Oxygen-dependent oxidations

The mechanism proposed for SSAO-catalyzed oxidations is shown in Scheme 1 [16], A molecular oxygen-dependent oxidation converts the reduced cofactor back to the quinone with the formation of hydrogen peroxide and ammonia. 

The formation of N-oxides of tertiary amines and demethylation of secondary amines are biochemical processes which may also be involved in detoxification. In the case of A, iV-dimethylaniline, the N-oxide is considered to be the possible intermediate in its NADPH- and oxygen-dependent oxidative demethylation. The formation of this A -oxide appears to be catalyzed via a flavoprotein oxidase (509). Tertiary amine A -oxides, thus formed, are often excreted in urine. In the case of A -methyl-4-aminoazobenzene (MAB), demethylation to 4-aminoazoben-zene (AB) appears to occur via a cytochrome P-450 mediated oxidation. Since an A -methyl substituent is regarded as essential for carcinogenic activity among derivatives of AB, this oxidative demethylation would be a specific mode of detoxifying this carcinogenic aromatic amine. 

Fig. 1. Schematic representation of a battery system also known as an electrochemical transducer where the anode, also known as electron state 1, may be comprised of lithium, magnesium, zinc, cadmium, lead, or hydrogen, and the cathode, or electron state 11, depending on the composition of the anode, may be lead dioxide, manganese dioxide, nickel oxide, iron disulfide, oxygen, silver oxide, or iodine.

Since SCC is often dependent on environmental factors other than stress and exposure to a specific corrodent, appropriate alteration of these other factors may be effective. For example, a reduction in metal temperature, a change in pH, or a reduction in the levels of oxygen or oxidizing ions may reduce or eliminate SCC. 

Nickel oxide, NiO, which is the only oxide formed by nickel during oxidation in air, has a very naiTow range of iroir-stoichiomen y, the maximum oxygeir/nickel ratio probably being 1.001. The oxygen dependence of the deviation from non-stoichiometry is and hence dre oxidation rate... 

In summary for non-metal transfer situations chemical thermodynamics is a useful guide to probable behaviour. The transfer of a non-metal, X, dissolved in a molten metal, M to another metal M", will depend on the relative free energies of formation of M X and M X (see Section 7.6). Thus sodium will give up oxygen to Zr, Nb, Ti and U, as the free energy of oxide formation of these metals is greater than that for sodium on the other hand, sodium will remove oxygen from oxides of Fe, Mo and Cu unless double oxides are formed. 

Although metal-catalyzed protein oxidation is undoubtedly a very effective oxidative process, the origin of free metal ions under in vivo conditions is still uncertain (see Chapter 21). However, protein oxidation can probably be initiated by metal-containing enzymes. Mukhopadhyay and Chatterjee [31] have shown that NADPH-stimulated oxidation of microsomal proteins was mediated by cytochrome P-450 and occurred in the absence of free metal ions. It is important that in contrast to metal ion-stimulated oxidation of proteins, ascorbate inhibited and not enhanced P-450-dependent protein oxidation reacting with the oxygenated P-450 complex. The following mechanism of P-450-dependent oxidation of the side chain protein amino acid residues has been proposed ... 

The conversion of a-meso-hydroxyheme to verdoheme is an oxygen-dependent process because the HO-l a-meso-hydroxyheme complex, whether obtained by reconstitution of the apoenzyme with synthetic a-meso-hydroxyheme or from oxidation of the heme complex with H2O2, is stable under anaerobic conditions 104, 105). EPR analysis of the... 

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