Effect of Oxidation of Glutathione and Membrane Thiol Groups on Mitochondrial Functions

Summary

Diamide [diazenedicarboxylic acid bis (N,N’-dimethylamide)] a thiol oxidant, does not titrate more than 25% of total liver mitochondrial thiol groups. These include the nonprotein thiols present in the inter-membrane and matrix spaces and vicinal pairs of protein membrane-bound -SH. Mitochondrial glutathione content is decreased by diamide only at concentrations above 0.5 mil. At concentrations below those required for GSH oxidation, diamide slightly stimulates ATPase activity of intact rat liver mitochondria and induces an efflux of endogenous Mg²⁺ dependent on coupled respiration. These effects are fully preventy by EGTA and ruthenium red, a known inhibitor of Ca²⁺ transport across mitochondrial inner membrane. The dependence of these phenomena on an energy-dissipating Ca²⁺ flux is also supported by a release of state 4 respiration. In the presence of low concentrations of diamide (< 0.25 mM) external Ca²⁺ are rapidly taken up and retained within mitochondria, while Mg²⁺ are released in an energy-dependent process. It is assumed that diamide-induced Mg²⁺ efflux is the consequence of a cyclic in and out movement of Ca²⁺, in which the passive efflux promoted by the oxidation of pairs of membrane-bound thiols is compensated by a continuous energy-dependent reuptake. At higher concentrations (> 0.5 mM) Ca²⁺ are initially taken up by liver mitochondria but subsequently released. Concomitantly the efflux of Mg²⁺, initially dependent on respiration and sensitive to ruthenium red, becomes “passive” and insensitive to ruthenium red in coincidence with Ca²⁺ release. It is assumed that the redox state of some pairs of mitochondrial -SH (those sensitive to low diamide concentrations) might control, through a modulation of Ca²⁺ flux, the binding of Mg²⁺ to mitochondrial membranes as well as their permeability properties.