Reversibility of self-phosphorylation of a histidine residue of adrenocortical cyclic nucleotide-independent protein kinase, SPK 380.

SPK 380 is a novel cyclic nucleotide-independent protein kinase that self-phosphorylates the tau-amino position of the histidine residue(s) residing in its 120,000-dalton subunit and specifically catalyzes the phosphorylation of serine residue(s) of the alpha-subunit of eukaryotic initiation factor 2 [Kuroda, Y. and Sharma, R.K. (1982) Arch. Biochem. Biophys. 217, 588-596]. Herein we demonstrate that this self-phosphorylation reaction is reversible. In the presence of ADP and Mg2+ or Mn2+, the phosphate bound to histidine is transferred from the enzyme to ADP, resulting in the formation of ATP. The rate of reversible reaction is rapid, and maximal value is reached in 30 seconds. The reaction is dependent on ADP concentration and shows a broad pH activity optimum in contrast to the optimal pH of 8 for the forward reaction.