Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: DrugBank:EXPT00568 (ascorbate)
 23,072 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The exchange of nitric oxide in nitrosylmyoglobin, the heme pigment of nitrite-cured meat, has been studied using nitrogen-15 labelling in aqueous solution under conditions (pH, concentration of ascorbate and nitrite) similar to those prevailing in meat during the curing process, and has been found to have a half-life of approximately 2 h at 40 degrees C. One nitric oxide molecule is coordinated to the iron(II) centre of a myoglobin molecule and, in weakly acidic aqueous solution under anaerobic conditions, the exchange rate of the bound nitric oxide is proportional to the concentration of nitrosylmyoglobin, nitrite and hydrogen ion. The rate of exchange has a moderate temperature dependence, corresponding to an activation barrier of delta H+- = 47 +/- 3 kJ.mol-1 at 25 degrees C and pH 5.9, a value dramatically lower than that found for the enthalpy of activation for the oxidation of nitrosylmyoglobin by molecular oxygen, delta H+- = 110 kJ.mol-1. The difference in temperature dependence between the exchange and the autoxidation is discussed in relation to the function of nitrosylmyoglobin as antioxidant in cured meat products.
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PMID:Nitric oxide exchange in nitrosylmyoglobin. 229 19

Macrophages and their immortalized cell lines can be activated to form nitrite and nitrate via oxidation of arginine and this is accompanied by the formation of N-nitroso compounds. The mechanism of nitrosamine formation has been investigated through the use of compounds which are known either to inhibit or enhance acid-catalyzed nitrosation. The range of nitrogen acceptors has been expanded to include ureas as well as amines of varying pKa and structure. The results are consistent with a mechanism in which NO is oxidized to N2O3 and N2O4, which are capable of nitrosating amines, but not ureas or amides, at neutral pH. This is in agreement with a recent observation that macrophage cell-free extracts can oxidize arginine to NO. The effect of ascorbic acid on intact activated macrophages is complex since nitrite formation is enhanced over a very wide range of ascorbate concentrations (5-500 microM) while nitrosation is inhibited at ascorbate concentrations greater than 50 microM.
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PMID:Nitrosation by stimulated macrophages. Inhibitors, enhancers and substrates. 249

The reaction of Octopus vulgaris hemocyanin with nitrite was studied under a variety of conditions in which the green half-met derivative is formed. Analytical evidence shows that the amount of chemically detectable nitrite in various samples of the derivative is not proportional to the cupric copper detected by EPR. The kinetics of oxidation of hemocyanin as a function of protein concentration and pH, in the presence of nitrite and ascorbate, is consistent with a scheme in which NO2 is the reactive oxidant. We suggest that the green half-methemocyanin contains a metal center with one cuprous and one cupric copper without an exogenous nitrogen oxide ligand.
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PMID:The oxidation of Octopus vulgaris hemocyanin by nitrogen oxides. 254 Aug 4

X-ray absorption spectroscopy has been used to investigate the local environment of the copper sites in bovine dopamine beta-hydroylase, the enzyme that catalyzes the conversion of dopamine to norepinephrine in the adrenal medulla and noradrenergic nerve cells. The marked similarity of the x-ray absorption edge features of the oxidized and ascorbate-reduced forms of the enzyme with those of the corresponding Cu(imidazole)4 complexes suggests that the ligation in both cases is very similar. Furthermore, this similarity is found for the extended x-ray absorption fine structure data, and analysis shows only nitrogen (or oxygen) ligation for both enzyme forms. Thus, four nitrogen atoms provide the best fit to the data at an average distance of 1.97 +/- 0.02 A for the oxidized enzyme and four nitrogen atoms at 2.05 +/- 0.02 A for the ascorbate-reduced form. The present data analysis also indicates that there is little change in the average copper ligand environment upon reduction of the enzyme-bound copper from Cu(II) to the Cu(I). The data for the oxidized form of the enzyme are in agreement with previous spin-echo EPR experiments that show three to four imidazole nitrogen ligands for each copper (McCracken, J., Desai, P. R., Papadopoulos, N. J., Villafranca, J. J., and Peisach, J. (1988) Biochemistry 27, 4133-4137). In addition, the data do not indicate the presence of any heavy atom (sulfur or chlorine) ligation to the ascorbate-reduced form of the enzyme as reported by Scott et al. (Scott, R. A., Sullivan, R. J., DeWolf, W. E., Jr., Dolle, R. E., and Kruse, L. I. (1988) Biochemistry 27, 5411-5417).
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PMID:X-ray absorption spectroscopic study of the active copper sites in dopamine beta-hydroxylase. 270 78

