Antimalarial quinones: redox potential dependence of methemoglobin formation and heme release in erythrocytes.

A number of quinones have been shown to be effective antimalarials. In addition some have been shown to have oxidant effects on glucose-6-phosphate dehydrogenase-deficient red cells. To clarify the mechanism of this oxidant effect and to determine whether it is a general property of antimalarial quinones, the effects of five compounds on red cells were studied. Two, 5-hydroxy-6-demethyl pentaquine (5H6DP) and menadione, caused marked methemoglobin production and the insertion of heme into the red-cell membrane. The other three, atovaquone, daphnetin, and menoctone, did not. The insertion of heme into membranes caused diminished deformability, and this might account for the hemolytic effects. Redox potentials of all five compounds were measured. The two quinones that caused heme release, menadione and 5H6DP, had redox potentials of -141 +/- 12 and -97 +/- 25 mV, respectively, which are similar to the reported redox potentials for hemoglobin. The other three quinones had redox potentials that were either significantly higher or lower. Thus only quinones of the appropriate redox potentials are likely to be toxic to red cells.

The electrochemical oxidation of 2,6-dichloro-1,4-phenylenediamine.

The electrochemical oxidation of 2,6-dichloro-1,4-phenylenediamine was studied at a glassy carbon electrode in a 50 vol% methanol-water mixture containing 0.1M HClO(4). Single sweep voltammograms are compared with curves obtained by digital simulation. Characteristic reaction parameters, such as formal potentials, charge transfer coefficients, rate constants of the electrochemical and the chemical steps, and diffusion coefficients, were determined from potential-step experiments in combination with a simulation-curvefitting routine.

pH-dissociation characteristics of cardiolipin and its 2'-deoxy analogue.

Cardiolipin (CL) is found in inner mitochondrial membranes and the plasma membrane of aerobic prokaryotes. CL is tightly bound to those transmembrane enzymes associated with oxidative phosphorylation. CL has earlier been reported to have a single pK at low pH. We have titrated CL in aqueous suspension (bilayers) and in solution in methanol/water (1:1, vol/vol) and found it to display two different pK values, pK1 at 2.8 and pK2 initially at 7.5 but shifting upwards to 9.5 as the titration proceeds. The unusually high pK2 might be explained by the formation of a unique hydrogen bond in which the free hydroxyl on the central glycerol forms a cyclic intramolecular hydrogen-bonded structure with one protonated phosphate (P-OH group). We have therefore chemically synthesized the 2'-deoxycardiolipin analogue, which lacks the central free hydroxyl group, and measured its pH-dissociation behavior by potentiometric titration, under the same conditions as those for CL. The absence of the hydroxyl group changes the titration dramatically so that the deoxy analogue displays two closely spaced low pK values (pK1 = 1.8; pK2 = 4.0). The anomalous titration behavior of the second dissociation constant of CL may be attributed to the participation of the central glycerol OH group in stabilizing the formation of a cyclic hydrogen-bonded monoprotonated form of CL, which may function as a reservoir of protons at relatively high pH. This function may have an important bearing on proton pumping in biological membranes.

Electrochemical study of methylcobalamin Determination of the reduction potential for a quasireversible system with a fast following reaction.

The reductive electrochemistry of methylocobalamin in nonaqueous solution is typical of many electrochemical mechanisms in that the initial electron transfer at the electrode is followed by a fast chemical reaction. The rate constant of the following chemical step, methyl radical cleavage, was measured by double potential step chronoamperometry to be 590 sec(-1) in a solvent mixture (DMF 40%, methanol 60%, at -30 degrees ). The cyclic voltammetric response in the slow scan-rate regime was analyzed by the simulation-fitting program CVFIT to extract the remaining parameters of the electrode reaction-chemical reaction mechanism: the formal reduction potential (E(0') = -1.529 +/- 0.004 V), the standard heterogeneous rate-constant (k(0) = 0.012 +/- 0.002 cm/sec), and the transfer coefficient (alpha = 0.78 +/- 0.02). This method of analysis allows for the rigorous determination of reduction potentials under conditions where the cyclic voltammetric response appears irreversible (no reverse peak is observed). A detailed analysis of the actual reversibility of the system and its effect on the apparent transfer coefficient is presented.

Hemin-catalyzed decomposition of artemisinin (qinghaosu).

Artemisinin (qinghaosu) and its derivatives represent an important new class of antimalarial drugs. Previous work suggests that the antimalarial activity of artemisinin may be mediated by a reaction with intraparasitic hemin. Using cyclic voltammetry, artemisinin and dihydroartemisinin were irreversibly reduced at approximately -1 V. In the presence of concentrations of hemin as low as 50 nM, the reduction took place at much lower potentials (-0.435 to -0.460 V). Both reductions took place after adsorption onto the electrode surface. The shift of the reduction potential to more positive values is indicative of a catalytic process similar to that seen with hydrogen peroxide. The catalytic decomposition of artemisinin may play a role in the antimalarial activity of artemisinin.

A PC-based general program for the simulation and analysis of cyclic voltammetric experiments.

A PC-based program, a General Program for Simulation of Cyclic Voltammetric Experiments (GPS-CV), is described. GPS-CV, written in Turbo Pascal 5.5, utilizes advances in simulation methodology that increase efficiency, allow for generality of mechanism, and include IR drop and capacitive current effects. An accessible user interface is used, and a graphical analysis program is provided. Cyclic voltammograms for nearly any mechanism can be simulated by the user. A companion program, CVFIT, combines the GPS-CV program with a least-squares fit by simplex minimization to give the best-fit parameters, with error estimates. The use of CVFIT is demonstrated with a three-parameter fit of experimental cyclic voltammograms.