Insulin-stimulated microtubule associated protein kinase is detectable by analytical gel chromatography as a 35-kDa protein in myocytes, adipocytes, and hepatocytes.

Insulin stimulates a novel Ser/Thr kinase, which phosphorylates microtubule associated protein-2 (MAP-2) in vitro. MAP kinase was studied in cell models of the principal insulin responsive tissues using analytical fast-protein liquid chromatography for partial purification of the enzyme. Stimulation of MAP kinase (1.3- to 2-fold) by insulin was readily detected in BC3H1 smooth and 23A2 skeletal muscle cells; 3T3-L1 adipocytes; and isolated rat hepatocytes and adipocytes. No phosphatase activity was detectable under the assay conditions used, proving that stimulation of a kinase, not inhibition of a phosphatase, is responsible for the increased incorporation of 32PO4 catalyzed by supernatants from insulin-treated 3T3-L1 cells. In H4 hepatoma cells, stimulation of MAP kinase was much less evident after gel filtration in comparison to the other cell types. The activated enzyme present in supernatants from insulin-treated cells migrated as a single peak of approximately 35 kDa apparent molecular mass (except in the case of isolated hepatocytes in which a shoulder was present). These results suggest that the insulin-stimulatable MAP kinase may be ubiquitous in insulin responsive cells.