A model phosphatase 2C --> phosphatase 1 activation cascade via dual control of inhibitor-1 (INH-1) and DARPP-32 dephosphorylation by two inositol glycan putative insulin mediators from beef liver.

Two inositol phosphoglycans (IPG) isolated from beef liver and designated as putative insulin mediators were demonstrated to reciprocally enhance the dephosphorylation of inhibitor-1 (INH-1) and DARPP-32, thus directly activating phosphatase 2C and disinhibiting phosphatase 1 in a potential protein phosphatase 2C --> phosphatase 1 cascade mechanism. One IPG termed pH 2.0, containing Dchiro-inositol and galactosamine, stimulated the dephosphorylation of INH-1 and DARPP-32 in a dose-dependent manner in the low micromolar range. A second, termed pH 1.3, containing myo-inositol glucosamine and mannose acted reciprocally to inhibit the cAMP-dependent protein kinase phosphorylation of INH-1 and DARPP-32 in a dose-dependent manner in the low micromolar range. These model experiments are discussed in terms of the observed dephosphorylation of INH-1 with insulin action documented in the literature and the activation of both phosphatase 1 and 2C described in intact cells and in vivo with insulin action.