Pyridoxamine oxidation

Vitamin B6 (pyridoxine) and its derivative pyridoxamine are apparently able to inhibit superoxide production, reduce lipid peroxidation and glycosylation in high glucose-exposed erythrocytes [353], It was suggested that the suppression of oxidative stress in erythrocytes may be a new mechanism by which these natural compounds inhibit the development of complication in diabetes mellitus. 

Pyridoxal Phosphate.—Analogues of pyridoxal and pyridoxamine 5 -phosphates have frequently been used to probe the size and shape of the active sites of a number of enzymes. For example, the apoenzyme of a tryptophanase from Bacillus alvei will bind pyridoxal 5 -phosphate as well as the 2-nor, 2 -methyl, 2 -hydroxy, 6-methyl, and A-oxide analogues.27 No analogue that has been modified at C-4 binds to the enzyme, confirming the absolute requirement for Schiff-base formation between the... 

In benzylamine oxidase there is evidence that the amine undergoes transamination with the pyridoxal prosthetic group to give a pyridoxamine, which is then oxidized by dioxygen to give H202 and NH3. The role for the copper is one of activation of the substrate.1346... 

Wada and Snell (150) have developed a method for the assay of pyridoxine and pyridoxamine phosphate, based on enzymatic oxidation to pyridoxal which is allowed to react with phenylhydrazine. 

Decarboxylation of an amino acid is an important reaction, catalyzed by a pyridoxal-dependent decarboxylase, that affords an amine as product (Scheme 2.6). It is very attractive to learn how to mimic this process to generate various amines from a-amino acids. Unfortunately, our previous studies established that treatment of a-alkyl amino acids with pyridoxal afforded only ketone and pyridoxamine as products, by a transamination-dependent oxidative decarboxylation process (pathway b in Scheme 2.5) [41]. Consequently, non-oxidative decarboxylation, using pyridoxals to generate amines, remains elusive. 

Three enzymes play an active role in the metabolism of vitamin B6 in human erythrocytes. Pyridoxal kinase uses ATP to phosphorylate pyridoxine, pyri-doxamine, and pyridoxal. Pyridoxamine oxidase oxidizes pyridoxamine-5 -phosphate and pyridoxine-5 -phosphate to pyridoxal-5 -phosphate. The phosphatase activity produces pyridoxal from pyridoxal-5 -phosphate. The assay of the three enzymes required separation of the semicarbazone derivatives of pyridoxal-5 -phosphate and pyridoxal. The mobile phase used by Ubbink and Schnell (1988) contained 2.5% acetonitrile. Detection was by fluorescence. 

Figure 6 The mechanism used in the oxidative decarboxylation of alpha disubstituted glycines by an enzyme, which, in mimics, solved the problem of converting pyridoxal species to pyridoxamine species in biomimetic transaminase systems.

Pyridoxine Hydrochloride, USP. Pyridoxine hydrochloride.. S-hydroxy-6-methyl-3.4-pyridinedimethanol hydrochloride. vitamin B, hydrochloride, rat antidermatitis factor. is a white, odorless, crystalline substance that is soluble l .S in water and 1 100 in alcohol and in.soluble in ether. It is relatively. stable to light and air in the solid form and in acid solutions at a pH no greater than S. at which pH it can be autoclaved at IS pounds at I20°C for 20 to 30 minutes. Pyridoxine is unstable when irradiated in aqueous solution.s at pH 6.8 or above. It is oxidized readily by hydrogen peroxide and other oxidizing agents. Pyridoxine is as stable in mixed vitamin preparations as riboflavin and nicotinic acid. A 1% aqueous solution has a pH of 3. The pK i values fur pyridoxine. pyridoxal. and pyridoxamine are S.OO. 4.22. and 3.40. respectively, and their pK 2 values are 8.96. 8.68. and 8.05. respectively. 

An early procedure which alfords a quick if inefficient route to toluquinone consists in steam distillation of a mixture of o-toluidine, manganese dioxide, and sulfuric acid. The reagent has been used to oxidize pyridoxine (1) to pyridoxal 2f and to oxidize pyridoxamine phosphate (3) to pyridoxal phosphate (4). ... 

