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P Horton

Publications and source records attributed to P Horton.

At least 73 records · Page 4Linked to original sources
PubMed1981

Perinatal echovirus infection: risk of transmission during a community outbreak.

During a community outbreak of enterovirus infection, seven of 194 pregnant women (3.6 per cent) were found to be excreting a prime strain of echovirus 11 at term. Each of the seven women possessed serum neutralizing echovirus 11 antibody in titers ranging from 1:20 to 1:320, and the cord serum of their seven infants had antibody in titers of 1:10 to 1:640. None of these seven infants became ill, but four were shedding virus from the respiratory or gastrointestinal tract by three days of age. In a previous study, four infants who died of generalized infection due to the same strain of echovirus 11 had no detectable antibody in cord serum. None of the infants of virus-negative mothers became infected, according to cultures at hospital discharge (151 infants) or at two weeks of age (135 infants). We conclude that passive transplacental acquisition of antibody prevents severe, systemic echovirus disease but does not prevent mucosal infection in the perinatally infected infant.

Antibodies, Viral↗
PubMed1981

Light-dependent quenching of chlorophyll fluorescence in pea chloroplasts induced by adenosine 5'-triphosphate.

Addition of ATP to chloroplasts causes a reversible 25-30% decrease in chlorophyll fluorescence. This quenching is light-dependent, uncoupler insensitive but inhibited by DCMU and electron acceptors and has a half-time of 3 minutes. Electron donors to Photosystem I can not overcome the inhibitory effect of DCMU, suggesting that light activation depends on the reduced state of plastoquinone. Fluorescence emission spectra recorded at -196 degrees C indicate that ATP treatment increases the amount of excitation energy transferred to Photosystem I. Examination of fluorescence induction curves indicate that ATP treatment decreases both the initial (F0) and variable (Fv) fluorescence such that the ratio of Fv to the maximum (Fm) yield is unchanged. The initial sigmoidal phase of induction is slowed down by ATP treatment and is quenched 3-fold more than the exponential slow phase, the rate of which is unchanged. A plot of Fv against area above the induction curve was identical plus or minus ATP. Thus ATP treatment can alter quantal distribution between Photosystems II and I without altering Photosystem II-Photosystem II interaction. The effect of ATP strongly resembles in its properties the phosphorylation of the light-harvesting complex by a light activated, ATP-dependent protein kinase found in chloroplast membranes and could be the basis of physiological mechanisms which contribute to slow fluorescence quenching in vivo and regulate excitation energy distribution between Photosystem I and II. It is suggested that the sensor for this regulation is the redox state of plastoquinone.

Adenosine Triphosphate↗
PubMed1981

The effect of redox potential on the kinetics of fluorescence induction in pea chloroplasts. I. Removal of the slow phase.

The effect of alteration of redox potential on the kinetics of fluorescence induction in pea chloroplasts has been investigated. Potentiometric titration of the initial (Fi) level of fluorescence recorded upon shutter opening gave a two component curve, with Em(*7) at -20 mV and -275 mV, almost, identical to results obtained using continuous low intensity illumination (Horton, P. and Croze, E. (1979) Biochim. Biophys. Acta 545, 188--201). The slow or tail phase of induction observed in the presence of DCMU can be eliminated by poising the redox potential at approx. 0 to +50 mV. At this potential Fi was increased by less than 10% and the higher potential quencher described above was only marginally reduced. The disappearance of the slow phase titrated as an n = 1 component with an Em(7) of +120 mV. Therefore it seems unlikely that the slow phase of fluorescence induction is due to photoreduction of the -20 mV quencher. These results are discussed with reference to current ideas concerning heterogeneity on the acceptor side of Photosystem II.

Chloroplasts↗
PubMed1980

Observation of two quenchers of chlorophyll fluorescence in chloroplasts at -196 degrees C.

