A novel low mol wt inositol phosphoglycan antagonist of insulin action of oxidative glucose metabolism in isolated rat adipocytes was partially purified from normal human plasma and shown to be increased in type II diabetic plasma. It was characterized chemically as a myo-inositol phosphoglycan containing a cyclic 1,2-phosphate. This antagonist, termed fraction V-3, is now shown to inhibit the action of an inositol glycan insulin pH 2.0 mediator that stimulates pyruvate dehydrogenase phosphatase in a similar manner to insulin. In addition, fraction V, inhibits stimulation of the pyruvate dehydrogenase (PDH) phosphatase by Mg2+, the enzyme's required metal, and by spermine, a polyamine. Fraction V-3 does not inhibit active PDH itself. The inhibitory effect is dose dependent and apparently noncompetitive or nonsurmountable for the insulin inositol glycan pH 2.0 mediator, thus comparing kinetically with its insulin antagonistic action on intact adipocytes. Its inhibitory action on PDH phosphatase is dose dependent and competitive for Mg2+ stimulation of the phosphatase. Additionally, fraction V-3 is shown to inhibit stimulation by Mg2+ of cloned recombinant PDH phosphatase catalytic subunit. Inhibition by fraction V-3 of Mg2+-stimulated PDH phosphatase and its cloned catalytic subunit helps explain its mechanism of action to inhibit insulin-stimulated oxidative glucose metabolism in adipocytes and its potential clinical significance in insulin resistance.