Metabolic Role. Riboflavin coenzymes are required for most oxidations of carbon-carbon bonds (Fig. 8.29). Examples include the oxidation of succinyl CoA to fumarate in the Krebs cycle and introduction of a,jS-unsaturation in /3-oxidation of fatty acids. Riboflavin is also required for the metabolism of other vitamins, including the reduction of 5,10-methylene tetrahydrofolate to 5-methyl tetrahydrofolate (Fig. 8.49), and interconversion of pyridoxine-pyridoxal phos-phate-pyridoxamine (Fig. 8.33). Because oxi-dation/reductions that use FAD or FMN as the coenzyme constitute a two-step process, some flavin coenzyme systems contain more than one FAD or FMN. 

How selective are pyridoxamine and lipophilic pyridoxamine analogs for scavenging y-ketoaldehydes compared to other reactive lipid peroxidation products The reactivity of pyridoxamine with a,p-unsaturated aldehydes is completely trivial, and pyridoxamine does not protect proteins from HNE adduction (Amarnath et ah, 2004 Davies et al., 2006). Pyridoxamine does react with a-ketoaldehydes such as methylglyoxal that form from the oxidative decomposition of carbohydrates and lipids (Voziyan et al., 2002), and these adducts can be detected in the mine of rodents fed pyridoxamine in their drinking water (Metz et... 

Fig. 2.7 Lipophilic IsoK/LG scavengers protect against cytotoxicity induced by oxidative stress. HepG2 cells were incubated with vehicle, pyridoxamine, pentyl-pyridoxamine, or salicylamine for 30 min prior to treatment with various concentrations of hydrogen peroxide (Davies et al., 2006). Viability was determined by detection of ATP using ATPlite luminescence assay and percent viability calculated relative to untreated cells (Mean SEM n = 8)...

The final step in this pathway is the oxidation of PNP to PLP and is carried out by PdxH. Tbe recombinant enzyme from E. coli has been studied in vitro and is a 51 kDa homodimer that utilizes flavin mononucleotide (FMN) as a cofactor. PdxH can use either PNP or pyridoxamine 5 -phosphate (PMP) as a substrate with a of 2 and 105 pM and cat of 0.8 and 1.7 s for PNP and PMP, respectively. The structures of the enzyme from E. coli as well as homologues from Mycobacterium tuberculosis and humans have been solved. The E. coli enzyme with PLP and FMN bound is shown in Figure 6. PdxH is involved in both the biosynthetic and the salvage pathways and is further discussed in a section describing the transport, salvage, and interconversion of the various forms of vitamin Bg. 

In pathway A (Figure 1), observed in Pseudomonas MA-1, pyridoxal (2) is produced either from pyridoxamine (15) by a transamination reaction with pyruvate catalyzed by pyridoxamine pyruvate transaminase,or from pyridoxine (1) by an oxidation reaction catalyzed by the FAD-dependent... 

Sequestration of redox-aetive eatalytie metal ions in vivo is eonsidered as a possible meehanism of modulation of the Maillard reaetion-promoted oxidative stress in diabetes by sueh drugs as pyridoxamine, a form of vitamin On the other... 

The major sales form of vitamin B6 is the hydrochlorid salt of the primary alcohol pyridoxine. Another vitamin B6 form introduced in the market is the dihydrochlo-rid salt of pyridoxamine. Both vitamin B6 forms are commercially produced via various straightforward chemical synthesis routes. The biologically active cofactor is the aldehyde pyridoxal-5 -phosphate, which is derived in human or animals from the vitamin B6 forms by oxidation or transamination before or after 5 phosphorylation by pyridoxal kinases. 

P3o-idine-I-oxides are comparatively resistant to reduction because of resonance stabilization by the aromatic system. Typical reagents that have been used for the formation of pyridones and pyridinols are Raney Nickel in methanol, palladium-on-charcoal, phosphorous trichloride, or phosphorus oxychloride in ethyl acetate. The N-oxides of pyridoxine, pyridoxal, and pyridoxamine have been deoxygenated catalytically. 4-Alkoxy-3-halopyri-dine-1-oxides are A-deoxygenated by phosphorous trichloride in chloroform. 2-Amino-3-pyridinol can be prepared ffom2-nitro-3-pyridinol-l-oxide (X1I450) in acetic acid by treatment with iron and mercuric chloride and then with zinc. 2-Halo-3-pyridinols can be prepared from XII-450 by treatment with phosphorous trihalides in chlorofiMm ... 

Snell and his associate [99] established that simple amination or oxidation of pyridoxine yields compounds (pyridoxamine and pyridoxal) with growth-promoting activity even greater than that of pyridoxine (see Fig. 4-27). Thus, three compounds possessing vitamin Bg activity are known to occur in nature pyridoxine, pyridoxal, and pyridoxamine. 

---