Phosphorlyation of tyrosine aminotransferase invitro by cyclic nucleotide-dependent protein kinases

Summary The phosphorylation of purified rat liver tyrosine aminotransferase was studied in vitro using a purified cGMP-dependent protein kinase (bovine lung) and the purified catalytic subunit from a cAMP-dependent protein kinase (rat liver). Both protein kinases catalyzed the transfer of 32 Pi from [γ- 32 P] ATP to tyrosine aminotransferase. However, the phosphorylation rates were much lower than observed with histones as substrates. Both protein kinases phosphorylated the same major site in tyrosine aminotransferase, corresponding to a serine residue in a tryptic peptide having a molecular weight of about 800. This site also appeared to be phosphorylated in hepatoma tissue culture (HTC) cells in vivo .