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31.
Mouse protoporphyrinogen oxidase. Kinetic parameters and demonstration of inhibition by bilirubin. 总被引:4,自引:1,他引:3
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The penultimate step of haem biosynthesis, the oxidation of protoporphyrinogen to protoporphyrin, was examined with purified murine hepatic protoporphyrinogen oxidase (EC 1.3.3.4) in detergent solution. The kinetic parameters for the two-substrate (protoporphyrinogen and oxygen) reaction were determined. The limiting Km for protoporphyrinogen when oxygen is saturating is 6.6 microM, whereas the Km for oxygen with saturating concentrations of protoporphyrinogen is 125 microM. The kcat. for the overall reaction is 447 h-1. The ratio of kcat. to the Km for protoporphyrinogen is approx. 20-fold greater than the kcat./Km,O2 ratio. The ratio of protoporphyrin formed to dioxygen consumed is 1:3. Ubiquinone-6, ubiquinone-10 and dicoumarol stimulate protoporphyrinogen oxidase activity at low concentrations (less than 15 microM), whereas coenzyme Q0 and menadione show no activation at these concentrations. Above 30 microM, all five quinones inhibit the enzyme activity. FAD does not significantly affect the activity of the enzyme. Bilirubin, a product of haem catabolism, is shown to be a competitive inhibitor of the penultimate enzyme of the haem-biosynthetic pathway, protoporphyrinogen oxidase, with a calculated Ki of 25 microM. The terminal enzyme of haem-biosynthetic pathway, namely ferrochelatase, is not inhibited by bilirubin at concentrations over double the Ki value for the oxidase. In contrast with other enzymic systems, the toxicity of bilirubin is not reversed by binding to albumin. 相似文献
32.
33.
Guerra F. A.; Savich R. D.; Wallen L. D.; Lee C. H.; Clyman R. I.; Mauray F. E.; Kitterman J. A. 《Journal of applied physiology》1988,64(5):2160-2166
To test the hypothesis that prostaglandin (PG) E2 is a respiratory depressant in the newborn lamb, 12 chronically catheterized, unanesthetized lambs (age 2-6 days) were infused with progressively increasing doses of PGE2 (0.1, 0.5, 1.0, and 5.0 micrograms.kg-1.min-1; 30 min for each dose) into the ascending aorta. PGE2 caused significant progressive decreases in ventilation (due to decreased tidal volume and breathing rate), heart rate, blood pressure, and percent of the time spent in low-voltage electrocortical activity (LVA). PGE2 also caused respiratory acidosis, hypoxemia, and increased frequency and duration of apneic events (greater than 3 s). During the infusion there was a dose-related increase in plasma concentration of PGE2. At 30 min postinfusion, all measured variables showed recovery, although arterial pH, CO2 tension, and plasma PGE2 remained significantly different from control values, and the percent time in LVA was even higher than during control. Infusion of the vehicle alone (n = 5) caused no significant changes in any of the measured variables. The results, taken in combination with previous fetal studies, indicate that PGE2 has marked inhibitory effects on breathing movements both before and after birth. 相似文献
34.
Andrew W. Thomas Jay Lewington Steve Hope Andrew W. Topping Andrew J. Weightman J. Howard Slater 《Archives of microbiology》1992,158(3):176-182
Favourable mutations involving the two dehalogenases (DehI and DehII) of Pseudomonas putida PP3 and derivative strains containing the cloned gene for DehI (dehI) occurred in response to specific environmental conditions, namely: starvation conditions; the presence of dehalogenase substrates (halogenated alkanoic acids — HAAs) which were toxic to P. putida; and/or the presence of a potential growth substrate. Fluctuation tests showed that these mutations were environmentally directed by the presence of HAAs. the mutations were associated with complex DNA rearrangements involving the movement of dehI located on a transposon DEH. Some mutations resulted in switching off the expression of either one or both of the dehalogenases, events which were effective in protecting P. putida from toxic compounds in its growth environment. Other mutations partially restored P. putida's dehalogenating capability under conditions where toxic substrates were absent. Restoration of the capability to untilize HAAs was favoured when normal growth substrates were present in the environment. 相似文献
35.
The distribution of holly ( Ilex aquifolium ) and its habitat preferences indicate a sensitivity to low temperature, particularly when exposed to high light. Experiments were conducted to determine whether photoinhibition of photosynthesis occurs in holly leaves in the field in United Kingdom during the winter. Photosynthetic efficiency was assessed in holly leaves that were exposed to or shaded from direct sunlight using measurements of photosynthetic oxygen evolution and chlorophyll fluorescence. Field measurements were conducted over 3 weeks during January and February. Correlation of the measurements of photosynthetic efficiency with weather conditions indicated that holly was suffering photoinhibition, particularly in leaves exposed to direct sunlight. Controlled environment studies demonstrated that exposure of leaves to low temperature and high light resulted in reductions in photosynthetic efficiency; however, leaves recovered rapidly when exposed to a higher temperature and reduced light level. 相似文献
36.
