Isolation, Structure, Synthesis, and Bioactivity of a Novel Putative Insulin Mediator. A Galactosamine chiro-Inositol Pseudo-Disaccharide Mn<SUP>2+</SUP> Chelate with Insulin-like Activity

We isolated from beef liver a putative insulin mediator termed INS-2, <BO>1</BO>. Its structure was determined to be a novel inositol glycan pseudo-disaccharide Mn²⁺ chelate containing <SCP>d</SCP>-chiro-inositol <BO>2a</BO> (as pinitol) and galactosamine. Purification methods were scaled up from those previously reported to isolate an inositol glycan with similar composition from rat liver.¹ Structure of the beef liver glycan was determined by degradative chemistry and 2D NMR spectroscopy and confirmed by chemical synthesis. Its structure is 4-O-(2-amino-2-deoxy-β-<SCP>d</SCP>-galactopyranosyl)-3-O-methyl-<SCP>d</SCP>-chiro-inositol <BO>1</BO> (INS-2, Figure 1). Its role as an insulin mimetic was demonstrated by its action in vivo to decrease elevated blood glucose injected to low-dose streptozotocin diabetic rats in a stereospecific and dose-dependent manner. The pseudo-disaccharide also stimulated [¹⁴C]glucose incorporation into [¹⁴C]glycogen in a dose-dependent manner in H4IIE hepatoma cells in the presence of insulin, thus enhancing insulin action. Only when chelated to Mn²⁺ did it activate pyruvate dehydrogenase phosphatase in vitro in a dose-dependent manner. To our knowledge, this is the first example of a β-1,4-linked inositol glycan consisting of <SCP>d</SCP>-chiro-inositol and galactosamine isolated from animal tissues with insulin mimetic actions.