Xanthine, nitration

Molybdenum. Molybdenum is a component of the metaHoen2ymes xanthine oxidase, aldehyde oxidase, and sulfite oxidase in mammals (130). Two other molybdenum metaHoen2ymes present in nitrifying bacteria have been characteri2ed nitrogenase and nitrate reductase (131). The molybdenum in the oxidases, is involved in redox reactions. The heme iron in sulfite oxidase also is involved in electron transfer (132). 

Xanthine, 8-ethyl-synthesis, 5, 574 Xanthine, 9-hydroxy-8-methyl-synthesis, S, 596 Xanthine, 1-methyl-deuterium-hydrogen exchange, S, 527 methylation, S, 533 occurrence, S, 598 reduction, 5, 541 synthesis, S, 589 Xanthine, 3-methyl-deuterium-hydrogen exchange, S, 528 methylation, S, 533 reduction, S, 541 synthesis, S, 595 Xanthine, 7-methyl-deuterium-hydrogen exchange alkylation, S, 527 reduction, S, 541 synthesis, S, 587 Xanthine, 8-methyl-synthesis, S, 574 Xanthine, 9-methyl-methylation, S, 533 nitration, 5, 538... 

Caffeine Xanthine derivatives Purines Purines, pyrimidines Purines Chloraminc-T Iron (111) chloride followed by iodine Silver nitrate followed by sodium dichromate Fluorescein Silver nitrate followed by bromophenol blue... 

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain. 

Others oxotransferases) (2 pyranopterins bonded to Mo) (8-10 members) Nitrate reduction dissimilatory terminal respiratory oxidase Pyridoxal oxidase Xanthine dehydrogenases Pyrogallol transhydrolase Nitrate to nitrite... 

Millar, T. M., Stevens, C. R., Benjamin, N., Eisenthal, R., Harrison, R., Blake, D. R., Xanthine oxidoreductase catalyses the reduction of nitrates and nitrite to nitric oxide under hypoxic conditions. FEBS Lett. 427 (1998), p. 225-228... 

NITRATE REDUCTASE NITRITE REDUCTASE PHENOL HYDROXYLASE PROLINE DEHYDROGENASE PUTRESCINE OXIDASE PYRUVATE OXIDASE SALICYLATE 1-MONOOXYGENASE SUCCINATE DEHYDROGENASE SULFITE REDUCTASE XANTHINE OXIDASE Falling ball viscometry,... 

METHOD OF CONTINUOUS VARIATION MOLYBDENUM COFACTOR (MoCo) Molybdenum-dependent reactions, ALDEHYDE OXIDASE MOLYBDOPTERIN NITRATE REDUCTASE NITROGENASE SULFITE OXIDASE XANTHINE DEHYDROGENASE MOLYBDOPTERIN... 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

Molybdoenzymes other than the nitrogenases are usually termed oxomolybdoenzymes. This prefix relates to the nature of the catalysis effected, i.e. the net effect of the conversion (xanthine to uric acid, sulfite to sulfate, nitrate to nitrite, or aldehyde to carboxylate) corresponds to the transfer of one oxygen atom to or from the substrate. Furthermore, molybdenum X-edge EXAFS studies have established that this metal is coordinated to one or more terminal oxo groups in each enzyme studied by this technique.204... 

Xanthine. An old Blk Pdr substitute proposed by Prof Schwarr of Gratz, Austria. It contained K nitrate 68.S, charcoal 4.1 and K xanthogen-ate 27.4% (xanthate de potasse, KS2COC2Hj). The latter material was prepd by adding an excess of K hydroxide and C disulfide to absolute ale... 

The recognition that the Mo in the molybdoproteins exists in organic cofactor forms came from studies of mutants of Aspergillus and Neurospora.650 In 1964, Pateman and associates discovered mutants that lacked both nitrate reductase and xanthine dehydrogenase. Later, it was shown that acid-treated molybdoenzymes released a material that would restore activity to the inactived nitrate reductase from the mutant organisms. This new coenzyme, a phosphate ester of molybdopterin (Fig. 15-17), was characterized by Rajagopalan and coworkers.650 651 A more complex form of the coenzyme, molybdopterin cytosine dinucleotide... 

