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Biomedical subjects

A Melis

Publications and source records attributed to A Melis.

At least 55 records · Page 3Linked to original sources
PubMed1990

Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition.

The effect of strong irradiance (2000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chlamydomonas reinhardtii was investigated. Low light (LL, 15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSII(alpha)) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (Q(b)-reducing centers). The smaller antenna (PSII(beta)) account for the remainder of PSII and exist in a state not yet able to photoreduce plastoquinone (Q(b)-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII(beta) content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSII(alpha) decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSII(beta) from a Q(b)-nonreducing to a Q(b)-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSII(alpha). The results suggest that PSII(beta) in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII(alpha).

Journal Article↗
PubMed1990

Hordeum vulgare Seedlings Amine Oxidase: Purification and Properties.

Although no amine oxidase could be detected in crude extracts, the enzyme has been purified to apparent homogeneity from Hordeum vulgare seedlings using ammonium sulfate precipitation and chromatography on DEAE cellulose, Hydroxylapatite, and Sephadex G200 columns. Gel filtration experiments indicate a molecular weight of about 150,000. The pH optimum of the enzyme was found to be 7.5 in potassium phosphate buffer. The spectrum of ultraviolet and visible regions were similar to Cuamine oxidase from Leguminosae.

Journal Article↗
PubMed1990

Response of the Photosynthetic Apparatus in Dunaliella salina (Green Algae) to Irradiance Stress.

The response of the photosynthetic apparatus in the green alga Dunaliella salina, to irradiance stress was investigated. Cells were grown under physiological conditions at 500 millimoles per square meter per second (control) and under irradiance-stress conditions at 1700 millimoles per square meter per second incident intensity (high light, HL). In control cells, the light-harvesting antenna of photosystem I (PSI) contained 210 chlorophyll a/b molecules. It was reduced to 105 chlorophyll a/b in HL-grown cells. In control cells, the dominant form of photosystem II (PSII) was PSII(alpha)(about 63% of the total PSII) containing >250 chlorophyll a/b molecules. The smaller antenna size PSII(beta) centers (about 37% of PSII) contained 135 +/- 10 chlorophyll a/b molecules. In sharp contrast, the dominant form of PSII in HL-grown cells accounted for about 95% of all PSII centers and had an antenna size of only about 60 chlorophyll a molecules. This newly identified PSII unit is termed PSII(gamma). The HL-grown cells showed a substantially elevated PSII/PSI stoichiometry ratio in their thylakoid membranes (PSII/PSI = 3.0/1.0) compared to that of control cells (PSII/PSI = 1.4/1.0). The steady state irradiance stress created a chronic photoinhibition condition in which D. salina thylakoids accumulate an excess of photochemically inactive PSII units. These PSII units contain both the reaction center proteins and the core chlorophyll-protein antenna complex but cannot perform a photochemical charge separation. The results are discussed in terms of regulatory mechanism(s) in the plant cell whose function is to alleviate the adverse effect of irradiance stress.

Journal Article↗
PubMed1990

Effect of volatile anesthetics on vecuronium-induced neuromuscular blockade in children.

We studied 60 children undergoing elective surgery to evaluate the effect of interactions between vecuronium and isoflurane or halothane on the potency and duration of neuromuscular blockade, as measured by electromyography. Vecuronium was first administered by a logarithm-based cumulative method (14, 22, 35, 56, 89 micrograms/kg) in 10 children anesthetized with thiopental (5 mg/kg), alfentanil (15 micrograms/kg first dose, then 10 micrograms/kg), and N2O/O2 (60:40) until a 95% +/- 2% twitch depression (ED95) was obtained. Thirty children given the same balanced anesthesia were then randomly assigned to three groups (n = 10 in each) to receive a single ED20 (21 micrograms/kg), ED50 (33 micrograms/kg), or ED80 (47 micrograms/kg) intravenous bolus of vecuronium calculated from the mean regression line of twitch responses of the first 10 children. In the second part of the study, 20 children were anesthetized with isoflurane (1.2%) or halothane (0.7%) and compared with the previous 10 children anesthetized with alfentanil-N2O. Potency of vecuronium determined by single-bolus or logarithm-based cumulative techniques was not significantly different. Isoflurane and halothane significantly decreased ED50 (22.3 +/- 1.6 and 25.4 +/- 1.4 micrograms/kg, respectively; mean +/- SE) and ED95 (41.5 +/- 3.3 and 46.7 +/- 3.2 micrograms/kg, respectively) compared with alfentanil-N2O (ED50: 32.8 +/- 0.8 micrograms/kg, ED95: 70.5 +/- 2.6 micrograms/kg). Recovery rate from vecuronium-induced neuromuscular blockade was significantly longer with isoflurane than with alfentanil-N2O or halothane. We conclude that in children single-bolus and logarithm-based cumulative techniques give similar potency estimates for vecuronium. Isoflurane and halothane increase by similar amounts the neuromuscular potency of vecuronium.(ABSTRACT TRUNCATED AT 250 WORDS)