Ronald A. Martin Steven P. Lynch Francis J. Schmitz Eva O. Pordesimo Steve Toth Rita Y. Horton 《Phytochemistry》1991,30(12):3935-3939
6′-O-(E-4-hydroxycinnamoyl) Desglucouzarin, the first cardenolide containing a cinnamoyl ester moiety, has been isolated from the ethanolic extract of the milkweed, Asclepias asperula. In addition, five known cardenolides were isolated and identified from A. asperula and A. viridis. 相似文献
37.
To study breed differences in ewes induced to parturition with dexamethasone (DXM), 15 Corriedale and 16 Polwarth ewes were injected with 15 mg of DXM four days before the expected lambing date (Days 144 and 145, respectively) in Experiment 1. Interval from treatment to parturition was twice as long for the Polwarth than for the Corriedale breed (57.9 vs 28.8 hours, P = 0.001). In Experiment 2, the effect of DXM on the interval from parturition to expulsion of the placenta and on biological parameters of the lambs (birth weight, ability to stand after birth and daily weight gain during the first week of life) was studied in 30 Corriedale ewes. On Day 144 of gestation, ewes were given 15 mg of DXM (n= 22) or 15 ml of saline (controls, n= 8). The DXM group lambed 33.8 +/- 2.7 hours after injection. Treatment caused an increase in the time of placental expulsion (4.3 vs 3.4 hours, P>/= 0.05) and on the percentage of ewes retaining the placenta beyond 6 hours (18% vs 0%, P>/= 0.05). Birth weight of lambs was lower in the induced group (3.9 vs 4.5 kg, P= 0.03), but this did not affect the other parameters of the lambs studied. 相似文献
38.
Coccidian oöcysts recovered from the faeces of eastern ringneck snakes, Diadophis punctatus arnyi, from Kansas, USA were found to represent a previously unreported eimerian. Oöcysts of Eimeria arnyi n. sp. are subspherical, 16.9×15.1 (15–18.5×13.5–16) m, with a thin, single-layered wall and a shape-index (length/width) of 1.1 (1.1–1.3). A micropyle and öocyst residuum are absent but a large polar granule is present. The sporocysts are elongate, 13.2×6.9 (12–14.5×6.5–7) m, with Stieda and substieda bodies and a shape-index of 1.9 (1.7–2.3). Each sporozoite contains one to two anterior and a single posterior refractile bodies. Sporulation was exogenous and complete within four days at 23°C. 相似文献
39.
Targeting of T7 RNA polymerase to tobacco nuclei mediated by an SV40 nuclear location signal 总被引:9,自引:0,他引:9
Michael W. Lassner Aubrey Jones Steve Daubert Luca Comai 《Plant molecular biology》1991,17(2):229-234
We have expressed two T7 RNA polymerase genes by electroporation into tobacco protoplasts. One of the genes was modified by inserting nucleotides encoding a viral nuclear localization signal (NLS) from the large T antigen of SV40. Both T7 RNA polymerase genes directed synthesis of a ca. 100 kDa protein in the electroporated protoplasts. T7 RNA polymerase activity was detected in extracts of protoplasts electroporated with both genes. Immunofluorescence analysis of these protoplasts indicated that only the polymerase carrying the NLS accumulated in the cell nucleus. These experiments suggest that mechanisms involved in the transport from the cytoplasm to the nucleus are similar in plant and animal cells. This system demonstrates the feasibility of T7 RNA polymerase-based approaches for the high-level expression of introduced genes in plant cells. 相似文献
40.
Michel A. Haring Steve Scofield Marianne J. Teeuwen-de Vroomen Gerjan S. Leuring H. John J. Nijkamp Jacques Hille 《Plant molecular biology》1991,17(5):995-1004
A Tam3 two-element system has been designed by combining an immobilized Tam3 element with a non-autonomous dTam3 element inserted into the HPT gene. The phenotypic assay employed, restored hygromycin resistance, indicated thattrans-activation of the non-autonomous dTam3 element occurred. Molecular analyses of the excision sites revealed that the ends of the dTam3 element remain in the empty donor sites. The predominant consequence of this type of excision appears to be that excised fragments fail to re-integrate into the tobacco genome. Only one case of dTam3 re-integration could be detected. The ends of this element had been degraded upon integration into the tobacco genome. Either the altered structure of the Tam3 derivatives or tobacco host factors are influencing thetrans-activation of a dTam3 element, resulting in aberrant excision. 相似文献