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1.
Immobilization was carried out of the lactate dehydrogenase (LDH) from rabbit muscle (EC 1.1.1.27), cross-linked through the bifunctional reactive glutar-aldehyde on to nylon tubing (1 m long, 53cm2 internal surface area). Immobilized LDH inactivation kinetics are of first order (t1/2 = 3·6 years, k = 5·4,e?4 day?1 to 5°C). The smaller effect of pH on activity than in the case of LDH in solution can be explained on the basis of limitation to proton diffusion towards the support. A limiting effect to free external diffusion of the substrate towards and products from the support was also observed, an effect which seems to determine the effective kinetic behaviour of immobilized LDH. The apparent optimum temperature is centred around 40°C, observing a clear inactivation (thermal denaturation) above this temperature. In the temperature range studied (10–40°C), the co-existence was seen of a kinetic control accompanied by another control, involving diffusional transport of substrates and products, on the global activity of the immobilized enzyme. This makes the Arrhenius profiles curvilinear. Both graphic and statistical non-linear regression analysis of the kinetic data—rate, v, versus substrate concentration [S]—carried out under conditions in which the diffusional limitations can be considered negligible (high recirculation flow rate), permitted investigation of the intrinsic kinetic behaviour of immobilized LDH. In this sense, it can be deduced that the rate equation to which these data seem to be fitted is of the polynomial quotient type in [S] of minimum degree 2:2. Although the diffusional limitations have a marked effect on the type of global kinetics shown by immobilized LDH, temperature was not found to affect its v[S] behaviour. The experimental evidence obtained thus indicates that the rate equation in the 10-40°C temperature range continues to be a rational equation of at least degree 2:2 in [S]. 相似文献
2.
José Luis Montañez-Soto Salvador Alegret Juan Alfredo Salazar-Montoya Emma Gloria Ramos-Ramírez 《European Food Research and Technology》2006,223(3):379-386
In this work a novel amperometric biosensor for fructose determination in solutions was developed. The device was constructed by the incorporation of a tetrathiofulvalene-tetracyanoquinodimethane organic conducting salt and fructose dehydrogenase enzyme, include in a polymeric matrix of epoxy resin and graphite powder. Because of the electrocatalytic function of the salt, the direct transfer of the electron between the reduced prosthetic group (PQQH2) of the enzyme and the transducing material, was verified at a low working potential (150 mV vs. Ag/AgCl), where the interfering reactions were minimized. The response time at 90% of the steady state value was less than 20 s. The current response was directly proportional to the D-fructose concentration from 0.01 to 0.3 mmol/l with a detection limit of 0.005 mmol/l (signal/noise of 3) and a sensitivity of 1.9985 μA/mmol. The biosensor sensitivity diminishes when its surface is not polished between successive determinations, and remains constant (rsd=1.85, n=10) when the surface is polished between determinations. The effects of temperature and pH on the biosensor response were studied and analyzed; also the properties of the enzyme (Km
ap, I
max, Q10) were determinate in this work. The biosensor was used to determine fructose in high fructose syrups and there were not significant differences between these results and those obtained by HPLC (p≤0.05). During 4 months, in intermittent determinations the biosensor kept 100% of its original sensitivity and after 18 months stored at 4°C, it only lost 32% of its sensitivity. The simplicity, low working potential, high stability and good performance of this biosensor shows a great potential for its use in the fructose determination. 相似文献
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Lysine succinylation is a post-translational modification which alters protein function in both physiological and pathological processes. Mindful that it requires succinyl-CoA, a metabolite formed within the mitochondrial matrix that cannot permeate the inner mitochondrial membrane, the question arises as to how there can be succinylation of proteins outside mitochondria. The present mini-review examines pathways participating in peroxisomal fatty acid oxidation that lead to succinyl-CoA production, potentially supporting succinylation of extramitochondrial proteins. Furthermore, the influence of the mitochondrial status on cytosolic NAD+ availability affecting the activity of cytosolic SIRT5 iso1 and iso4—in turn regulating cytosolic protein lysine succinylations—is presented. Finally, the discovery that glia in the adult human brain lack subunits of both alpha-ketoglutarate dehydrogenase complex and succinate-CoA ligase—thus being unable to produce succinyl-CoA in the matrix—and yet exhibit robust pancellular lysine succinylation, is highlighted. 相似文献
5.