Fluorescence induction at -196 degrees C has been monitored in chloroplasts rapidly frozen after poising at different redox potentials at room temperature. It was found that, as at room temperature, the initial level of fluorescence observed upon shutter opening (Fo), relative to the final level observed after 10 seconds of illumination (Fm) increased as the redox potential of the chloroplasts was lowered. Redox titration and revealed the presence of two quenching components with Em, 7.8 at -70 mV and -275 mV accounting for approx. 75% and 25% of the variable fluorescence (Fv). Parallel observation of fluorescence yield at room temperature similarly gave two components, with Em, 7.8 at -95 mV and -290 mV, also accounting for appox. 75% and 25%. Simultaneous measurement of fluorescence emission at -196 degrees C at 695 nm and 735 nm indicated that both emissions are quenched by the same redox components.

Chlorophyll↗
PubMed1979

Characterization of two quenchers of chlorophyll fluorescence with different midpoint oxidation-reduction potentials in chloroplasts.

The properties of two redox quenchers of chlorophyll fluorescence in chloroplasts at room temperature have been investigated. (1) Redox titration of the fluorescence yield reveals two n = 1 components with Em7.8 at--45 and --247 mV, accounting for approx. 70 and 30% of the total yield, respectively. (2) Neutral red, a redox mediator often used at redox potentials below --300 mV, preferentially quenches the fluorescence controlled by the --247 mV component. Titrations using neutral red artifactually create an n = 2 quenching component with Em7.8 = --375 mV. (3) Analysis of fluorescence induction curves recorded at different redox potentials indicates that both the --45 and --247 mV components can be photochemically reduced. The reduction of the --247 mV component corresponds to a fast phase of the induction curve whilst the slower reduction of the 45 mV component accounts for the tail phase. (4) The excitation spectra for the fluorescence controlled by the two quenchers show small differences in the ratio of chlorophyll a and b. (5) Whereas the --247 mV component readily shows a 60 mV per pH unit dependency on solution pH, the ability of the --45 mV component to respond to pH change is restricted. (6) Triton Photosystem II particles contain both quenchers but the --247 mV component accounts for approx. 70% of the fluorescence and the high component has an Em7.8 of +48 mV. The relative merits of sequential and parallel models in explaining the presence of the two quenchers are considered.

Chlorophyll↗
PubMed1978

Interactions between photosystem II components in chloroplast membranes. A correlation between the existence of a low potential species of cytochrome b-559 and low chlorophyll fluorescence in inhibited and developing chloroplasts.

1. Chloroplasts inhibited by incubation with hydroxylamine in the light exhibit a low fluorescence yield upon illumination in the presence of dithionite sufficient to completely reduce the primary acceptor, Q. In the absence of magnesium ions, the fluorescence yield is the same as in control chloroplasts, suggesting that the reason for the low yield is a defect in the mechanism by which Mg2+ enhances the fluorescence. These chloroplasts were previouly shown to contain only low potential (Em7.8 = +80 mV) cytochrome b-559 (Horton, P. and Croze, E (1977) Biochim. Biophys. Acta 462, 86-101). 2. In Photosystem II particles, in heat-treated chloroplasts and in trypsin-digested chloroplasts, high potential cytochrome b-559 is absent and the variable fluorescence yield is again low. 3. Peas grown under intermittent light contain only one-fifth of the content of high potential cytochrome b-559 seen in fully greened plants, yet show high rates of water to methyl viologen electron transport. Aquisition of the high potential cytochrome b-559 accompanies synthesis of chlorophyll b, the onset of Mg-stimulated fluorescence and an increased variable yield of fluorescence. A similar correlation was seen during greening of dark-grown barley. 4. It is proposed that the high potential state of cytochrome b-559 is due to the same membrane properties which allow cation enhanced variable fluorescence, so that the presence of low potential cytochrome b-559 is accompanied by a decrease in variable fluorescence yield.

