Mitochondrial phosphoprotein metabolism

1. 1. Rat-liver mitochondria incubated at 20° in a medium of KCl, Tris, Versene, and containing ³²P, rapidly take up the tracer into ATP and phosphotein fractions. Phospholipids are not labelled under these conditions. 2. 2. Inhibitors of electron transport and of oxidative phosphorylation reduce the rate of labelling of the phosphoproteins. In the presence of these inhibitors the phosphoproteins fail to reach control levels of radioactivity at any time. 3. 3. The turnover of mitochondral ATP is also reduced by inhibitors of electron transport and oxidative phosphorylation. In contrast to the phosphoproteins, mitochondrial ATP reaches the same isotopic equilibrium both in the presence and absence of inhibitors within a few minutes. 4. 4. Oligomycin, however, does not inhibit phosphoprotein turnover at concentrations at which the labelling of ATP is greatly reduced. 5. 5. It is proposed that mitochondrial phosphoproteins lie on a cyclic pathway between intermediates of the oxidative phosphorylation system and ATP.