The native mitochondrial F1-inhibitor protein complex carries out uni- and multisite ATP hydrolysis

Pharmacological profile of the selective mitochondrial F1F0 ATP hydrolase inhibitor BMS-199264 in myocardial ischemia

The mitochondrial F1F0 ATP synthase is responsible for the majority of ATP production in mammals and does this through a rotary catalytic mechanism. Studies show that the F1F0 ATP synthase can switch to an ATP hydrolase, and this occurs under conditions seen during myocardial ischemia. This ATP hydrolysis causes wasting of ATP that does not produce work. The degree of ATP inefficiently hydrolyzed during ischemia may be as high as 50-90% of the total. A naturally occurring, reversible inhibitor (IF-1) of the hydrolase activity is in the mitochondria, and it has a pH optimum of 6.8. Based on studies with the nonselective (inhibit both synthase and hydrolase activity) inhibitors aurovertin B and oligomycin B reduce the rate of ATP depletion during ischemia, showing that IF-1 does not completely block hydrolase activity. Oligomycin and aurovertin cannot be used for treating myocardial ischemia as they will reduce ATP production in healthy tissue. We generated a focused structure-activity relationship, and several compounds were identified that selectively inhibited the F1F0 ATP hydrolase activity while having no effect on synthase function. One compound, BMS-199264 had no effect on F1F0 ATP synthase function in submitochondrial particles while inhibiting hydrolase function, unlike oligomycin that inhibits both. BMS-199264 selectively inhibited ATP decline during ischemia while not affecting ATP production in normoxic and reperfused hearts. BMS-191264 also reduced cardiac necrosis and enhanced the recovery of contractile function following reperfusion. These data also suggest that the reversal of the synthase and hydrolase activities is not merely a chemical reaction run in reverse.

Effect of denaturants on multisite and unisite ATP hydrolysis by bovine heart submitochondrial particles with and without inhibitor protein

The effect of guanidinium hydrochloride (GdnHCl) on multisite and unisite ATPase activity by F0F1 of submitochondrial particles from bovine hearts was studied. In particles without control by the inhibitor protein, 50 mM GdnHCl inhibited multisite hydrolysis by about 85%; full inhibition required around 500 mM. In the range of 500-650 mM, GdnHCl enhanced the rate of unisite catalysis by promoting product release; it also increased the rate of hydrolysis of ATP bound to the catalytic site without GdnHCl. GdnHCl diminished the affinity of the enzyme for aurovertin. The effects of GdnHCl were irreversible. The results suggest that disruption of intersubunit contacts in F0F1 abolishes multisite hydrolysis and stimulates of unisite hydrolysis. Particles under control by the inhibitor protein were insensitive to concentrations of GdnHCl that induce the aforementioned alterations of F0F1 free of inhibitor protein, indicating that the protein stabilizes the global structure of particulate F1.

The IF(1) inhibitor protein of the mitochondrial F(1)F(0)-ATPase

Recent studies on the IF(1) inhibitor protein of the mitochondrial F(1)F(0)-ATPase from molecular biochemistry to possible pathophysiological roles are reviewed. The apparent mechanism of IF(1) inhibition of F(1)F(0)-ATPase activity and the biophysical conditions that influence IF(1) activity are summarized. The amino acid sequences of human, bovine, rat and murine IF(1) are compared and domains and residues implicated in IF(1) function examined. Defining the minimal inhibitory sequence of IF(1) and the role of conserved histidines and conformational changes using peptides or recombinant IF(1) is reviewed. Luft's disease, a mitochondrial myopathy where IF(1) is absent, is described with respect to IF(1) relevance to mitochondrial bioenergetics and clinical observations. The possible pathophysiological role of IF(1) in conserving ATP under conditions where cells experience oxygen deprivation (tumor growth, myocardial ischemia) is evaluated. Finally, studies attempting to correlate IF(1) activity to ATP conservation in myocardial ischemic preconditioning are compared.

When beef-heart mitochondrial F1-ATPase is inhibited by inhibitor protein a nucleotide is trapped in one of the catalytic sites

Inactivation of the isolated ATPase portion of ATP synthase from beef-heart mitochondria (F1) by its natural inhibitor protein (IP) during steady-state ATP hydrolysis is accompanied by a trapping of 1 mol nucleotide/mol F1 in one of the catalytic sites. The trapped nucleotide is not released during incubation of IP-inhibited F1 in the presence of MgATP at pH 8.0 for at least 20 min, indicating a very low turnover rate of the IP.F1 complex. The ATP/ADP ratio of the trapped nucleotides is higher than that found for transitorily bound nucleotides under the same conditions but in the absence of IP. The IP impairs the acceleration of ATP hydrolysis and product release steps that results from the binding of ATP to an alternate catalytic site. It also inhibits ATP hydrolysis by a single catalytic site or shifts the equilibrium toward ATP formation from bound ADP and Pi. At high pH, an active acidic form of the free IP is transformed to the inactive basic one with a half-time of 3-4 s. This process seems to be prevented by IP binding to F1. The inactive basic form of IP does not compete with the active acidic IP for the binding to F1. The data do not favor the existence of a long-lived catalytically active IP.F1 intermediate during IP action on F1. The reactivation of IP-inhibited membrane-bound F1 by energization may be due to a conformational change in the IP.F1 complex allowing the transformation of IP into an inactive basic state that rapidly dissociates.