Identification and characterization of endogenous inhibitors of polyamine-responsive protein kinase activity

Polyamine-responsive protein kinase activity was inhibited by two heat stable inhibitors which were purified from Morris hepatoma 3924A. They were of low molecular weight (inhibitor I-1,600 to 2,000, inhibitor II-600 to 800 daltons). The inhibitors, when purified, passed through a DEAE-cellulose column; however, the crude complex of inhibitors and protein kinase activity bound to the DEAE-cellulose column. This suggests that the inhibitors are bound to the polyamine-responsive protein kinase in vivo. Both inhibitors completely inhibited the polyamine stimulated activity, but did not affect the basal enzymatic activity. They were not affected by treatment at 90 degrees for 2 min. These results demonstrate the presence of two new protein kinase inhibitors in mammalian cells.