The interaction of 'soluble' ribonucleic acid, magnesium ions and sulphydryl groups in the control of amino acid-dependent pyrophosphate-exchange reactions.

During an attempt to purify a leucine-activating enzyme from pig-liver-protein fractions low in 'soluble' ribonucleic acid (Hele & Finch, 1960), some fractions became very unstable and lost over half their activity when kept overnight at 15°. In some instances, enzyme activity has been found to be preserved by the presence of 'soluble' ribonucleic acid. In this paper the effect has been further studied with preparations from rat liver. The evidence suggests that the terminal nucleotide sequence of the 'soluble' ribonucleic acid (Hecht, Stephenson & Zamecnik, 1959) is particularly involved in conferring protection upon the enzyme system. Also, removal of 'soluble' ribonucleic a,cid from preparations of the leucine-activating enzyme greatly enhances the pyrophosphate exchange at high concentrations of Mg2+ ion, and the terminal nucleotide sequence (Hecht et al. 1959) again seems to be particularly concerned. The work has also been extended to the activation of isoleucine and lysine. Several investigators (Rendi & Hultin, 1959; Hoagland, Keller & Zamecnik, 1956; Novelli, 1958) have found little or