Synthesis of C-glycoside analogues of beta-galactosamine-(1-->4)-3-O-methyl-D-chiro-inositol and assay as activator of protein phosphatases PDHP and PP2Calpha.

The glycan beta-galactosamine-(1-4)-3-O-methyl-D-chiro-inositol, called INS-2, was previously isolated from liver as a putative second messenger-modulator for insulin. Synthetic INS-2 injected intravenously in rats is both insulin-mimetic and insulin-sensitizing. This bioactivity is attributed to allosteric activation of pyruvate dehydrogenase phosphatase (PDHP) and protein phosphatase 2Calpha (PP2Calpha). Towards identification of potentially metabolically stable analogues of INS-2 and illumination of the mechanism of enzymatic activation, C-INS-2, the exact C-glycoside of INS-2, and C-INS-2-OH the deaminated analog of C-INS-2, were synthesized and their activity against these two enzymes evaluated. C-INS-2 activates PDHP comparable to INS-2, but failed to activate PP2Calpha. C-INS-2-OH was inactive against both phosphatases. These results and modeling of INS-2, C-INS-2 and C-INS-2-OH into the 3D structure of PDHP and PP2Calpha, suggest that INS-2 binds to distinctive sites on the two different phosphatases to activate insulin signaling. Thus the carbon analog could selectively favor glucose disposal via oxidative pathways.