The properties of particulate phosphoprotein phosphatase.

The very high metabolic activity of protein-bound phosphorus was first discovered by Davidson et al. (1) in P32 uptake experiments and subsequently confirmed in other laboratories. A possible explanation of these findings is that the primary step in oxidative phosphorylation results in the formation of a phosphorylated protein with subsequent transfer of the high energy phosphate to adenosine triphosphate. The existence of a high energy phosphate complex before adenosine triphosphate has been suggested by a number of investigators (see (2) for review). An enzyme, phosphoprotein phosphatase, which liberates inorganic phosphate from phosphoproteins, was first found by Harris (3) in frog eggs. Some of the properties of the enzyme as it occurs in rat (4, 5) and ox (6) tissues were subsequently reported on. A study of this enzyme was undertaken to test the hypothesis that the activity of some uncoupling agents results from their ability to activate a mitochondrial phosphoprotein phosphatase. Although the enzyme was present in the cytoplasmic particles of liver,’ it was not activated by any of the uncoupling agents tested. In the course of this work, a number of additional properties of the enzyme were examined; the enzyme was found to be metal activated, and a study of the kinetics suggested a rather unusual role for the metal.