A Putative Mediator of Insulin Action Which Inhibits Adenylate Cyclase and Adenosine 3′,5′-Monophosphate- Dependent Protein Kinase: Partial Purificaton from Rat Liver: Site and Kinetic Mechanism of Action*

A novel putative mediator of insulin action which acts to inhibit adenylate cyclase and cAMP-dependent protein kinase has been purified from livers of insulin-treated streptozotocin-diabetic rats. It was increased by short term (5-min) insulin injections in vivo and purified several thousand-fold by Sephadex and HPLC. Its mol wt was somewhat larger (2500) than previous mediators identified, and it was more hydrophobic in character. Its mechanism of action or adenylate cyclase was determined and found to be chiefly directed against the catalytic subunit. Its action on the cAMP-dependent protein kinase was found to be competitive with regard to protein substrate, but noncompetitive with regard to ATP and cAMP. Its relationship to other putative insulin mediators and the mechanism of insulin action is discussed.