MiP2005: Session 5

Mitochondrial Physiology Network 10.9: 55 (2005) - download pdf

 

cAMP-dependent phosphorylation of catalytic subunit I of cytochrome c oxidase switches off enzyme activity.

I Lee, AR Salomon, S Ficarro, I Mathes, F Lottspeich, LI Grossman, Maik Hüttemann

Wayne State Univ, School Medicine, Molecular Medicine Genetics, 540 East Canfield, Detroit, MI 48201, USA. - maik@genetics.wayne.edu

    Signaling pathways targeting mitochondria are poorly understood. To examine whether the cAMP-dependent pathway affects cytochrome c oxidase (COX), the terminal enzyme of the electron transport chain, cow liver COX was purified in the presence of theophylline, a phospho-diesterase inhibitor, leading to high cAMP levels. Using anti-phospho antibodies, we showed that COX subunit I is tyrosine-phosphorylated in the presence of theophylline but not in its absence. The site of phosphorylation, identified by mass spectrometry, was Tyr-304 of COX subunit I. Subunit I phosphorylation leads to decrease of Vmax and shifts the reaction kinetics from hyperbolic to sigmoidal such that COX is fully inhibited up to 10 mM cytochrome c substrate concentrations, even in the presence of allosteric activator ADP. To assess our findings with the isolated enzyme in a physiological context, we tested whether treatment with the physiological starvation signal glucagon leads to COX inactivation. Measurements using cow liver tissue or fed human HepG2 cells revealed dramatic inhibition of COX activity upon treatment with glucagon. Similar results were obtained with cow tissue. Interestingly, cells starved overnight already showed sigmoidal kinetics in the presence of ATP. Thus, the glucagon receptor/G-protein/cAMP pathway regulates COX activity. We tested the effect of asthma drug theophylline on cow lung tissue applying concentrations used in therapy and observed COX inhibition and decreased ATP levels. Our findings may provide a mechanism for theophylline action during asthma, where airway constriction, which requires energy, is weakened due to COX inhibition, leading to decreased ATP production through oxidative phosphorylation.


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