The Effects of Superoxide and the Peripheral Benzodiazepine Receptor Ligands on the Mitochondrial Processing of Manganese-Dependent Superoxide Dismutase

The mitochondrion imports and processes the vast majority of the proteins that constitute its structural elements and metabolic pathways. To study mitochondrial precursor processing in the context of the cellular environment, we employed the baculovirus expression system to overexpress the prototypical precursor protein, human manganese-dependent superoxide dismutase (hMn-SOD). It was found that superoxide produced by hyperoxic culture conditions (95% O₂atm) or the redox cycling agent paraquat caused a lesion of the import/processing of precursor hMn-SOD in the baculovirus model. The oxidation of key sulfhydryl groups as a component of the mitochondrial processing lesion was implicated by the observation that the sulfhydryl reducing agent dithiothreitol was completely effective in preventing the block of hMn-SOD processing induced by paraquat. Interestingly, the peripheral benzodiazepine receptor (PBzR) agonists PK11194, Ro5-4864, and protoporphyrin IX were all found to enhance mitochondrial processing of the hMn-SOD precursor protein, suggesting a role for the PBzR in the regulation of mitochondrial import of proteins. Collectively, our results suggest a possible redox-regulated mechanism of mitochondrial protein import that may lead to less efficient precursor protein uptake by mitochondria under severely oxidizing conditions.