Oxidation, enzymic with nitric acid

Inosinic acid (1), the first nucleotide to be discovered, was isolated over a century ago from beef extract by Liebig. Mild, acid hydrolysis of this nucleotide yielded a ribose phosphate (2) which, by oxidation with nitric acid, gave a ribonic acid phosphate (3), but not a ribaric acid phosphate. These studies by Levene and his associates showed that the phosphoric (phospho) group is bound to the 5-hydroxyl group of the ribosyl moiety in 1 hence, inosinic acid is inosine 5 -phosphate. Adenylic acid (4), isolated from muscle, was converted enzymically (by adenylic... 

As shown in Scheme 1, NHase catalyzes the conversion of nitriles to amides 4), The active site contains either a non-corrin cobalt(III) or non-heme iron(III). Amino acid sequence comparisons have shown that the primary coordination sphere is conserved regardless of the identity of the metal center and consists of a -C-S-L-C-S-C- motif (5). EPR studies on Fe-NHase revealed the iron center maintains a low-spin Fe(III) state throughout the catalytic cycle, and that the iron center has a variable coordination site for substrate interaction (6). These findings are consistent with the hypothesis that the enzyme functions solely as a hydrolytic (i.e., redox-inactive) catalyst. Incubation of the enzyme with nitric oxide in the dark inactivates the enzyme. Exposure to light was found to reinstate activity with concomitant loss of NO, thus revealing a novel photo-regulatory mechanism (7-70). 

Recently, Gunther et al. [41] proposed that nitric oxide may directly react with enzymes without intermediate formation of peroxynitrite. It is known that the oxidation of arachido-nic acid by prostaglandin H oxidase is mediated by the formation of enzyme tyrosyl radical (see Chapter 26). Correspondingly, it has been suggested that NO is able to react with this radical to form the tyrosine iminoxyl radical and then nitrotyrosine. Therefore, the NO-dependent nitration of protein tyrosine residue may occur without the formation of peroxynitrite or other nitrogen oxides. 

After identification, the next step was to determine how NO was made in the endothelium. As with other bodily substances, NO manufacture involves a pathway that requires other chemical entities. The substrate, or raw material required, is an amino acid (a building block of protein), called L-arginine, henceforth in this book simply arginine. A special enzyme, endothelial nitric oxide synthase (eNOS), must be present for NO to be fabricated from... 

Chemical techniques have also been used to prepare nanotube-encapsulated materials. In this context, carbon nanotubes were oxidatively opened by boiling aqueous nitric acid, then filled with oxides of Ni [145], Co [145], Fe [145], U [145],Mo [146],Sn [147,148],Nd [149],Sm [149],Eu [149],La [149],Ce [149], Pr [149], Y [149], Zr [149], Cd [149], pure metals such as Pd [150], Ag [151], Au [151], proteins or enzymes (e.g. Zn2Cd5-metallothionein, cytochrome c(3), /1-lactamaseI,andDNA complexes) [152,153],andAu2S3 [149],AuCl [151],CdS [149] by wetting techniques (see Fig. 23 also [148,149]).In addition, other capillarity wetting methods, such as heating the elements with open-ended nanotubes, have led to the introduction of the elements Pb, Bi, Cs, S and Se into nanotubes [29-31]. From these studies, it was concluded that only low surface tension substances can be introduced into nanotubes. 

In the biochemical context, it has been well noted that the chemistry of NO (and of its conjugated acid, FINO (i8)) is quite distinct from that of NO (22). Its occurrence in catalytic cycles of NO synthase, nitrite reductase, and nitric oxide reductase (NOR) has been postulated (23). For example, in the multi-iron containing enzyme NOR two NO molecules are converted reductively to nitrous oxide, N2O, with nitroxyl (NO ), and hyponitrite (N202 ) (24) as putative intermediates (23). 

Nitrosothiols are difficult to control as donors, since the release of NO is catalyzed by traces of free metal ions, in particular copper(I) Cu" ". The endogenous synthesis of NO may be enhanced by activating existing nitric oxide synthases with supplementation of cofactors like tetrahydrobiopterin, with folic acid or vitamin C, or by phosphorylation of certain serine groups of the enzyme. Alternatively, the expression of NOS enzyme in tissues or cell cultures may be artificially enhanced by gene therapy, i.e., genetic transfection with viral vectors. Finally, under favorable conditions NO levels may be increased by blocking the... 

Complete hydrolysis of h3 uronic acid with mineral adds produces D-ghicosamine, acetic add, and carbon dioxide, the last named indicating the presence of a uronic acid. The latter compound was identified as d-g ucuronic acid by oxidation with nitric add, which led to the formation of D-glucaricacid. D-Glucuronic add was subsequently isolated after enzymic degradation. The carboxyl group of the uronic moiety is free, as shown by salt formation, titration, and the positive hydroxamic test after ester formation. 

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