Donald C. Borg

Cytotoxic reactions of free radical species of oxygen.

Abstract— We report on autooxidation reactions related to selective cell killing in vivo and present oxygen uptake and EPR data on the molecular nature of these changes, with a note on a reaction used to test for singlet oxygen.

THE CHEMISTRY OF PORPHYRIN π‐CATIONS*

The redox properties of metalloporphyrins have been studied extensively, and particular attention has been paid to the changes in oxidation states of the metal.’-” In general, the most stable metalloporphyrins are those in which the metal is in the + 2 oxidation state. However, most of the transition metalloporphyrins show a variety of oxidation states. Thus Co(L1) porphyrins can, like the corresponding cobalt-containing vitamin B I Z , be oxidized to the CO(LII),~ or be reduced to the Co(1) complex; and like the Co(1) containing vitamin BI2(BL2 s), the Co(1) porphyrin is nucleophilic and undergoes oxidative addition with alkyl halides to give the corresponding alkyl cobalt porphyrins. Manganese porphyrins show + 2, + 3, and +4 oxidation states,6 and it has recently been shown’ that Pb(I1) porphyrins can be oxidized to Pb(1V) while the more stable Sn(Lv) can be reduced to Sn(LL) ~ y s t e r n s . ~ ~ ~ A particularly striking example of the stability of the divalent complexes is that of the Ag(1L) porphyrins.*O Few examples of divalent silver complexes are known, but the planar tetradentate porphyrin ligand stabilized this unusual oxidation state which can, however, be oxidized to the trivalent Although the redox properties of metalloporphyrins are fascinating and important in their own right, the initial focus and interest in the oxidation states of these systems stems from the redox properties of the cytochromes (which are enzymes containing iron porphyrins, and function catalytically via theFe(I1) Fe(LLL)couple) and the function of hemoglobin (an Fe(I1) porphyrin which, unlike simple Fe(I1) porphyrins, is not oxidized by oxygen to Fe(LI1) but reversibly binds oxygen at the Fe(LL) oxidation level) as well as from the incompletely determined role of valence changes of iron in the peroxidase/catalase class of heme enzymes. The central role of the iron in these naturally occurring systems and the considerable efforts that have been expended on elucidating the roles of metals in these and other metalloporphyrins resulted in the widely held assumption that the macrocyclic porphyrin ligand serves merely to modify the redox potentials of the metals, and to act as a convenient bridge between the metal and the protein-an assumption which is far from true.

Prooxidant action of desferrioxamine: Fenton-like production of hydroxyl radicals by reduced ferrioxamine

It is common practice in biochemical research to assume that iron bound to desferrioxamine (DFO) to form ferrioxamine (FOA) has been rendered inactive to subsequent redox chemistry within the range of physiological redox potentials, both in vitro and in vivo. However, plants and microorganisms can make iron metabolically available from ferrioxamine and closely related trihydroxamate siderophores, and at neutral pH, cyclic voltammetry of FOA demonstrates a reversible one-electron reduction at about -0.42 to -0.45 V (vs. normal hydrogen electrode), which is within the range of a number of reducing enzymes. We present evidence for the Fenton-like ability of FOA reduced by paraquat cation radicals to consume H2O2 and produce hydroxyl radicals (OH.) in the process. Similar reactions may explain previously reported potentiation of the oxidizing toxicity of paraquat in rats by high doses of DFO, as well as several other examples of prooxidant actions of DFO in vivo. We present the hypothesis that biphasic antioxidant/prooxidant behavior of DFO as a function of dose may be common with iron-catalyzed oxidizing reactions when mobile strong reducing agents are present. Hence, the real possibility of amplifying oxidizing damage must be considered when planning treatment with DFO, and failure of DFO to inhibit a particular response to oxidizing stress or its enhancement by DFO cannot, by itself, be considered sufficient evidence to rule out an iron-dependent process.

Failure of desferrioxamine to modify the toxicity of paraquat in rats

The feasibility of using desferrioxamine (DF), an iron chelator, as a therapeutic agent against paraquat (PQ++) toxicity in male Sprague-Dawley rats was explored, based on the rationale of limiting toxic hydroxyl radical production from hydrogen peroxide by removing redox-active iron. Body weights, mortality, and lung histopathology were followed for periods up to 14 days after intraperitoneal injection of PQ++ (20 or 25 mg/kg body weight) with or without concurrent daily subcutaneous injections of DF (300 mg/day). Animals receiving PQ++ showed the expected typical patterns of mortality and of lung histopathology, namely: marked edema, subpleural hemorrhage, acute inflammation, perivascular mononuclear cell infiltrates, sloughing of alveolar and bronchiolar lining cells, and diffuse interstitial fibrosis. Desferrioxamine alone was non-toxic. Surprisingly, results when both PQ++ and DF were administered indicated a failure of DF to ameliorate toxic effects of PQ++ in the lung, and even suggested an accentuation of PQ++-induced damage by DF. Mortality data showed that PQ++/DF animals died in greater numbers (20 mg PQ++/kg) or died earlier (25 mg PQ++/kg) than animals receiving DF alone. Qualitative histopathology in PQ++/DF animals was comparable to PQ++ animals in early stages, but damage was more severe in both incidence and severity of lesions in PQ++/DF animals, particularly at the 25 mg PQ++/kg dose level. After 14 days, surviving animals receiving PQ++ alone showed almost complete resolution of previous inflammation and other acute effects, whereas in the only surviving PQ++/DF animal initial fibrosis had persisted and become more generalized. Thus, chelation therapy with DF may not be straightforward in its effects on PQ++ toxicity.

An electrochemical study of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and its oxidation products☆

Cyclic voltammetric and chronoamperometric experiments were performed on the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and its oxidative metabolites, 1-methyl-4-phenyl-2,3-dihydropyridinium ion (MPDP+) and 1-methyl-4-phenylpyridinium ion (MPP+). In neutral phosphate buffer electrolyte, MPTP underwent a 2-electron, 1-proton electrooxidation to MPDP+ according to an ECE mechanism where U2o < U1o. Further oxidation could be achieved in alkaline solution. MPDP+ could be reduced to the radical state (Uo ⩽ −0.560 V versus SHE) followed by dimerization and other side reactions. It was also shown that MPDP+ gradually disproportionates to produce MPTP and MPP+. With an Uo = −1.067±0.010 V, MPP+ underwent a 1-electron reduction, followed by dimerization. The redox potentials of MPP+ and possibly even MPDP+ were beyond the limits of known physiological reducing potentials; therefore, the primary cytotoxic action of MPP+ does not proceed via redox cycling of its reduction radical, as do other pyridinium-based toxins such as methyl viologen.

Common Molecular Mechanisms

The present workshop is the second of two coordinated conferences on mechanisms of carcinogenesis. The first meeting highlighted the molecular mechanisms common to radiation and chemical agents. Here we will place greater emphasis on epidemiological considerations and on dose-response models used in risk assessment to extrapolate from experimental data obtained at high doses to the effects from long-term, low-level exposures. Nevertheless, we feel so strongly that the sorting out of biologically realistic dose-response models must follow from mechanistic insight that we will open Session I with reports from the NCI symposium/workshop. It is my charge to deal with common molecular mechanisms; Michael Fry will be the rapporteur on cellular and animal models.