On the Function of the Natural ATPase Inhibitor Protein in Intact Mitochondria

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Abstract
The recently described methodology to extract the [bovine heart] mitochondrial ATPase along with other mitochondrial proteins into organic solvents, and their subsequent incorporation into liposomes was employed to estimate the number of ATPases that contain the natural ATPase inhibitor protein in its inhibitory site in intact mitochondria incubated in various metabolic states. In the presence of electrochemical gradients about 50% of the ATPases are without inhibitor protein in its inhibitory site (active ATPase). In the transition from state 4 to state 3 the percentage of active ATPase diminishes from about 50 to .apprx. 20%. This indicates that during steady-state phosphorylation only a limited number of ATPases are in the active catalytic state, and that not only during active hydrolysis does the inhibitor protein interact with its inhibiting site; rather the inhibitor seems to interact with an intermediate state of the enzyme that appears either during the synthetic or hydrolytic reactions. In addition ATP, with or without uncoupler, induces the interaction of the inhibitor protein in more than 80% the ATPases through an oligomycin-sensitive process. Thus, notwithstanding other factors the interaction may account for the low hydrolytic activity of mitochondria.

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