Role of sulfhydryl groups in benzoquinone-induced Ca²⁺ release by rat liver mitochondria

Abstract

Incubation of rat liver mitochondria with benzoquinone derivatives in the presence of succinate plus rotenone has been shown to cause NAD(P)H oxidation followed by Ca²⁺ release. Further investigation revealed: (1) p-Benzoquinone-induced Ca² release was not initiated by a collapse of the mitochondrial membrane potential. However, Ca²⁺ release and subsequent Ca²⁺ cycling caused limited increased membrane permeability. (2) p-Benzoquinone-induced NAD(P)H oxidation and Ca²⁺ release were prevented by isocitrate, 3-hydroxybutyrate, and glutamate but not by pyruvate or 2-oxoglutarate. (3) Inhibition of pyruvate and 2-oxoglutarate dehydrogenases by p-benzoquinone was attributed to arylation of the SH groups of the cofactors, CoA and lipoic acid. Isocitrate dehydrogenase was also inhibited by p-benzoquinone, but the cofactors NAD(P)H and Mn²⁺ protected the enzyme. Glutamate dehydrogenase was not inhibited by p-benzoquinone. (4) Arylation of mitochondrial protein thiols by p-benzoquinone was associated with an inhibition of state 3 respiration, which was attributed to the inactivation of the phosphate translocase. In contrast, state 4 respiration, and the F₁ · F₀-ATPase and $\text{ATP}\text{ADP}$ translocase activities were not inhibited. It was concluded that inhibition of mitochondrial NAD(P)H dehydrogenases by arylation of critical thiol groups will decrease the NAD(P)⁺-reducing capacity, and possibly lower the $\text{NAD(P)H}\text{NAD(P)}{}^{\mathrm{+}}$ redox status in Favor of Ca²⁺ release.