A phosphoprotein phosphatase from ox brain.

1. The pyrophosphate-exchange reactions which are catalysed by rat-liver preparations and depend upon leucine or isoleucine are profoundly modified by 'soluble' ribonucleic acid and by changes in magnesium concentration. The preponderant influence is exerted by the terminal nucleotide sequence of the 'soluble' ribonucleic acid. Lysinedependent pyrophosphate exchange occurs only at relatively high magnesium concentrations and is not greatly influenced by 'soluble' ribonucleic acid. The transfer of [14C]lysine to 'soluble' ribonucleic acid is catalysed more rapidly by 'recombined' systems than by pH 5 0 fractions and may be related to the inability of the enzyme to react with low concentrations of magnesium, and to the presence of an inhibitor. 2. The terminal nucleotide sequence of 'soluble' ribonucleic acid may be concerned in the protection of a labile, essential site associated with the activating enzyme. The leucineand lysineactivating enzymes show highly individual responses to activators and inhibitors of sulphydryl groups. The possible role of the labile site in the reaction mechanism is discussed.