Cell Membrane↗
PubMed1977

The relationship between the activity of chloroplast photosystem II and the midpoint oxidation-reduction potential of cytochrome b-559.

The role of cytochrome b-559 in Photosystem II reactions has been investigated using hydroxylamine treatment of chloroplast membranes. Incubation of chloroplasts with hydroxylamine in darkness resulted in inhibition of water oxidation and a decrease in the amplitude of cytochrome b-559 reducible by hydroquinone. The loss of water oxidizing activity perfectly correlated with the decrease in amplitude of cytochrome b-559 reduction. Potentiometric titration of cytochrome b-559 after hydroxylamine treatment revealed a component with Em7.8 at +240 mV in addition to a lower potential species at +90 mV. This compared to control chloroplasts in which cytochrome b-559 exists in the typical high potential state, Em7.8 = +383 mV, in addition to some of the low potential (Em7.8 = +77 mV) form. Photosystem II activity could be further inhibited by incubation with hydroxylamine in the light. In these chloroplasts only low rates of photooxidation of artificial electron donors were observed compared to 'dark' chloroplasts. In addition, the hydroxylamine light treatment caused a further change in cytochrome b-559 redox properties; a single component, Em7.8 = 90 mV is seen in titration curves. The role of cytochrome b-559 in Photosystem II functioning is discussed on the basis of these observations which suggest a dependence of photooxidizing ability of Photosystem II on the redox properties of this cytochrome.

Chloroplasts↗
PubMed1976

Stimulation of photosystem I-induced oxidation of chloroplast cytochrome b-559 by pre-illumination and by low pH.

(1) The proportion of higher plant chloroplast cytochrome b-559 oxidizable during illumination by low intensity 732 nm light increases as the pH is decreased below 6.5. At pH 5.0-5.3 total oxidation is seen and subsequent red light can cause reduction of up to 2/3 of the oxidized cytochrome. The oxidation by far red light at pH 5 is inhibited by 2 muM 2,5-dibromo-3-methyl-6-isopropyl-rho-benzoquinone whereas the red light-induced reduction is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In this pH range ferricyanide-oxidized cytochrome b-559 exists in a form not reducible by ferrocyanide. (2) An increase in the amplitude of far-red induced oxidation also occurs at higher pH (up to pH 7.8) after pre-treatment of chloroplasts with substantially higher levels of light (approx. 10(6) ergs-cm-2-s-1). The degree of light activation is pH dependent, being more pronounced at lower pH. After light activation, cytochrome b-559 can be completely oxidized by far-red light in a manner reversible by red light up to pH values of 6, and the curve describing the amplitude of far-red oxidation as a function of pH is shifted by 0.5-1.0 pH unit toward higher pH. Far-red oxidation and red light reduction are again inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, respectively. (3) Light activation at pH 5.2-6.0 is also manifested in a small decrease in the amplitude of subsequent dark ferrocyanide reduction, and this decrease is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (10 muM). (4) The effect of intramembranal acidity on the effective redox potential of cytochrome b-559 and its function is discussed.

Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone↗
PubMed1975

Light-induced turnover of chloroplast cytochrome b-559 in the presence of N-methylphenazonium methosulphate.

In the presence of 0.1-5 muM N-methylphenazonium methosulphate approx. 50-70% oxidation of cytochrome b-559 can be induced by far-red light. The oxidation is best observed with long wavelength far-red light (732 nm) of moderate intensities (approx. 10(4) ergs/cm2 per s) and is reversed by subsequent illumination with red light. Concentrations of N-methylphenazonium methosulphate above 5 muM are inhibitory probably due to cyclic electron flow. The far-red oxidation is inhibited by low concentrations of the plastoquinone antagonist 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, while 3-(3,4-dichlorophenyl)-1,1-dimethylurea inhibits red light reduction and increases the amplitude of far-red oxidation. The effect of N-methylphenazonium methosulphate is mimicked by N-methyl-phenazonium ethosulphate, but not by pyocyanine or diaminodurene. Low concentrations (2-3 muM) of N-methylphenazonium methosulphate also stimulate a 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone-inhibitable red light reduction of cytochrome f.

