A cytosolic protein tyrosine kinase in rat adipocytes

Abstract

Previous studies suggested that insulin receptor tyrosine kinase (IRTK) is the sole tyrosine kinase in rat adipocytes. We now report that this cell type also contains a cytosolic soluble protein tyrosine kinase (CytPTK) which is not related to IRTK. The enzyme phosphorylated PolyGlu₄Tyr whith high efficiency at a rate of 20 ± 2 pmoI PTyr/20μg PolyGlu₄ Tyr/20 min/μg cytosolic protein. Upon gel filtration chromatography 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 staurosporine (ID₅₀ = 3 nM) as opposed to IRTK (ID₅₀ = 8μM). In addition, CytPTK (but not IRTK) was largely activated by vanadate ions. Agents which affect the serine/threonine phosphorylation state of cell proteins did not alter CytPTK activity when subjected to intact adepocytes. In a cell-free system CytPTK activity was largely reduced by pretreatment with immobilized alkaline phosphatase at physiological pH. The possibility that CytPTK participates in insulin-independent regulation of glucose metabolism is suggested.