Alfentanil↗
PubMed1990

[Trans-membrane cationic flow and hemodialysis].

Abnormalities of intracellular ion concentrations and transmembrane fluxes were reported in uremia. In RBC from 12 chronically hemodialyzed patients (age 41 + 12, 7 men, 5 women; mean dialysis duration 31 + 24 months), we evaluated the acute effects of hemodialysis on intracellular Na and K concentrations, ouabain sensitive Na/K pump, furosemide sensitive Na/K cotransport, Na/Li countertransport, and passive permeability to Na. Six patients were normotensive and 6 were taking antihypertensive drugs which were withdrawn before the study. When compared to our normal reference group, uremic patients showed a significant increase in intracellular K concentration and a significant decrease in ouabain-sensitive Na/K pump. Intracellular sodium was not increased. No correlation was found between the activity of sodium-potassium pump and the duration of hemodialysis. The other transport systems were comparable to normal. No significant change was observed between the values measured before and after dialysis. Ouabain sensitive Na/K pump was lower in hypertensive as compared to normotensive patients, but this difference was not significant. Our data support the existence of ion transport derangements in uremia, which are not acutely affected by hemodialysis.

Adult↗
PubMed1990

Renal response to furosemide in normal subjects during hyperinsulinaemic clamp.

The study has been performed in an attempt to provide further data on the supposed direct action of insulin on the kidney, based on the assumption that any effect of insulin on sodium reabsorption via co-transport should shift the dose-response curve to the furosemide administration. In five normal male volunteers plasma insulin concentration was changed by means of a hyperinsulinaemic euglycaemic clamp. The diuresis and natriuresis following intravenous injections of furosemide (in increasing doses) were measured in basal conditions and during clamp. No significant difference was found between the experiments performed in the two conditions. Our study has not identified a direct sodium-retaining effect of insulin on the furosemide-sensitive mechanisms of the renal tubule.

Adult↗
PubMed1990

Effects of furosemide on blood pressure in anephric rats.

This study has been performed in order to evaluate whether furosemide can induce changes in blood pressure independently of its diuretic effects,and whether its pressor effects are connected with the ability to synthetize renal prostaglandins. The experiments were performed in four groups of volume-expanded rats: the first (n.5) had bilateral ligation of the renal vessels; the second (n.8) had bilateral ligation of the ureters; the third group (n.6) had ureters ligation after pre-treatment with indomethacin; the fourth group (n.2) received a pre-treatment with captopril. After blood pressure stabilization, furosemide was administered i.v. (0.125 mg/100 g. of body weight). Indirect readings of the blood pressure were obtained at 1, 2, 3, 4, 5, 7.5, 10, 15, 20 min. After furosemide administration, blood pressure fell down quickly in the rats with ureteral ligation and in those with captopril pretreatment, while the tensive response to furosemide was blunted by the indomethacin treatment. In conclusion, furosemide can reduce blood pressure by a mechanism non related with its diuretic properties, which, however, requires the integrity of the renal connections with the circulatory system.

Animals↗
PubMed1990

[Insulin resistance in the uremic patient: effects of hemodialysis].

