Steven R. Meshnick

Scavenger enzymes and resistance to oxygen mediated damage in Trichinella spiralis

In order to understand why different stages of Trichinella spiralis vary in their susceptibility to killing by leukocytes, the effects of artificially generated oxidants on different stages of this parasite were compared. More than 90% newborn larvae were killed after incubation in acetaldehyde-xanthine oxidase or glucose-glucose oxidase. On the other hand, fewer than 10% of adult worms or muscle larvae were killed when incubated under identical conditions. Thus, only the stages which are resistant to killing by leukocytes are resistant to killing by oxidants. The larvicidal effect of acetaldehyde-xanthine oxidase was blocked by the addition of either superoxide dismutase or catalase and was partially inhibited by radical scavengers and singlet oxygen quenchers. The oxidant resistant adults and muscle larvae contained 3-5 times more superoxide dismutase and at least five times more glutathione peroxidase than the oxidant sensitive newborn larvae. In contrast, all 3 stages lacked detectable amounts of catalase and contained roughly equivalent amounts of reduced glutathione. Accordingly, adults and muscle larvae may be more resistant to killing by leukocytes than newborn larvae because they contain better oxidant defenses.

Leishmanial superoxide dismutase: A possible target for chemotherapy

Abstract Leishmania tropica promastigotes stimulate macrophages to produce activated oxygen as measured by luminol-enhanced chemiluminescence. Exogenous superoxide dismutase and catalase inhibit this by 95%, implying that both superoxide and hydrogen peroxide are generated. Whereas leishmania have undetectable levels of catalase, and very little glutathione peroxidase, they have relatively high amcunts of superoxide dismutase (23 units/mg protein). The leishmanial superoxide dismutase is cyanide-insensitive but azide- and peroxide-sensitive, suggesting that the enzyme may be iron-containing. Furthermore, the leishmanial superoxide dismutase is insensitive to diethyldithiocarbamate, which inhibits vertebrate enzymes. Thus, leishmania may contain a superoxide dismutase which is different from its hosts enzyme. A specific inhibitor of this enzyme might serve as an antileishmanial agent.

An iron-containing superoxide dismutase in Tritrichomonas foetus

Dialysed extracts of Tritrichomonas foetus were found to have superoxide dismutase at substantially higher levels than those found in trypanosomatids and mouse red blood cells. The activity was sensitive to inhibition by H2O2 but not by cyanide, suggesting that this organism has iron-containing superoxide dismutase(s). Three isozymes were seen by isoelectric focusing which appeared to be sensitive to inhibition by H2O2.

Modifying intracellular redox balance: an approach to improving therapeutic index

It was postulated that the therapeutic index of very toxic, oxidative drugs could be improved by concurrent treatment with other agents, such as methylene-blue, which affect the concentration of intracellular reducing agents. In support of this hypothesis, methylene-blue was found to protect mice against the toxic effects of doxorubicin without reducing doxorubicins antineoplastic activity. Because the clinical usefulness of doxorubicin, the most commonly used and broadly active chemotherapeutic agent, is severely limited by acute and cumulative toxic effects, clinical trials of the effect of methylene-blue on the toxic therapeutic ratio of doxorubicin and other oxidative quinone drugs should be considered.

Trypanosomatid iron-superoxide dismutase inhibitors: Selectivity and mechanism of N1, N6-BIS (2,3-dihydroxybenzoyl)-1,6-diaminohexane

Crithidia, like trypanosomes and leishmania, has an iron-containing superoxide dismutase. The iron chelator N1,N6-bis (dihydroxybenzoyl)-1,6-diaminohexane proved to be a potent inhibitor of this enzyme. Inhibition of the crithidial superoxide dismutase by this compound was dependent on the presence of oxygen and associated with the formation of a complex which could not be dissociated by gel-filtration chromatography. We propose that this biscatecholic inhibitor is first oxidized to a quinone which then covalently modifies a nucleophilic residue on the enzyme. This compound was less effective as an inhibitor of a mammalian copper- and zinc-containing superoxide dismutase. Thus, this inhibitor could serve as a prototype for the design of antiparasitic agents.