A thermostable protein inhibitor of protein kinase from the swine brain. Isolation of the inhibitor and interaction with the enzyme

A heat-stable protein inhibitor of cAMP-dependent protein kinase has been isolated from pig brain tissue. During gel filtration the protein is eluted in three peaks corresponding to the tetramer, dimer and monomer. The monomer fraction was purified 609-fold. The molecular mass of the monomeric form as determined by gel filtration and electrophoresis is equal to 11,000 Da and 8000 Da, respectively. The inhibition of the phosphotransferase reaction with respect to ATP occurs via a non-competitive mechanism, while that for histone--via a competitive mechanism. A formal kinetic analysis of various modes of the inhibitor binding to different protein kinase forms, e. g., the cAMP-dependent protein kinase catalytic subunit, protein kinase holoenzyme in the presence and absence of cAMP as well as of holoenzyme preparations modified by dimethylsuberimidate and cupric 1,10-phenanthroline, has been carried out. It was demonstrated that 3-4 inhibitor molecules are involved in the interaction with protein kinase.