High tyrosine kinase activity in normal nonproliferating cells.

Protein phosphorylation at serine and threonine residues has been implicated in the regulation of many cellular processes. More recently, tyrosine residue phosphorylation has been shown to be associated with stimulation of cell proliferation, including viral transformation and stimulation by epidermal growth factors (EGF), platelet-derived growth factor (PDGF) and other compounds related to cellular growth such as insulin and dimethyl sulphoxide. To compare protein kinases and phosphoproteins of normal and leukaemic human haematopoietic cells in vivo and in vitro, we first have investigated the percentages of phosphoserine, phosphothreonine and phosphotyrosine obtained after hydrolysis of proteins from different blood cell fractions phosphorylated in vitro. We report here that phosphotyrosine formed less than 1% of the soluble fractions from polymorphonuclear cells, mononuclear cells (80% circulating lymphocytes, 20% monocytes), blood platelets and red blood cells (not shown). Surprisingly, high percentages of phosphorylated tyrosine were found only in the particulate fractions from non-proliferating anuclear cells, platelets and red blood cells.