Molecular Basis for the Interaction of Catalase with d-Penicillamine: Rationalization of Some of Its Deleterious Effects.

d-Penicillamine (d-Pen) is a sulfur compound used in the management of rheumatoid arthritis, Wilson's disease (WD), and alcohol dependence. Many side effects are associated with its use, particularly after long-term treatment. However, the molecular basis for such side effects is poorly understood. Based on the well-known oxidase activity of hemoproteins and the participation of catalase in cellular HO redox signaling, we posit that d-Pen could inactivate catalase, thus disturbing HO levels. Herein, we report on the molecular basis that could partly explain the side effects associated with this drug compound, and we demonstrate that it induces the formation of compound II, a temporarily inactive state of the enzyme, through two distinct mechanisms. Initially, d-Pen reacts with native catalase and/or iron metal ions, used to mimic non-heme iron overload observed in long-term treated WD patients, to generate thiyl radicals. These radicals partake in a futile redox cycle, thus producing superoxide radical anions O and hydrogen peroxide HO. Then, either HO unexpectedly reacts with reduced CAT-Fe(II) to produce compound II or both aforementioned reactive oxygen species intervene in compound II generation through compound I formation and then reduction. These findings support the evidence that d-Pen could perturb HO redox homeostasis through transient but recurring catalase inactivation, which may in part rationalize some deleterious effects observed with this therapeutic agent, as discussed.