Hexokinase autophosphorylation: identification of a new dual specificity protein kinase

Hexokinase 1 (HK1) purified from rat brain exhibits protein kinase activity, including autophosphorylation and phosphorylation of other protein substrates. The amino acid specificity of rat brain autophosphorylation was analyzed with monoclonal antibodies directed against phosphotyrosine and by acid hydrolysis of the phosphorylated enzyme. The results show that serine, threonine, and tyrosine residues are phosphorylated after incubation with ATP. The stoichiometry of this phosphorylation was 0.2 mole phosphate per mole hexokinase after 30 min of incubation. Evaluation of freshly isolated HK1 with monoclonal anti-phosphotyrosine antibody indicates that the enzyme is phosphorylated at a basal level in its native state. We concluded that rat brain HK1 is a dual specificity protein kinase that is phosphorylated physiologically.