Primary AML cells and leukemic cell lines have lower spare reserve capacity in their respiratory chain enzymes than normal hematopoietic cells. Spare reserve capacity measured by OCR of primary AML samples and normal hematopoietic cells (A) and leukemic cell lines and solid tumor cell lines (B) after the sequential addition of oligomycin and FCCP. (C) Primary AML and normal hematopoietic cells were treated with increasing concentrations of antimycin and changes in oxygen consumption were measured. A representative graph is shown. Primary AML and normal hematopoietic cells (D-E) and leukemia, MCF-7 breast, and OVCAR-3 ovarian cancer cells (F-H) were treated with increasing concentrations of inhibitors of complex I (rotenone) (D,F), complex III (antimycin) (D,G), or complex V (oligomycin) (D,H). The concentration of the complex inhibitor required to reduce OCR by 50% (EC₅₀) was determined. Data for cell lines represent the mean complex activity ± SD from representative experiments performed in triplicate. Experiments with cell lines were performed at least 3 times. (E) Primary AML cells and normal hematopoietic cells were treated with increasing concentrations of complex IV inhibitor, sodium azide (NaN₃), and changes in oxygen consumption were measured. A representative graph is shown. The concentration of NaN₃ required to reduce OCR by 25% (EC₂₅) was determined. In all panels, *P < .05; **P < .001; ***P < .0001 as determined by the unpaired Student t test.