The carbohydrate metabolism abnormalities present in uremia have been attributed to a combination of peripheral resistance to insulin and inhibition of insulin release secondary to beta cells insensitivity. Previous studies evaluated the chronic effects of hemodialysis on glucose metabolism, while acute effects were not examined. In 12 uremic subjects undergoing hemodialysis (3 times a week) the fasting serum levels of glucose, insulin and C-peptide were measured, and the glucose/insulin ratio was calculated as an index of peripheral sensitivity to insulin before and after dialysis. While glucose did not change, insulin and C-Peptide rose significantly after dialysis. The rise in insulin and C-Peptide was directly correlated with the fall in body weight due to the fluid loss. This might indicate that the increase in insulin and C-peptide was due to a concentration phenomenon. However, it could reflect a reaction to hypoglycemia. The glucose/insulin ratio exhibited trend towards a fall after hemodialysis. Although not significant, this might indicate an acute worsening of the peripheral sensitivity to insulin, induced by the procedure.

Adult↗
PubMed1990

Insulin resistance and beta-cell hypersecretion in essential hypertension.

To determine whether a decreased sensitivity to insulin is involved in the pathogenesis of essential hypertension, fasting blood glucose, serum insulin, serum C peptide, the glucose:insulin ratio and the insulin:C-peptide ratio were measured in 14 lean normotensives, 17 overweight normotensives, 17 lean hypertensives and 20 overweight hypertensives. Compared with the lean normotensives, the patients who were overweight, those with hypertension and those who were both overweight and hypertensive showed increased fasting serum insulin and C-peptide levels, and a lower glucose:insulin ratio. No significant difference between the normotensive and the hypertensive subjects was found in the insulin:C-peptide ratio. Diastolic blood pressure was directly correlated with serum insulin (P less than 0.01) and with C-peptide levels (P less than 0.01), and inversely correlated with the glucose:insulin ratio (P less than 0.02). We conclude that insulin resistance is present in both essential hypertensive and overweight subjects. Since the present study showed that hepatic insulin clearance was normal in hypertensives, the hyperinsulinaemia in essential hypertension appears to be due to beta-cell hypersecretion in response to a defective peripheral action of the hormone.

Adult↗
PubMed1989

[Disseminated intravascular coagulation and acute hepatic necrosis at the end of pregnancy. A case report].

A 30-year-old woman in the 36th week of her second pregnancy, suddenly developed jaundice with remarkable liver necrosis, accompanied by generalized bleeding due to disseminated intravascular coagulation (DIC). She underwent a caesarean and a dead foetus was extracted from the uterus. Heparin and frozen plasma infusion resulted in a prompt recovery from the haemostatic disorder. The course of the disease involved the successive appearance of haemorrhagic shock, intestinal ileus and pulmonary embolism all of which she recovered from. The liver biopsy showed severe cholestasis without derangement of the lobular structure. Hypotheses of acute veno-occlusive disease caused by the DIC, and acute fatty liver of pregnancy are discussed.

Acute Disease↗
PubMed1988

Compensatory Alterations in the Photochemical Apparatus of a Photoregulatory, Chlorophyll b-Deficient Mutant of Maize.

Characterization of the functional organization of the photochemical apparatus in the light sensitive chlorophyll b-deficient oil yellow-yellow green (OY-YG) mutant of maize (Zea mays) is presented. Spectrophotometric and kinetic analysis revealed substantially lower amounts of the light harvesting complex of photosystem II (LHCII-peripheral) in high light-grown OY-YG thylakoids. However, accumulation of a tightly bound LHCII appears unaffected by the lesion. Changes in photosystem (PS) stoichiometry include lower amounts of PSII with characteristic fast kinetics (PSII(alpha)) and a substantial accumulation of PSII centers with characteristic slow kinetics (PSII(beta)) in the thylakoid membrane of the OY-YG mutant. Thus, PSII(beta) is the dominant photosystem in the mutant chloroplasts. In contrast to wild type, roughly 80% of the mutant PSII(beta) centers are functionally coupled to the plastoquinone pool and are probably localized in the appressed regions of the thylakoid membrane. These centers, designated PSII(beta)-Q(B)-reducing (Q(B) being the secondary electron quinone acceptor of PSII), are clearly distinct from the typical PSII(beta)-Q(B)-nonreducing centers found in the stroma lamellae of wild-type chloroplasts. It is concluded that the observed changes in the stoichiometry of electron-transport complexes reflect the existence of a regulatory mechanism for the adjustment of photosystem stoichiometry in chloroplasts designed to correct any imbalance in light absorption by the two photosystems.

Journal Article↗
PubMed1987

Protonophores induce plastoquinol oxidation and quench chloroplast fluorescence: Evidence for a cyclic, proton-conducting pathway in oxygenic photosynthesis.

