR = 2.2) and 53% fewer hospitalizations (P = 0.064, RR = 2.1). By multivariate analysis, proton-pump inhibitor therapy independently predicted a reduced prevalence of patients experiencing chest pain, emergency department visits, and hospitalizations [OR = 0.09 (0.04-0.21); 0.15 (0.06-0.40); 0.14 (0.05-0.40); all P < 0.001]. CONCLUSIONS: Proton-pump inhibitor therapy for veteran coronary artery disease patients is associated with fewer chest pain episodes, emergency department visits and hospitalizations for chest pain.

Aged↗

PubMed2006

Continuous treatment of Barrett's oesophagus patients with proton pump inhibitors up to 13 years: observations on regression and cancer incidence.

BACKGROUND: There is little evidence that treatment of patients with Barrett's oesophagus with proton pump inhibitors over periods up to 6 years results in major regression of Barrett's oesophagus. AIM: To determine if longer periods of treatment with proton pump inhibitors lead to significant regression of Barrett's oesophagus, and to determine the incidence of oesophageal adenocarcinoma in the proton pump inhibitor-treated patients. METHODS: We analysed prospectively-collected data on Barrett's oesophagus patients treated with proton pump inhibitors for 1-13 years. RESULTS: 188 patients with Barrett's oesophagus and intestinal metaplasia, were treated for 1-13 years with a proton pump inhibitor (966 years of treatment; mean 5.1 years). No change in length was seen during treatment but 48% of patients developed squamous islands (25% after 1-3 years; 100% at 12-13 years). Squamous islands correlated with treatment duration and male sex but not with proton pump inhibitor dose or patient age. Six patients developed dysplasia and three males developed adenocarcinoma during treatment (cancer incidence 0.31%). CONCLUSIONS: Proton-pump inhibitor treatment over 1-13 years does not shorten the Barrett's oesophagus segment but squamous islands appear in many patients. The incidence of oesophageal adenocarcinoma was low in these proton pump inhibitor-treated patients compared with published series.

2-Pyridinylmethylsulfinylbenzimidazoles↗

PubMed1982

Kinetic studies of proton transfer in the microenvironment of a binding site.

Excitation of 8-hydroxypyrene 1,3,6-trisulfonate to its first electronic singlet state converts the compound from weak base (pK degrees = 7.7) into a strong acid (pK* = 0.5). The dissociation of the proton in water or dilute salt solution is a very fast reaction, K12 = 1 X 10(10) S-1. In concentrated salt solutions the dissociation is slowed as an exponential function of the chemical activity of the water in the solution. This kinetic parameter has been used to gauge the properties of the microenvironment of the binding sites of bovine serum albumin at which this compound is bound. Time-resolved fluorometry reveals two distinct steps: a rapid dissociation of the proton with tau = 300 +/- 40 ps which lasts approximately 0.5 ns, followed by a slower reaction with tau = 3.3 ns. The first rapid phase represents proton dissociation taking place in the binding site. From the rate constant K = 3.3 X 10(9) s-1 we estimate that the ability of the water molecules in the site to hydrate the ejected proton is equivalent to a salt solution with water activity of 0.85. The slow phase represents the escape of the proton from the binding site. The rate of the escape, 1.4 X 10(8) s-1, is significantly slower than diffusion-controlled dissociation. It is concluded that the shape of the site or its lowered proton conductivity do not allow a rapid escape of the proton to the bulk. Still it should be remembered that the escape of the proton is 10(5)-10(6)-times faster than a typical turnover of an enzyme.U

Arylsulfonates↗

PubMed1990

Dicyclohexylcarbodiimide-binding proteins related to the short circuit of the proton-pumping activity of photosystem II. Identified as light-harvesting chlorophyll-a/b-binding proteins.

In photosynthesis of higher plants, photosystem II drives electron transfer from the water-oxidizing manganese centre at the lumenal side to bound plastoquinone at the stromal side of the thylakoid membrane. Proton release into the lumen and proton uptake from the stroma, i.e. net proton pumping, follows as consequence of vectoral electron transport. The proton pumping activity can be short circuited by covalent modification with N,N'-dicyclohexylcarbodiimide (cHxN)2C of certain proteins in the 20-28-kDa range. After modification, protons from water oxidation are no longer released into the thylakoid lumen, but instead transferred through the photosystem complex to protonate the photoreduced bound quinone at the other side of the membrane [Jahns, P., Polle, A. & Junge, W. (1988) EMBO J. 7, 589-594]. Here we identify the pertinent (cHxN)2C-binding proteins by amino acid sequence analysis and localize (cHxN)2C-binding sites within their primary structure. The proteins that are associated with the proton short circuit are light-harvesting chlorophyll-a/b-binding proteins. Our results imply that in addition to acting as antennae they may serve another function: the funneling into the thylakoid lumen of protons, which are liberated in the water-oxidizing Mn centre.

Amino Acid Sequence↗

PubMed1993

The ratio of protons translocated/hydride ion equivalent transferred by nicotinamide nucleotide transhydrogenase in chromatophores from Rhodospirillum rubrum.

