Crypto - OH. radical production by nitrofurantoin

Mechanism of lysozyme inactivation and degradation by iron

The site-specific lysozyme damage by iron and by iron-catalysed oxygen radicals was investigated. A solution of purified lysozyme was inactivated by Fe(II) at pH 7.4 in phosphate buffer, as tested on cleavage of Micrococcus lysodeikticus cells; this inactivation was time- and iron concentration-dependent and was associated with a loss of tryptophan fluorescence. In addition, it was reversible at pH 4, as demonstrated by lysozyme reactivation and by the intensity of the 14.4-kD-band on SDS-PAGE. Desferal (1 mM) and Detapac (1 mM) added before iron, prevented lysozyme inactivation, while catalase (100 micrograms/ml), superoxide dismutase (100 micrograms/ml) and bovine serum albumin (100 micrograms/ml) gave about 30 to 40% protection by competing with lysozyme for iron binding. The denaturing effect of iron on lysozyme was studied in the presence of H2O2 (1 mM) and ascorbate (1 mM); under these conditions the enzyme underwent partly irreversible inactivation and degradation different to that produced by gamma radiolysis-generated .OH. Catalase almost fully protected lysozyme; in contrast, mannitol (10 mM), benzoate (10 mM), and formate (10 mM) provided no protection because of their inability to access the site at which damaging species are generated. In this system, radical species were formed in a site-specific manner, and they reacted essentially with lysozyme at the site of their formation, causing inactivation and degradation differently than the hydroxyl radical.

Iron and oxygen: a biologically damaging mixture

Iron is a remarkably useful metal in Nature, but iron ions not safely sequestered in storage or transport proteins are hazardous because they can stimulate damaging free radical reactions. Biological examples of these are Fenton Chemistry leading to the formation of highly reactive species, such as the hydroxyl radical (.OH) and the ferryl ion (FeO2+), and lipid peroxidation. The need to conserve body iron stores has closely evolved with an essential requirement for antioxidant protection and, several 'acute-phase' proteins involved in iron metabolism such as caeruloplasmin, haptoglobins and haemopexin in collaboration with the iron binding proteins transferrin and lactoferrin contribute to our defense against oxidative damage.

Free radical formation by antitumor quinones

Quinones are among the most frequently used drugs to treat human cancer. All of the antitumor quinones can undergo reversible enzymatic reduction and oxidation, and form semiquinone and oxygen radicals. For several antitumor quinones enzymatic reduction also leads to formation of alkylating species but whether this involves reduction to the semiquinone or the hydroquinone is not always clear. The antitumor activity of quinones is frequently linked to DNA damage caused by alkylating species or oxygen radicals. Some other effects of the antitumor quinones, such as cardiotoxicity and skin toxicity, may also be related to oxygen radical formation. The evidence for a relationship between radical formation and the biological activity of the antitumor quinones is evaluated.

Transcriptional and posttranscriptional regulation of manganese superoxide dismutase biosynthesis in Escherichia coli, studied with operon and protein fusions

Protein and operon fusions between the manganese superoxide dismutase (MnSOD) gene, sodA, and genes of the lactose operon were constructed in an attempt to explore the effects of various factors on MnSOD expression and the level at which they operate. In sodA-lacZ protein fusions, induction of beta-galactosidase perfectly mimicked MnSOD induction (i.e., beta-galactosidase was not expressed in anaerobiosis and was induced by oxygen, redox-cycling compounds in aerobiosis, and iron chelators in anaerobiosis). In tac-sodA operon fusions, MnSOD induction was monitored only by the lactose operon inducer isopropyl-beta-D-thiogalactopyranoside. Various plasmids carrying part or all of the sodA regulatory and structural region inhibited aerobic beta-galactosidase induction in sodA-lacZ fusions. This included plasmids carrying only the transcription start and upstream region and also plasmids which did not contain this region and in which MnSOD was under foreign transcriptional control. The role of metal ions was also investigated. Addition of Mn(II) enhanced MnSOD activity but did not affect induction. The anaerobic expression of MnSOD from the oxygen-insensitive tac promoter was enhanced threefold by iron-chelating agents, implying a posttranscriptional or most likely a posttranslational modulation of enzyme activity via metal ions. To accommodate all these data, multiregulation of MnSOD is proposed.