The photosynthetic apparatus converts light into chemical energy by a series of reactions that give rise to a coupled flow of electrons and protons that generate reducing power and ATP, respectively. A key intermediate in these reactions is plastoquinone (PQ), the most abundant electron and proton (hydrogen) carrier in photosynthetic membranes (thylakoids). PQ ultimately transfers electrons to a terminal electron acceptor by way of the Rieske Fe-S center of the cytochrome bf complex. In the absence of a terminal acceptor, electrons accumulate in the PQ pool, which is reduced to plastoquinol (PQH(2)), and also on a specialized PQ, Q(A), which is reduced to an unprotonated semiquinone anion (Q(A) (-)). The accumulation of Q(A) (-) is measured by a rise in fluorescence yield and the accumulation of PQH(2) is measured by absorption difference spectrometry. We have found that in the absence of a terminal electron acceptor, two chemically diverse proton-conducting ionophores (protonophores), 2,6-di-t-butyl-4-(2',2'-dicyanovinyl)phenol (SF 6847) and carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP), induced oxidation of PQH(2) and quenching of chloroplast fluorescence, signifying oxidation of Q(A) (-). The two protonophores produced the same effects even when the only recognized pathway of PQH(2) oxidation by way of the cytochrome bf complex was inhibited by dibromothymoquinone. Two other uncouplers, gramicidin and nigericin, which are not protonophores but facilitate proton movement across membranes by other mechanisms, were ineffective. These findings are consistent with the operation in the oxygen-generating photosystem (photosystem II) of a cyclic, proton-conducting pathway.

Journal Article↗
PubMed1987

Changes in Photosynthetic Capacity and Photosynthetic Protein Pattern during Tomato Fruit Ripening.

Levels of polypeptides participating in the photosynthetic light and dark reactions have been measured during fruit ripening in tomato. Photosynthetic proteins were identified by Western blot analysis with heterologous antibodies. The concentrations of proteins of photosystem (PS) I (14 kilodaltons), of PSII (47-kilodalton reaction center protein, 32-kilodalton ;Q(B) binding' protein and light harvesting complex proteins), of the photosynthetic electron transport chain (ferredoxin-NADP-oxidoreductase and plastocyanin), and of the stroma (ribulose-1,5-bisphosphate carboxylase) decrease during the ripening process. The 32-kilodalton protein and plastocyanin were detectable in pericarp protein preparations of ripe tomato fruits. Absorbance difference spectrophotometry provided information on the relative concentrations of PSII and PSI reaction centers in leaf and green fruit tissue of tomato. These results indicate that green fruit pericarp of tomato is photosynthetically active. Photosynthetic activity decreases during chloroplast/chromoplast differentiation. This is consistent with changes that occur at the transcript level of photosynthesis-specific proteins during the differentiation process.

Journal Article↗
PubMed1987

Photosystem Stoichiometry and Excitation Distribution in Chloroplasts from Surface and Minus 20 Meter Blades of Macrocystis pyrifera, the Giant Kelp.

The photochemical apparatus organization in the thylakoid membrane of Macrocystis pyrifera, the giant kelp, was investigated. Chloroplasts were isolated from surface and minus 20 meter blades. Photosynthetic electron-transport complex quantitation revealed ratios of photosystem (PS) II/cytochrome b(6)-f/PSI = 1.8:3.3:1.0 in surface and 2.2:2.3:1.0 in minus 20 meter blades. The apparent photosynthetic unit size of chloroplasts from minus 20 meter blades (chlorophyll/P700 = 1485:1) was about 45% larger than that of surface blades (chlorophyll/P700 = 1025:1). The larger photosynthetic unit size of minus 20 meter blades is attributed to the substantially lower intensity of sunlight reaching the minus 20 meter habitat. In different chloroplast preparations, the effective absorption cross section of PSI and PSII to 670 nanometer light (chlorophyll a) and 481 nanometer light (chlorophyll c and fucoxanthin) was investigated. The results showed larger functional antenna size for PSII (about 90%) and for PSI (about 50%) in minus 20 meter than in surface blades. Moreover, the efficiency of utilization of 481 nanometer light by Macrocystis chloroplasts was equal to that of 670 nanometer light. It is concluded that the chlorophyll c-fucoxanthin complex in brown algae enables the highly efficient utilization of blue-green wavelengths of the nearshore marine environment and contributes to the dominance of M. pyrifera in this habitat.