The reduction of acetylpyridine adenine dinucleotide (AcPdAD+, an NAD+ analogue) by NADPH, in chromatophores treated with valinomycin, was accompanied by alkalinisation of the external medium, as measured by the absorbance change of added cresol red, a simple, non-binding pH indicator. Experiments with a stopped-flow spectrophotometer showed that initial (linear) rates of alkalinisation persisted for 1-2s. From the results of experiments in which H+ uptake was driven by a series of short flashes of light, the dependence of the outward proton leak on the extent of H+ uptake was established. Thus, the proton leak was subtracted from the initial rate of alkalinisation during transhydrogenation to give the true proton-uptake rate. The correction factor was usually about 10%. The ratio of protons translocated/H- transferred from NADPH to AcPdAD+ (the H+/H- ratio) was 0.60 +/- 0.06. The transhydrogenation reaction between NAD+ and NADPH was measured in the presence of a regeneration system for NAD+ (pyruvate and lactate dehydrogenase). In addition to the accompanying proton-translocation reaction, scalar H+ consumption linked to the regeneration system was observed and permitted internal checks on the calibration of the cresol red absorbance changes. After correction for the proton leak and scalar proton uptake, an H+/H- ratio of 0.60 +/- 0.30 was calculated from the initial rates. The water-soluble polypeptide of transhydrogenase (Ths) was washed from a sample of chromatophores to inhibit transhydrogenation activity and the accompanying H+ uptake. Re-addition of purified Ths to depleted chromatophores led to recovery of transhydrogenation activity and of H+ uptake. In this reconstituted system the H+/H- was similar to that in the native membranes. These results make it unlikely that the H+/H- ratio is artefactually low because chromatophores have a population of transhydrogenase which is not coupled to proton translocation. Further evidence that the mechanistic H+/H- ratio of chromatophore transhydrogenase is less than 1 was provided by an analysis of the kinetics of alkalinisation of the medium during reduction of AcPdAD+ by NADPH. It was shown that the progress of the transhydrogenation-induced alkalinisation was fitted by the sum of H+ uptake (the rate of transhydrogenation multiplied by the H+/H- ratio) plus the H+ leak, when the ratio was 0.6 but not when it was 1.0. The results are discussed in terms of the possible mechanism of energy coupling by transhydrogenase.(ABSTRACT TRUNCATED AT 400 WORDS)

Bacterial Chromatophores↗

PubMed2005

Characterization of a proton-activated, outwardly rectifying anion channel.

Anion channels are present in every mammalian cell and serve many different functions, including cell volume regulation, ion transport across epithelia, regulation of membrane potential and vesicular acidification. Here we characterize a proton-activated, outwardly rectifying current endogenously expressed in HEK293 cells. Binding of three to four protons activated the anion permeable channels at external pH below 5.5 (50% activation at pH 5.1). The proton-activated current is strongly outwardly rectifying, due to an outwardly rectifying single channel conductance and an additional voltage dependent facilitation at depolarized membrane potentials. The anion channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) rapidly and potently inhibited the channel (IC50: 2.9 microm). Flufenamic acid blocked this channel only slowly, while mibefradil and amiloride at high concentrations had no effect. As determined from reversal potential measurements under bi-ionic conditions, the relative permeability sequence of this channel was SCN-> I-> NO3-> Br-> Cl-. None of the previously characterized anion channel matches the properties of the proton-activated, outwardly rectifying channel. Specifically, the proton-activated and the volume-regulated anion channels are two distinct and separable populations of ion channels, each having its own set of biophysical and pharmacological properties. We also demonstrate endogenous proton-activated currents in primary cultured hippocampal astrocytes. The proton-activated current in astrocytes is also carried by anions, strongly outwardly rectifying, voltage dependent and inhibited by DIDS. Proton-activated, outwardly rectifying anion channels therefore may be a broadly expressed part of the anionic channel repertoire of mammalian cells.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗

PubMed2000

Proton loss model for therapeutic beam dose calculations.

A transport algorithm called the proton loss (PL) model is developed for proton pencil beams of therapeutic energies. The PL model takes into account inelastic nuclear reactions, pathlength straggling, and energy-loss straggling and predicts the 3D dose distribution from a proton pencil beam. In proton beams, the multiple scattering and ionizational energy loss processes approach their diffusional limit where scattering and energy loss probability densities become Gaussian. Therefore we chose to derive the PL model from the Fermi-Eyges diffusional multiple scattering theory and the Gaussian theory of energy straggling. We first introduce a generalization of the Fermi-Eyges equation for proton pencil beams, labeled the proton loss (PL) transport equation. This new equation includes terms that model inelastic nuclear reactions as a depth-dependent absorption and pathlength straggling as a quasi-absorption. Then energy straggling is taken into account by using a weighted superposition of a discrete number of elementary pencil beams. These elementary pencil beams have different initial energies and lose energy according to the CSDA, thus they have different ranges of penetration. A final solution for the proton beam transport is obtained as a linear combination of elementary pencil beam solutions with weights defined by the Gaussian evolution of the proton energy spectrum with depth. A numerical comparison of the dose distribution predictions of the PL model with measurements and PTRAN Monte Carlo simulations indicates the model is both computational fast and accurate.

Algorithms↗