Factors Affecting the Species-Homologous and Species-Heterologous Binding of Mitochondrial ATPase Inhibitor, IF1, to the Mitochondrial ATPase of Slow and Fast Heart-Rate Hearts

Abstract

We examined the effects of a variety of conditions upon the IF1-mediated inhibition of the ATPase in both intact and sonicated mitochondria and in IF1-depleted submitochondrial particles (SMP) in species-homologous and species-heterologous combinations of IF1 and ATPase. IF1-mediated ATPase inhibition occurred in intact rabbit heart mitochondria at low matrix pH and low membrane potential, but not in intact pigeon and rat heart mitochondria under the same conditions. IF1-mediated ATPase inhibition was, however, demonstrable in both the rabbit and pigeon heart systems in sonicated mitochondria incubated at low ionic strength. The rat heart system failed to exhibit significant IF1-mediated ATPase inhibition in either intact or sonicated mitochondria due to the low amount of IF1 present. When rabbit heart IF1-containing extracts were incubated with IF1-depleted rabbit heart SMP over a range of KCl concentrations, increasing the [KCl] to 100 mM had little effect on IF1-mediated ATPase inhibition. When pigeon heart IF1-containing extracts were incubated with IF1-depleted pigeon heart SMP under the same conditions, increasing [KCl] to 100 mM nearly completely blocked IF1-mediated ATPase inhibition. While the species-endogenous level of rat heart IF1 (i.e., 1× IF1) inhibited IF1-depleted rat heart SMP virtually not at all at any [KCl] examined, the 8× rat heart IF1 was nearly as inhibitory as the 1× rabbit heart IF1 at varying ionic strengths. When rabbit, pigeon, or rat heart IF1 was bound to rabbit versus pigeon IF1-depleted SMP, the effect of varying ionic strength on IF1-mediated ATPase inhibition was related to the species source of the IF1, not to the species source of the enzyme; 1× bovine heart IF1 purified to homogeneity behaved much like 1× crude rabbit heart IF1 when binding to either the rabbit or the pigeon heart enzyme. This suggests that an IF1-ATPase complex stabilizing factor such as has been isolated from baker′s yeast cells is neither lacking in the pigeon heart system nor required for the more ionic-strength-resistant binding of IF1 observed in slow heart-rate mammalian heart mitochondria.