Journal Article↗
PubMed1986

Differential detergent-solubilization of integral thylakoid membrane complexes in spinach chloroplasts. Localization of photosystem II, cytochrome b6-f complex and photosystem I.

Progressive solubilization of spinach chloroplast thylakoids by Triton X-100 was employed to investigate the domain organization of the electron transport complexes in the thylakoid membrane. Triton/chlorophyll ratios of 1:1 were sufficient to disrupt fully the continuity of the thylakoid membrane network, but not sufficient to solubilize either photosystem I (PSI), photosystem II (PSII) or the cytochrome b6-f(Cyt b6-f) complex. Progressive with the Triton concentration increase (Triton/Chl greater than 1:1), a differential solubilization of the three electron transport complexes was observed. Solubilization of the Cyt b6-f complex from the thylakoid membrane preceded that of PSI and apparently occurred early in the solubilization of stroma-exposed segments of the chloroplast lamellae. The initial removal of chlorophyll (up to 40% of the total) occurred upon solubilization of PSI from the stroma-exposed lamella regions in which PSI is localized. The tightly appressed membrane of the grana partition regions was markedly resistant to solubilization by Triton X-100. Thus, solubilization of PSII from this membrane region was initiated only after all Cyt b6-f and PSI complexes were removed from the chloroplast lamellae. The results support the notion of extreme lateral heterogeneity in the organization of the electron transport complexes in higher plant chloroplasts and suggest a Cyt b6-f localization in the membrane of the narrow fret regions which serve as a continuum between the grana and stroma lamellae.

Binding Sites↗
PubMed1986

Light quality regulates expression of chloroplast genes and assembly of photosynthetic membrane complexes.

The concentrations of photosystem I (PSI) and photosystem II (PSII) reaction centers and the level of chloroplast reaction center gene transcripts were determined in pea plants grown under different light-quality regimes. In plants grown in light primarily absorbed by PSI ("red" light), the PSII/PSI reaction center ratio was 2-fold greater than that in plants grown in PSII-sensitizing ("yellow") light. In addition, the ratio of a PSII gene (psbB) transcript to a PSI gene (psaA) transcript was 2.6 times greater in red-grown plants relative to yellow-grown plants. Thus, a differential reaction-center concentration in the thylakoid membrane was accompanied by a differential expression of reaction center genes, suggesting that the synthesis of chloroplast membrane complexes and the assembly of photosystems are regulated by light quality at the transcriptional and/or post-transcriptional level.

Journal Article↗
PubMed1986

Cyanobacterial Acclimation to Photosystem I or Photosystem II Light.

The organization and function of the photochemical apparatus of Synechococcus 6301 was investigated in cells grown under yellow and red light regimes. Broadband yellow illumination is absorbed preferentially by the phycobilisome (PBS) whereas red light is absorbed primarily by the chlorophyll (Chl) pigment beds. Since PBSs are associated exclusively with photosystem II (PSII) and most of the Chl with photosystem I (PSI), it follows that yellow and red light regimes will create an imbalance of light absorption by the two photosystems. The cause and effect relationship between light quality and photosystem stoichiometry in Synechococcus was investigated. Cells grown under red light compensated for the excitation imbalance by synthesis/assembly of more PBS-PSII complexes resulting in high PSII/PSI = 0.71 and high bilin/Chl = 1.30. The adjustment of the photosystem stoichiometry in red light-grown cells was necessary and sufficient to establish an overall balanced absorption of red light by PSII and PSI. Cells grown under yellow light compensated for this excitation imbalance by assembly of more PSI complexes, resulting in low PSII/PSI = 0.27 and low bilin/Chl = 0.42. This adjustment of the photosystem stoichiometry in yellow light-grown cells was necessary but not quite sufficient to balance the absorption of yellow light by the PBS and the Chl pigment beds. A novel excitation quenching process was identified in yellow light-grown cells which dissipated approximately 40% of the PBS excitation, thus preventing over-excitation of PSII under yellow light conditions. It is hypothesized that State transitions in O(2) evolving photosynthetic organisms may serve as the signal for change in the stoichiometry of photochemical complexes in response to light quality conditions.

Journal Article↗