MtDNA and nuclear mutations affecting oxidative phosphorylation: correlating severity of clinical defect with extent of bioenergetic compromise.

Rates of ATP synthesis were studied in cultured skin fibroblasts treated with digitonin. In fibroblasts from patients with complex I deficiency, complex IV and complex V deficiency rates of ATP synthesis were decreased below the levels found in controls. In mitochondria isolated from cultured lymphoblasts, ATP synthesis was also decreased by 35-50% in cases of Leigh's disease due to complex I, complex IV, or complex V deficiency. Calculating the effect of the mutations in the various complexes on the overall efficiency of oxidative phosphorylation, we show that the mtDNA 8993 mutation which affects the activity of the F1F0 ATPase (complex V) has the strongest effect.