Cystine oxidase

In addition, Medes and Floyd (86) have presented evidence that in the liver of rats two enzyme systems exist which bring about the oxidation of cystine. One of them is cysteine oxidase B, which was described on page 388, and which acts likewise on cystine to form cysteic acid the rate of this oxidation is slightly less than the corresponding oxidation of cysteine. The other is a cysteine oxidase which results in the formation of cystine disulfoxide, RSOSOR. Greenstein and Leuthardt (55) have studied the distribution of cystine oxidase in the different organs of mice. Only liver, kidney, and pancreas, in order of decreasing activity, contain this enzyme. On the other hand, no noticeable activity of cystine oxidase has been revealed in tumors in the same animals. 

Inhibition of monoamine oxidase has been proposed as a possible mechanism underlying the hydrogen sulfide-mediated disruption of neurotransmission in brain stem nuclei controlling respiration (Warenycia et al. 1989a). Administration of sodium hydrosulfide, an alkali salt of hydrogen sulfide, has been shown to increase brain catecholamine and serotonin levels in rats. It has also been suggested that persulfide formation resulting from sulfide interaction with tissue cystine and cystinyl peptides may underlie some... 

Since sulphur compounds are important in the off-flavour of UHT milk, attempts to improve its flavour have focused on reducing the concentration of these, e.g. by adding thiosulphonates, thiosulphates or cystine (which react with mercaptans) or sulphydryl oxidase, an indigenous milk enzyme (which oxidizes sulphydryls to disulphides Chapter 8). 

Studies of intramolecular ET in oxidases provide interesting examples of how pulse radiolysis is employed to obtain insights into both (1) these enzymes respective mechanisms of action and (2) electron transfer along protein polypeptide matrices that were most probably selected by evolution (9,10, 30-32). Thus, early attempts to study the electron uptake mechanism by the blue oxidase, ceruloplasmin, showed that a diffusion-controlled decay process of the eaq in solutions of this protein is paralleled by the formation of transient optical absorptions due to electron adducts of protein residues, primarily of cystine disulfide bonds (30). The monomolecular decay of the latter absorption was found to have the same rate constant as that at which the type 1 Cu(II) absorption band was reduced. These results were interpreted as being the combined result of the high reactivity of the e q and the relatively inaccessible type 1 Cu(II) site, yielding an indirect, intramolecular electron transfer pathway from surface-exposed residues (30). 

The oxidation of cysteine to cystine (reaction 6) occurs readily under aerobic conditions in the presence of cytochrome c and cytochrome oxidase (Keilin, 1930). The NADH-dependent reduction of cystine to cysteine by crude pea seed preparations has been reported by Romano and Nickerson (1954) and Mapson (1953). This enzyme (E.C. 1.6.4.1) has never been purified and no further studies of this reaction have been reported. 

Reaction 1, the oxidation of cysteine to cystine, is known to be catalyzed by cytochrome oxidase in the presence of cytochrome It is a moot question whether this enzyme system is essential for this oxidation to proceed in the living organism, as the oxidation of cysteine by oxygen... 

The oxidation of cysteine to cystine is catalysed by cytochrome-c and cytochrome-oxidase, whilst the reduction of C3rstine is accomplished by a number of reducing agents HjS, glutathione, SH-enzymes, etc. 

Fig. 15.34. Reactions involved in flour improvement by ascorbic acid (according to Grosch and Wieser, 1999) Asc, ascorbic acid DHAsc, dehydroascorbic acid AO, ascorbic acid oxidase GSH-DH, glutathione dehydrogenase GSH, reduced glutathione GSSG, oxidized glutathione CSH, cysteine CSSC, cystine PSSP, gluten proteins...

The study of the initial stages of the enzymic oxidation of cysteine is complicated by the susceptibility of this thiol to catalytic oxidation by trace metals. Working with tissue slices, Pirie (6) and Medes (7) found that 3% of cysteine sulfur is oxidized all the way to SO 65 to 89% is recovered as cystine, while the rest is present as other or nic sulfur compounds. It is probable that the oxidation of cysteine to cystine is brought about to a large ertent by cytochrome c-cytochrome oxidase as... 

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