A CYTOSOLIC PROTEIN TYROSINE KINASE IN RAT ADIPOCYTES

Previous studies suggested that insulin receptor tyrosine kinase (IRTK ) is the sole tyrosine kinase in rat adipocytes. We now report that th is cell type also contains a cytosolic soluble protein tyrosine kinase (CytPTK) which is not related to IRTK. The enzyme phosphorylated Poly Glu4Tyr with high efficiency at a rate of 20 +/- 2 pmol PTyr/20-mu-g P olyGlu4 Tyr/20 min/mu-g cytosolic protein. Upon gel filtration chromat ography the enzyme activity was eluted as a single peak corresponding to a molecular mass of 53 +/- 3 kDa. Unlike IRTK, CytPTK activity was supported by Co2+ rather than by Mn2+, and it was not inactivated by N -ethylmaleimide. The enzyme was extremely sensitive to inhibition by s taurosporine (ID50 = 3 nM) as opposed to IRTK (ID50 = 8-mu-M). In addi tion, CytPTK (but not IRTK) was largely activated by vanadate ions. Ag ents which affect the serine/threonine phosphorylation state of cell p roteins did not alter CytPTK activity when subjected to intact adipocy tes. In a cell-free system CytPTK activity was largely reduced by pret reatment with immobilized alkaline phosphatase at physiological pH. Th e possibility that CytPTK participates in insulin-independent regulati on of glucose metabolism is suggested.