Cautions on the use of the heat stable inhibitor of protein kinase: studies with S49 lymphoma cells.

When crude preparations of protein kinase inhibitor (PKI) are added to homogenates of S49 lymphoma cells, protein kinase activity increases rather than decreases. This is not an increase in the activity of cyclic AMP-dependent protein kinase, which is completely inhibited by PKI, nor an increase in either cyclic GMP-dependent or Ca ++-dependent kinases. The increased kinase activity is not due to PKI itself but to one or more protein contaminants which serve as substrates for an S49 cell kinase which is cyclic nucleotide- and Ca ++ -independent. These contaminating substrates can be readily separated from PKI by DEAE cellulose chromatography. Although the crude PKI preparations sold by the major biochemical supply houses may be satisfactory for some purposes, we suggest that crude PKI be further purified when used to assess kinase activity in systems as complex as homogenates of whole cells. Otherwise, the presence of contaminating proteins in crude preparations of PKI can lead to erroneous conclusions about the type and quantity of protein kinase in the cell.