Catalytic activities of mitochondrial ATP synthase in patients with mitochondrial DNA T8993G mutation in the ATPase 6 gene encoding subunit alpha

We:investigated the biochemical phenotype of the mtDNA T8993G point mutatio n in the ATPase 6 gene, associated with neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP), in three patients from two unrelated fami lies. All three carried >80% mutant genome in platelets and were manifestin g clinically various degrees of the NARP phenotype, Coupled submitochondria l particles prepared from platelets capable of succinate-sustained ATP synt hesis were studied using very sensitive and rapid luminometric and fluoresc ence methods. A sharp decrease (>95%) in the succinate-sustained ATP synthe sis rate of the particles was found, but both the ATP hydrolysis rate and A TP-driven proton translocation (when the protons flow from the matrix to th e cytosol) were minimally affected. The T8993G mutation changes the highly conserved residue Leu(156) to Arg in the ATPase 6 subunit (subunit a). This subunit, together with subunit c, is thought to cooperatively catalyze pro ton translocation and rotate, one with respect to the other, during the cat alytic cycle of the F1F0 complex. Our results suggest that the T8993G mutat ion induces a structural defect in human F1F0-ATPase that causes a severe i mpairment of ATP synthesis. This is possibly due to a defect in either the vectorial proton transport from the cytosol to the mitochondrial matrix or the coupling of proton flow through F-0, to ATP synthesis in F-1, Whatever mechanism is involved, this leads to impaired ATP synthesis. On the other h and, ATP hydrolysis that involves proton flow from the matrix to the cytoso l is essentially unaffected.