This study was undertaken with the aim of providing adequate calories for optimal growth in low birth weight (LBW) by fortifying human milk with medium chain triglycerides and sugar. Twenty-one LBW babies with birth weight between 1.0 and 1.75 kg and gestational age 28-36 weeks constituted the study material. They were administered expressed human milk, initially with gavage and then by spoon. Coconut oil and sugar were added to increase the caloric density to (0.8 cal/ml). The aim was to achieve a caloric intake of 200 cal/kg. This was achieved between 6 and 11 days of birth. Additionally, vitamin C (50 mg) and vitamin E (25 IU/kg/day) were administered. Weight was recorded daily to the nearest 50 g. Head circumference was measured weekly using a non-stretch tape measure. Blood urea nitrogen was measured once the neonate started taking high calorie feeds. Stools were examined daily for the presence of fat globules and reducing substances and for the pH. All but one neonate tolerated the feeds well and there were no complications, such as vomiting, diarrhoea, abdominal distension, or necrotizing enterocolitis. The weight gain recorded was 17.29 +/- 5.30 g/day or 13.95 +/- 5.52 g/kg/day. The study demonstrates that optimal growth can be achieved within the metabolic tolerance of low birth weight infants by administering fortified high calorie breast milk.
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PMID:Fortified high calorie human milk for optimal growth of low birth weight babies. 272

The transport rates of radiolabeled ascorbic acid and dehydroascorbic acid, as well as, labeled 3-O-methyl-D-glucose and L-glucose from a central plasma compartment into aqueous and vitreous humors and cerebrospinal fluid were studied in vivo. Normal, male albino Sprague-Dawley rats and English Short Haired guinea pigs were used to explore the mechanism of ascorbic acid entry into ocular humors in a species that can produce ascorbate (the rat) and one that cannot and, like humans, is dependent on dietary sources (the guinea pig). In vivo kinetic studies allowed for the calculation of entry rate constants, Ki (min-1), in double-labeled experiments using L-glucose as an internal passive control. Parallel TLC chromatographic studies were performed to monitor intraocular labeled molecules deriving from the plasma-introduced test molecule. In addition, resting levels of ascorbic acid and D-glucose were determined in order to obtain more reliable data than previously available. Resting levels of D-glucose revealed a consistent pattern of lower levels in aqueous and vitreous humors and CSF than found in plasma for both rat and guinea pig. However, ascorbate levels differed significantly, with the guinea pig demonstrating high ascorbate levels in the aforementioned humors: 58, 77 and 22, respectively, times the circulating plasma value of 0.2 +/- 0.2 mg/dl. In contrast, the rat, like the guinea pig, had low plasma ascorbate levels (3.3 +/- 0.8 mg/dl) compared to glucose (162 +/- 8 mg/dl), with even lower aqueous and vitreous values in a pattern similar to that of D-glucose. In vivo aqueous, vitreous and CSF transport results from the guinea pig indicate active transport mechanisms for ascorbic acid that prefer the ascorbate over the dehydroascorbate moiety and are probably different from the carrier-facilitated diffusion mechanisms for D-glucose, which do not move molecules against a concentration gradient. TLC studies, performed under nitrogen, revealed that only (14C)-ascorbic acid was present in aqueous or vitreous humors regardless of whether the radiolabeled pulse was of ascorbic or dehydroascorbic acid. The rat demonstrated little or no carrier involvement, with ascorbic acid crossing into ocular humors at rates very close to those of L-glucose, which is similar in size and is considered to cross the barriers studied via passive diffusion. Saturation studies with unlabeled glucose and glucose inhibitor drugs phloretin (10(-3) M) and phloridzin (10(-1)) had no apparent effect on ocular entry rates. Dehydroascorbic acid movement was also found to be passive.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:A comparative study of ascorbic acid entry into aqueous and vitreous humors of the rat and guinea pig. 280 90

Isolated haemosiderin contained iron and nitrogen in a weight ratio of 6.75, with phosphorus and no detectable haem. Considerably more iron was released from haemosiderin under acidic conditions than under neutral conditions in the presence of ascorbate, nitrilotriacetate or dithionite. Unlike the situation with ascorbate, chelators such as citrate, ADP or succinate induced the release of only some iron, with almost no pH-dependence. Dehydroascorbate (the oxidized form of ascorbate with no reducing capacity) behaved like citrate, ADP, succinate or desferal, rather than like ascorbate itself, in releasing iron. GSH had less effect on the release of iron than these chelators, but in the presence of a small amount of chelator the release of iron increased, especially under acidic conditions. Thus reduction, chelation and pH were all found to be important factors involved in the release of iron from haemosiderin. Investigation by e.p.r. of hydroxyl-radical production by the released iron showed high radical productivity at an acidic pH. However, at a physiological pH, almost no radical formation was detected, except in the presence of nitrilotriacetate. These findings suggested that, under physiological conditions, haemosiderin was not an effective iron donor and was almost not involved in radical production. Under acidic conditions, however, such as in inflammation, hypoxia and in a lysosomal milieu, it could possibly be an iron donor and is thought to be implicated in radical production and tissue damage in iron-overloaded conditions.
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PMID:Iron release from haemosiderin and production of iron-catalysed hydroxyl radicals in vitro. 283 49

