Hydroxylamines enzymic reduction

A modified form of cellulose acetate containing some periodate-oxidized D-glucopyranosyl residues has been prepared. Fibres of cellulose acetate have been used to immobilize enzymes. Selectively oxidized cellulose (dialdehydo-cellulose) has been treated with hydroxylamine, bisulphite, and acetic acid. Dialdehydocellulose provides a matrix suitable for coupling with trypsin, giving an active immobilized enzyme. Reduction of the unchanged aldehydo-groups on the matrix stabilized the enzyme. 

Enzymes can also undergo other side reactions under conditions that divert a chemically reactive intermediate from its usual catalytic function. Again, glutamine synthetase is an excellent example (see figure above), because its side reactions include acyl-phosphate reduction by borohydride, pyroglutamate formation, and the formation of y-glutamyl hydroxamate in the presence of hydroxylamine and arsenate. 

The enteric bacterium Enterobacter cloacae produces a nitroreductase that reduces nitrofurans, nitroimidazoles, nitrobenzene derivatives, and quinones (Bryant DeLuca, 1991). This oxygen-insensitive enzyme has been purified and is known to require FMN to transfer reducing equivalents from NAD(P)H to the nitroaromatic compounds, TNT being the preferred substrate. Aerobically, this enzyme reduces nitrofurazone through the hydroxylamine intermediate, which then tautomerizes to yield an oxime end-product. Anaerobically, however, the reduction proceeds to the fully reduced amine adduct. When E. cloacae was grown in the presence of TNT, the nitroreductase activity increased five- to tenfold. 

The enzymes from green plants and fungi are large multifunctional proteins,80 which may resemble assimilatory sulfite reductases (Fig. 16-19). These contain siroheme (Fig. 16-6), which accepts electrons from either reduced ferredoxin (in photosynthetic organisms) or from NADH or NADPH. FAD acts as an intermediate carrier. It seems likely that the nitrite N binds to Fe of the siroheme and remains there during the entire six-electron reduction to NH3. Nitroxyl (NOH) and hydroxylamine (NH2OH) may be bound intermediates as is suggested in steps a-c of Eq. 24-14. 

Nitrite reduction in assimilatory nitrate-reducing Neurospora crassa, Torulopsis nitratophila, Azotobacter vinelandii, and Azotobacter chro-ococcum appears to be catalyzed by enzyme systems which require flavin and metals. The enzyme from N. crassa has been partially purified, and its molecular weight has been estimated to be 300,000 (344, 346, 351, 367). The enzyme reduces both nitrite and hydroxylamine to ammonia and utilizes NADH or NADPH as electron donor. It is reported to be a FAD-dependent enzyme and to contain iron, copper, and active thiol (346, 367). Three moles of NADH are oxidized per mole of nitrite reduced to ammonia. It has been suggested that the reduction of nitrite occurs in three steps, each involving two electrons. Thus, hyponitrite and hydroxylamine have been proposed as successive intermediates in the re-... 

The nitrite reductase of Torulopsis nitratophila is specific for NADPH and FAD, and can utilize reduced benzyl or methyl viologen as electron donor, but not reduced flavins (34S). With NADPH as electron donor, nitrite reduction is inhibited by cyanide and mercurials. Michaelis constants for FAD and nitrite have been reported to be 45 nM and 19 ftiW, respectively. Unlike the Neurospora enzyme, the nitrite reductase of T. nitratophila could not reduce hydroxylamine in the presence of NADPH and FAD. 

That hydroxylamine might not be an obligatory intermediate, or occur as a free intermediate, in the reduction of nitrite to ammonia is suggested by the properties of nitrite reductases of Azotobacter chroococcum and Escherichia coli. The former is an adaptive enzyme, the formation of which requires nitrate or nitrite in the culture (31,2). It is FAD-depen-dent and presumably contains metals and p-mercuribenzoate inhibitable... 

The E. coll enzyme can reduce nitrite and hydroxylamine to ammonia at the expense of NADPH (339). However, with the use of N-nitrite it was shown that hydroxylamine was not an intermediate in the reduction of nitrite. No cofactor requirements were shown for the E. coli enzyme, but similar to other flavin and metal requiring nitrite reductases it was inhibited by cyanide and mercurials. 

The latter inhibition is reversed by light. Urea inactivation-reaclivation studies showed parallel loss and recovery of nitrite and hyroxylamine reductase activities, and nitrite was shown to inhibit hydroxylamine reduction. These results have suggested that the enzyme has a common binding site for nitrite and hydroxylamine. The absorption spectra of the A. fischeri enzyme (oxidized, reduced, and reduced plus nitrite or hydroxylamine) are shown in Fig. 39. 

Treatment of E. coli sulfite reductase with p-mercuriphenyl sulfonate results in the specific release of FMN from the enzyme (390). FMN-depleted sulfite reductase can be prepared also by photodestruction of FMN. The enzyme-FMN dissociation constant is 10 nAf at 25°, and light irradiation can deplete the enzyme of FMN by destroying the released flavin. These treatments do not lead to removal or destruction of other components of the enzyme. The FMN-depleted enzyme is no longer capable of NADPH-dependent reduction of sulfite, nitrite, hydroxylamine. 

Oxidation of ammonia to nitrite, N02, and nitrate, N03, is called nitrification the reverse reaction is ammonification. Reduction from nitrite to nitrogen is called denitrification. All these reactions, and more, occur in enzyme systems, many of which include transition metals. A molybdenum enzyme, nitrate reductase, reduces nitrate to nitrite. Further reduction to ammonia seems to proceed by 2-electron steps, through an uncertain intermediate with a -fl oxidation state (possibly hyponitrite, N202 ) and hydroxylamine ... 

The types of nitrogen-containing compounds that are most frequently involved in reductive biotransformation are those containing nitro, azo, and N-oxide functional groups. Similar enzymes are involved that are generally located in the endoplasmic reticulum or cytosol of the liver or in the intestinal microflora. Complete reduction of a nitro compound to the primary amine involves a six-electron transfer and proceeds through nitroso and hydroxylamine intermediates [Eq. (16)]. 

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