Potassium Collapses the ΔPin Yeast Mitochondria While the Rate of ATP Synthesis Is Inhibited Only Partially: Modulation by Phosphate

Addition of increasing concentrations of K+to yeast mitochondria in the presence of 0 to 400 μmphosphate and 200 μmMg2+led to uncoupled respiration and decreased protonmotive force (ΔP): at 0 K+ΔP= 213 mV, negative inside, where Δψ = 180 mV and ΔpH = 33 mV, while at 20 mmK+ΔP= 28 mV, where Δψ = 16 mV and ΔpH = 12 mV. In contrast, the synthesis of ATP resulted in smaller values for theKmand theVmaxin 400 μmPi and increasing ADP: in 0 K+,Km= 18.6 μmandVmax= 75.4 nmol (min·mg protein)−1, while in 20 mmK+,Km= 5.2 μmandVmax= 46.0 nmol (min·mg protein)−1, i.e., when K+depleted most of the ΔP, and at ADP concentrations below theKm, the rate of ATP synthesis was essentially the same as in the absence of K+. At saturating ADP, the rate of ATP synthesis in the presence of K+was about 60% of the rate observed without K+. The synthesis of ATP by yeast mitochondria was inhibited by oligomycin or uncouplers. K+had no effects on rat liver mitochondria. Adenylate kinase activity was much smaller in yeast mitochondria than in rat liver mitochondria and thus did not account for the synthesis of ATP observed in the presence of K+. The effects of K+on the ΔPof yeast mitochondria were prevented by increasing concentrations of phosphate (1 to 4 mm). At 4 mmphosphate, the ΔPwas always above 200 mV and the kinetics of ATP synthesis were as follows: 0 K+Km= 10.0 μmandVmax= 88.3 nmol (min·mg protein)−1. At 20 mmK+,Km= 7.4 μmandVmax= 133 nmol (min·mg protein)−1.