Ascorbate reversibly inhibits catalase, and this inhibition is enhanced and rendered irreversible by the prior addition of copper(II)-bishistidine. In the absence of copper, the inhibition was prevented and reversed by ethanol, but not by superoxide dismutase, benzoate, mannitol, thiourea, desferrioxamine, or DETAPAC. In the presence of the copper complex mannitol, benzoate, and superoxide dismutase still had no effect, but thiourea, desferrioxamine, DETAPAC, or additional histidine decreased the extent of inactivation to that seen in the absence of copper. In the presence of copper, ethanol protected at [ascorbate] less than 1 mM, but was ineffective at [ascorbate] greater than 2 mM, even in the absence of oxygen. Although in the absence of copper, complete removal of oxygen provided full protection against inactivation by ascorbate, this protection was not seen if the catalase was briefly preincubated with H2O2 prior to flushing with nitrogen, or if copper was present. In fact, if copper was present, inactivation was enhanced by the removal of oxygen. Increasing the concentration of oxygen from ambient to 100% slowed the inactivation, whether or not copper was present. It is concluded that the initial reversible inactivation involves reaction with H2O2 to form compound I, followed by one electron reduction of compound I to compound II. In the presence of added copper, the initial (reversible) inactivation allows H2O2 to accumulate sufficiently to permit irreversible inactivation. Since in the presence of copper oxygen is not required, and neither the reversible nor the irreversible inactivation was prevented by conventional scavengers of active forms of oxygen, the inactivation is likely mediated by semidehydroascorbate, and/or it may involve site-specific generation of the damaging intermediates.
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PMID:Mechanism of the inhibition of catalase by ascorbate. Roles of active oxygen species, copper and semidehydroascorbate. 300 60

Dopamine beta-hydroxylase is inactivated by phenyl-, phenethyl-, benzyl-, and methylhydrazine, but not by hydrazine itself. With phenyl-, methyl-, and phenethylhydrazine, the rate of inactivation decreases in the presence of ascorbate and increases in the presence of tyramine. Reduction of the enzyme-bound copper occurs with all of the hydrazines tested. In the presence of the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone the carbon-centered radicals generated from each compound are trapped. This is consistent with reduction of the enzyme-bound copper by the hydrazine-containing compounds, resulting in formation of the hydrazine cation radical. Homolytic cleavage of the carbon-nitrogen bond then generates a carbon-centered radical which reacts with the enzyme, resulting in inactivation. Inactivation with [14C]phenylhydrazine results in the incorporation of 0.94 molecule of label per enzyme subunit. Benzylhydrazine behaves as a mechanism-based inhibitor of the enzyme. Both benzyl- and phenethylhydrazine are substrates for dopamine beta-hydroxylase. The second-order rate constant for inactivation of dopamine beta-hydroxylase by benzylhydrazine in the presence of ascorbate is increased about 4-fold when the benzylic hydrogens are replaced with deuterium. The apparent Vmax shows an observed deuterium kinetic isotope effect of 13 +/- 2. The partition ratio for product formation versus inactivation is 11-fold less for alpha,alpha-d2-benzylhydrazine. These results are interpreted in terms of a model where inactivation is due to abstraction of an electron from nitrogen instead of abstraction of a hydrogen atom from the benzylic carbon.
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PMID:The mechanism of inactivation of dopamine beta-hydroxylase by hydrazines. 300 60

A mitochondrial preparation from duck adrenal gland was used, under aerobic conditions, to show that the oxygen requirement for the last step of aldosterone biosynthesis (transformation of 18-hydroxycorticosterone into aldosterone) is at the cytochrome P-450 level only. Vitamin C and tetramethyl-p-phenylene-diamine (TMPD) were used to increase oxygen consumption at the cytochrome a3 level, thereby decreasing its availability to cytochrome P-450. The vitamin C plus TMPD system acts as an 'oxygen trap'. Results show that despite reducing equivalents provided by L-malate, vitamin C plus TMPD strongly inhibits aldosterone biosynthesis from 18-hydroxycorticosterone (89%). Moreover, we used KCN in order to block oxygen consumption, even in the presence of vitamin C plus TMPD. Under these conditions, the inhibition of aldosterone biosynthesis from 18-hydroxycorticosterone is reduced by 51%. The reversal of this inhibition by KCN was evident but only partial. According to polarographic and electron microscopy studies, the reversal of inhibition can only be explained by an increased availability of oxygen at the cytochrome P-450 level. Experiments performed under aerobic conditions, without a nitrogen atmosphere, show that oxygen is required in the transformation of 18-hydroxycorticosterone into aldosterone, at the cytochrome P-450 level. This suggests that a classical hydroxylating mechanism is involved.
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PMID:Stimulation of oxygen consumption at the cytochrome A3 level inhibits aldosterone biosynthesis from 18-hydroxycorticosterone. 302 Dec 36


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