Addition of increasing concentrations of K+ to yeast mitochondria in the presence of 0 to 400 mu M phosphate and 200 mu M Mg2+ led to uncoupled respiration and decreased protonmotive force (Delta P): at 0 K+ Delta P = 213 mV, negative inside, where Delta psi = 180 mV and Delta pH = 33 mV, while at 20 mM K+ Delta P = 28 mV, where Delta psi = 16 mV and Delta pH = 12 mV, In contrast, the synthesis of ATP resulted in smaller values for the K-m and the V-max in 400 mu M Pi and increasing ADP: in 0 K+, K-m = 18.6 mu M and V-max = 75.4 nmol (min.mg protein)(-1), while in 20 mM K+, K-m = 5.2 mu M and V-max = 46.0 nmol (min.mg protein)(-1), i.e., when K+ depleted most of the Delta P, and at ADP concentrations below the K-m, 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 Delta P of yeast mitochondria were prevented by increasing concentrations of phosphate (1 to 4 mM), At 4 mM phosphate, the Delta P was always above 200 mV and the kinetics of ATP synthesis were as follows: 0 K+ K-m = 10.0 mu M and V-max = 88.3 nmol (min.mg protein)(-1). At 20 mM K+, K-m = 7.4 mu M and V-max = 133 nmol (min.mg protein)(-1). (C) 1997 Academic Press.