Study by X-ray absorption spectroscopy of the extended X-ray absorption fine structure (EXAFS) has provided estimates of both the nature and the number of the nearest neighboring atoms around the Mo. The EXAFS spectra of xanthine dehydrogenase and of nitrate reductase from Chlorella confirmed the... 

Xanthine oxidase also has a site that will bind anions such as nitrate. This appears to be the substrate-binding site on molybdenum. 

Li H, Cui H, Liu X, Zweier JL. Xanthine oxidase catalyzes anaerobic transformation of organic nitrates to nitric oxide and nitrosothiols characterization of this mechanism and the link between organic nitrate and guanylvl cyclase activation. 7 Biol Chem. 2005 280 16594-16600. 

Sfilfire Oxid,W Assituilatory Nitrate Rftfi.iciase Xanthine Oxidase/AIdchydic OxicUircdnciisc... 

It is the (MPT)Mo(0)2 cofactor variant that likely reconstitutes the nitrate reductase activity in the Nit-1 mutant [29,31], Although the (MPT)Mo(0)2 unit could be provided directly by sulfite oxidase, nitrate reductase, and even xanthine... 

Although molybdenum and tungsten enzymes carry the name of a single substrate, they are often not as selective as this nomenclature suggests. Many of the enzymes process more than one substrate, both in vivo and in vitro. Several enzymes can function as both oxidases and reductases, for example, xanthine oxidases not only oxidize purines but can deoxygenate amine N-oxides [82]. There are also sets of enzymes that catalyze the same reaction but in opposite directions. These enzymes include aldehyde and formate oxidases/carboxylic acid reductase [31,75] and nitrate reductase/nitrite oxidase [83-87]. These complementary enzymes have considerable sequence homology, and the direction of the preferred catalytic reaction depends on the electrochemical reduction potentials of the redox partners that have evolved to couple the reactions to cellular redox systems and metabolic requirements. 

The half reactions for the oxidation of formate, sulfate, and xanthine the reduction of nitrate and the reduction of C02 via the carbamate of methanofuran are listed in Table 3b. The potentials for several relevant redox partners are listed in Table 4. On the basis of the respective E° values for the substrate reactions and their physiological redox partners, it is clear that most (if not all) reactions... 

Shown in Figures 5-7 are the redox pathways for xanthine oxidase, sulfite oxidase, and nitrate reductase (assimilatory and respiratory), respectively. These schemes address the electron and proton (hydron) flows. The action of the molyb-doenzymes is conceptually similar to that of electrochemical cells in which half reactions occur at different electrodes. In the enzymes, the half reactions occur at different prosthetic groups and intraprotein (internal) electron transfer allows the reactions to be coupled (i.e., the circuit to be completed). In essence, this is the modus operandi of these enzymes, which must be determined before intimate mechanistic considerations are seriously addressed. 

Protein sequence homology suggests that sulfite oxidase and assimilatory nitrate reductase are members of the same molybdenum enzyme subfamily [31]. Consistent with this classification, the cofactors of sulfite oxidase and assimilatory nitrate reductase differ significantly from those in dmso reductase, aldehyde oxido-reductase, xanthine oxidase (see Section IV.E.), and even respiratory nitrate reductase (Section IV.D). The EXAFS of both sulfite oxidase [132-136] and assimilatory nitrate reductase [131,137,138] and x-ray studies of sulfite oxidase (chicken liver) [116] confirm that the molybdenum center is coordinated by two sulfur atoms from a single MPT ligand and by the sulfur atom of a cysteine side chain. The Movl state is bis(oxido) coordinated (Figure 14). 

As with xanthine oxidase, the sulfido ligand of the active form of aldehyde oxidoreductase is readily replaced by an oxido ligand to yield a cofactor with a structure that resembles that of oxidized sulfite oxidase and assimilatoiy nitrate reductase. Both x-ray and EXAFS data are available for the bis(oxido) form, and, with the exception of the oxido replaced sulfido ligand, few changes are obvious in the overall structure of the oxidized form of the desulfo cofactor. Upon reduction of the enzyme the oxido ligand is presumably reduced to hydroxido, an observation that is supported by EPR data for the Mov state. 

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