A Novel and Simple Method to Assay the Activity of Individual Protein Kinases in a Crude Tissue Extract

Protein kinases and phosphatases play an important role in a variety of cellular functions such as cell growth, development, and gene expression (1 ). It is estimated that one-third of the proteins in a typical mammalian cell are phosphorylated and about 200 protein kinases and 100 protein phosphatases have been identified. In addition, perhaps 2-3% of the genes in the entire genome of an eukaryotic cell may code for protein kinases and as many as 5% of the human genes may encode protein kinases and phosphatases (2 ). The fact that these protein kinases and phosphatases have multiple substrates in vivo may explain their diverse physiological functions (1 -3 ). Thus, it is of considerable interest to develop an assay system that is specific for certain protein kinases and simple enough to be used by both the novice as well as the expert in the field.