Modelling changes in glutathione homeostasis as a function of quinone redox metabolism

Abstract

Redox cycling is an understated mechanism of toxicity associated with a plethora of xenobiotics, responsible for preventing the effective treatment of serious conditions such as malaria and cardiomyopathy. Quinone compounds are notorious redox cyclers, present in drugs such as doxorubicin, which is used to treat a host of human cancers. However, the therapeutic index of doxorubicin is undermined by dose-dependent cardiotoxicity, which may be a function of futile redox cycling. In this study, a doxorubicin-specific in silico quinone redox metabolism model is described. Doxorubicin-GSH adduct formation kinetics are thermodynamically estimated from its reduction potential, while the remainder of the model is parameterised using oxygen consumption rate data, indicative of hydroquinone auto-oxidation. The model is then combined with a comprehensive glutathione metabolism model, facilitating the simulation of quinone redox cycling, and adduct-induced GSH depletion. Simulations suggest that glutathione pools are most sensitive to exposure duration at pharmacologically and supra-pharmacologically relevant doxorubicin concentrations. The model provides an alternative method of investigating and quantifying redox cycling induced oxidative stress, circumventing the experimental difficulties of measuring and tracking radical species. This in silico framework provides a platform from which GSH depletion can be explored as a function of a compound's physicochemical properties.

Figure 1

(a) Quinone redox cycling, ROS formation and GSH-based detoxification. A schematic of the single electron reduction of a quinone (Q) to a semiquinone radical anion (SQ^.−), followed by complete reduction to the hydroquinone (H₂Q). The figure shows the concomitant reduction of molecular oxygen by SQ^.− to form the ROS, superoxide (O₂^.−), followed by its dismutation into hydrogen peroxide (H₂O₂), which is detoxified by glutathione (GSH) into harmless H₂O through the glutathione peroxidase (GPx) reaction. GSH is regenerated from its oxidised form (GSSG), catalysed by the glutathione reduction (GR) reaction. Finally, the glutathione-quinone adduct (GS-H₂Q) formation represents the reductive addition (Michael reaction) between GSH and the Q electrophile. (b) Chemical structure of doxorubicin. The anthracycline contains both the quinone (red) and hydroquinone (blue) moieties within its chemical structure. The hydroquinone is the site of auto-oxidation for doxorubicin.

Figure 2

Doxorubicin-quinone redox cycling model schematics. Three variations of quinone redox cycling (reduced, triad and comproportionation) are described. Each model comprises of a single compartment and a selection of the following species: quinone (Q); semiquinone radical (SQ^.−); hydroquinone (H₂Q); superoxide radical (O₂^.−); molecular oxygen (O₂); and hydrogen peroxide (H₂O₂). The corresponding reaction rate equations (R_1–5) are described in Table 1.

Figure 3

Oxygen consumption rate (OCR) profiles for doxorubicin at 50, 37.5, 25, 12.5 and 10 μM. Each data point in represents the OCR immediately after a 3-minute solution mix within the well, measured in the transient microchamber. Compound injection occurs at t = 16 min (between measurements 1 and 2). Each dataset is the average of n = 3 experiments expressed with its standard deviation.

Figure 4

Normalised sensitivity measures for the comproportionation model reaction rate constants, expressed as main and total effects.

Figure 5

Triad model fitting and simulation. Comparison of simulated and experimental OCR data for 50, 37.5, 25, 12.5 and 10 µM of doxorubicin (Fig. 3).

Figure 6

Model simulations for doxorubicin and ROS metabolism. The fate of a single doxorubicin exposure (50 µM) was simulated over a 30-minute time-span in order to glean the resulting transformations between Q, SQ^.− and H₂Q (top panel). The resulting superoxide and hydrogen peroxide formation and detoxification profiles are illustrated in the bottom left and right panels, respectively.

Figure 7

The effects of doxorubicin quinone-based metabolism on glutathione and cysteine model homeostasis. The resulting simulated changes in blood and cytosolic GSH and cysteine after a single or constant exposure to 50 μM of doxorubicin are shown in (A and B) respectively, for a 10-hour time-span. % of normal represents the percentage difference of the variable compared to its simulated steady state value.

Figure 8

The effects of doxorubicin quinone-based metabolism on glutathione homeostasis. The resulting simulated changes in cytosolic GSH following single (A) or constant exposure (B) to a range of doxorubicin concentrations (0–50 μM) are shown in (A and B) respectively, for a 20-hour time-span. The 70% reduction threshold is indicated in (B) with a black dashed line.