Tyrosyl protein kinases in normal rat liver: identification and partial characterization.

Rat livers were fractionated and subcellular components were assayed for tyrosyl protein kinase activity. About 60% of the kinase activity in the cytoplasm sedimented with the microsomal fraction, whereas 40% remained in the supernatant. Purification of cytosolic and microsomal kinases by ion-exchange and gel filtration chromatography resolved a major species whose molecular mass was 75 kilodaltons (referred to as TPK 75) and a minor one whose molecular mass was greater than 160 kilodaltons. Partially purified TPK 75 phosphorylated a protein of the same molecular mass on tyrosine residues. The activity associated with TPK 75 was not stimulated by growth factors and was sensitive to thiol re-agents.