Chlorophyll↗
PubMed1975

The effect of adenosine 5'-triphosphate on the shibata shift and on associated structural changes in the conformation of the prolamellar body in isolated maize etioplasts.

Isolated maize (Zea mays var. kelvedon glory) etioplasts have been used to investigate the relationships between the spectral shifts and ultrastructural changes which occur during light-induced chloroplast development. After primary photoconversion, the Shibata shift was observed as a change from 680 to 670 nm in the chlorophyllide absorption maximum. When 1.5 nm ATP was added to the incubation medium the maximum was 675 nm even after 3.5 hours of illumination. Difference spectra for this effect indicate ATP inhibition of the Shibata shift. Two bands with maxima at 682 and 669 nm can be used to fit spectra of both ATP-treated and control etioplasts, the estimated proportions of chlorophyllide 682 being 36% and 6%, respectively. Quantitative analysis of electron micrographs of the etioplasts showed that the frequency of untransformed prolamellar bodies was also higher in the presence of ATP (73% untransformed compared to 22% in the absence of ATP). A similar correlation was observed when transformation was measured for two etioplast fractions which show the shift to different extents. These results imply that the Shibata shift and prolamellar body transformation are related events, both being inhibited by the presence of ATP. ATP may therefore have an important role in regulating the early stages of plastid development.

Journal Article↗
PubMed1975

The Effect of ATP on the Photoconversion of Protochlorophyllide in Isolated Etioplasts of Zea mays.

The transformation of protochlorophyllide (PChle) into chlorophyllide (Chle) has been studied in isolated etioplasts from Zea mays. ATP (1.5mm) prevented the transformation of photoconvertible PChle 650 to PChle 630 in aged etioplasts. Curve analysis indicated that the ATP effect on photoconvertibility could be entirely accounted for by changes in the proportions of PChle 630 and PChle 650 and examination of the isolated pigments revealed that only unphytylated PChle could be activated for photoconversion by ATP. In etioplasts aged for 5 hours, ATP also stimulated photoconversion of PChle 630 into Chle 670. The process was temperature-sensitive and involved PChle 650 and Chle 680 as intermediates. AMP alone had no effect, but inhibited ATP retardation of PChle loss. ADP had a similar but lesser effect than ATP. The ADP response, but not the ATP response, was considerably enhanced in the presence of an ATP-generating system (phosphoenolpyruvate/pyruvate kinase). UTP, GTP, and CTP gave 40 to 50% of the ATP response with intact etioplasts. In envelope-free etioplasts, ATP gave the greatest response but the other nucleotides were now 80% as effective as ATP. After primary photoconversion, ATP stimulated resynthesis of PChle 650. It is proposed that ATP both gives the holochrome the ability to bind to the PChle molecule and enables additional association of the pigment-protein complex to form PChle 650.

Journal Article↗
PubMed1973

Remission of thyrotoxicosis during treatment with propranolol.

Twenty-eight thyrotoxic patients were treated with propranolol. In seven patients the drug had to be discontinued after one or two months, but in the remaining 21 clinical improvement was observed. Serial clinical studies and tests of thyroid function performed at monthly intervals showed that in four patients thyrotoxicosis remitted and all indices of thyroid function returned to normal. A fifth patient shows distinct evidence of remission with the 20-minute (132)I uptake falling to normal, although the free-thyroxine index remains slightly raised. It is likely that these remissions reflect the natural tendency of the disease to remit since propranolol is not considered to have any direct in-vivo effect on thyroid function.However, because of failure to gain adequate control of symptoms in all patients treated, and the fact that circulating thyroid hormone levels were often not restored to normal, propranolol is considered an unsatisfactory alternative to conventional antithyroid drugs for routine treatment.

Female↗