The reversal of humic matter-induced inhibition of callus growth and metabolism by 2,4-dichlorophenoxyacetic acid (2,4-D) was studied in Pinus laricio. Two forest humic fractions (relative molecular mass (Mr) > 3500), derived from soil under Fagus sylvatica (Fs) and Abies alba (Aa) plantation, were used. Pinus laricio callus was grown for a subculture period (4 weeks) on Basal Murashige and Skoog (MS) medium plus forest humic matters (Fs or Aa), at a concentration of 1 mg C/l, and then was transferred, for an additional four weeks, to a MS medium culture without humic matter, but with different hormones: indole-3-acetic acid (IAA, 2 mg/1) or 2,4-dichlorophenoxyacetic acid (2,4-D, 0.5 mg/1) and/or 6-benzylaminopurine (BAP, 0.25 mg/1). Growth of calluse, glucose, fructose, and sucrose contents, and activities of soluble and bound invertases, glucokinase, phosphoglucose isomerase, aldolase, and pyruvate kinase were monitored. The results show a negative effect of humic fractions on callus growth, due to decreased utilization of glucose and fructose, and decreased activities of glycolytic enzymes. The effects are reversible. Substitution of humic fractions with 2,4-D+BAP or 2,4-D is followed by an increase of glycolytic enzyme activities and, consequently, by the utilization of glucose and fructose that induces a restart of growth. In contrast, the inhibitory effects of humic fractions persist when they are substituted with BAP alone, indicating that only the auxin 2,4-D is capable of reversing the negative effects. A possible competitive action on the auxin-binding site between 2,4-D and the chemical structures in the forest humic fractions is suggested. 相似文献
6.
7.
Xiaobao Li Richard P. K. Wells Peter B. Wells Graham J. Hutchings 《Catalysis Letters》2003,89(3-4):163-167
The premodification of a 5 wt% Pt/-Al2O3 catalyst with cinchonidine (0.01 and 0.2 g g-1
catalyst) is described and discussed. Premodification is carried out by treating the catalyst with a solution of cinchonidine followed by solvent removal. Catalysts premodified in this way give the same ee and initial rate of reaction for the enantioselective hydrogenation of ethyl pyruvate as those using the standard in situ modification procedure. Investigations of different solvents for premodification and reaction (dichloromethane, ethanol) show that it is the solvent used for the reaction that controls the observed enantioselection. Premodified catalysts also display the initial transient behavior typically observed with in situ modified catalysts in which the ee increases with conversion in the early part of the reaction. Premodified catalysts show an enhanced rate of reaction when ethanol is used as the reaction solvent compared with in situ modified catalysts under the same conditions. Premodification using aerobic conditions gives the best results and premodified catalysts can be stored prior to use for up to a week without loss of catalytic performance. 相似文献
8.
The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and -isocinchonine-modified Pt/Al2O3 catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmoldm–3, 1 bar H2, 23°C, 6% AcOH in toluene), and 76% ee in the case of -isocinchonine (0.14 mmoldm–3, 1 bar H2, –10°C, 6% AcOH in toluene). Since -isocinchonine of rigid structure exists only in anti-open conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of anti-open conformation in these cinchona-modified enantioselective hydrogenations. 相似文献
9.
10.
丙酮酸脱氢酶复合酶系研究进展 总被引:4,自引:1,他引:4
王琦环 《包头钢铁学院学报》2004,23(1):93-96
对丙酮酸脱氢酶复合酶系的组成、功能及特性进行了简要的综述.丙酸酸脱氢酶系是1个定位在线粒体中的多酶复合体.它是由3种酶:丙酮酸脱氢酶(E1)、二氢硫辛酸乙酰转移酶(E2)、二氢硫辛酸脱氢酶(E3)和调节它的活性的丙酮酸脱氢酶激酶、一种丙酮酸脱氢酶磷酸酶以及功能未知的蛋白X,还有一些辅助因子组成.在能量代谢调控过程中,丙酮酸脱氢酶系是1个主要的酶系.丙酸脱氢酶转化来自于碳水化合物和一些氨基酸的丙酮酸成为乙酰辅酶A.乙酰辅酶A是三羧酸循环的底物.丙酮酸脱氢酶系活性的调节可以通过PDH的磷酸化和脱磷酸化来调节平衡在许多组织中的源于碳水化合物和脂肪酸的乙酰辅酶A.丙酮酸脱氢酶发生突变后而不能行使正常功能,乳酸集结引起乳酸中毒. 相似文献