Reactivity of hydroxyl and hydroxyl-like radicals discriminated by release of thiobarbituric acid-reactive material from deoxy sugars, nucleosides and benzoate

Hydroxyl radicals (OH.) can be formed in aqueous solution by a superoxide (O2.-)-generating system in the presence of a ferric salt or in a reaction independent of O2.- by the direct addition of a ferrous salt. OH. damage was detected in the present work by the release of thiobarbituric acid-reactive material from deoxy sugars, nucleosides and benzoate. The carbohydrates deoxyribose, deoxygalactose and deoxyglucose were substantially degraded by the iron(II) salt and the iron(III) salt in the presence of an O2.- -generating system, whereas deoxyinosine, deoxyadenosine and benzoate were not. Addition of EDTA to the reaction systems producing radicals greatly enhanced damage to deoxyribose, deoxyinosine, deoxyadenosine and benzoate, but decreased damage to deoxygalactose and deoxyglucose. Further, OH. scavengers were effective inhibitors only when EDTA was present. Inhibition by catalase and desferrioxamine confirmed that H2O2 and iron salts were essential for these reactions. The results suggest that, in the absence of EDTA, iron ions bind to the carbohydrate detector molecules and bring about a site-specific reaction on the molecule. This reaction is poorly inhibited by most OH. scavengers, but is strongly inhibited by scavengers such as mannitol, glucose and thiourea, which can themselves bind iron ions, albeit weakly. In the presence of EDTA, however, iron is removed from these binding sites to produce OH. in 'free' solution. These can be readily intercepted by the addition of OH. scavengers.

Mechanisms of oxygen activation by nitrofurantoin and relevance to its toxicity

Purified ferredoxin-(cytochrome c)-NADP+ oxidoreductase and xanthine oxidase were found to catalyse the reduction of nitrofurantoin to the free radical. Under aerobic conditions, the nitrofurantoin radical underwent autoxidation to regenerate the parent compound with the concomitant production of superoxide and eventually hydrogen peroxide. The nitrofurantoin radical was also shown to react with hydrogen peroxide to generate a highly reactive species which was capable of oxidising methionine to ethylene. This active oxygen radical appeared to be identical with the crypto-OH . radical, previously proposed as being formed from the analogous reaction of the methyl viologen radical with hydrogen peroxide [R.J. Youngman and E.F. Elstner, FEBS Lett. 129, 265 (1981)]. Catalase inhibited nitrofurantoin-dependent ethylene formation in both enzyme systems, whereas superoxide dismutase was only inhibitory in the xanthine oxidase mediated reaction. Although the primary function of the respective enzyme systems is to generate the nitrofurantoin radical, the xanthine oxidase reaction is markedly more complex than that of ferredoxin-(cytochrome c)-NADP+ oxidoreductase. The differences between the two enzyme reactions appear to be due to the endogenous autoxidation of xanthine oxidase. The aerobic activation of nitrofurantoin by xanthine oxidase involved the superoxide anion as an intermediate, whereas the nitrofuran was directly reduced by ferredoxin-(cytochrome c)-NADP+ oxidoreductase without a requirement for active oxygen species.

Superoxide-dependent formation of hydroxyl radicals and lipid peroxidation in the presence of iron salts. Detection of 'catalytic' iron and anti-oxidant activity in extracellular fluids

Synovial fluid from rheumatoid patients and normal cerebrospinal fluid contains micromolar concentrations of non-protein-bound iron salts that can promote lipid peroxidation and also the superoxide-dependent formation of hydroxyl radicals from hydrogen peroxide. These iron catalysts of oxygen radical reactions cannot be detected by conventional assays unless interfering high-molecular-weight substances, probably proteins, are removed by ultrafiltration or inactivated by exposure to low pH values. The bleomycin assay for ;catalytic' iron [Gutteridge, Rowley & Halliwell (1981) Biochem. J.199, 263-265] does not suffer from these artifacts.