Insulin mimetic effect of a tungstate cluster. Effect of oral administration of homo-polyoxotungstates and vanadium-substituted polyoxotungstates on blood glucose level of STZ mice

Aqueous vanadate and aqueous tungstate have been known to mimic all or most of the actions of insulin in intact cell systems with respect to normalization of the blood glucose level. By carrying out oral administration in vivo experiments on the blood glucose level of streptozotocin (STZ)-induced diabetes (STZ mice), the insulin-mimetic (IM) effects of metal-oxide clusters of all-inorganic composition were examined using many types of polyoxometalates (POM) with and without vanadium substitution. Several homo-POM and vanadium-substituted POM showed hypoglycemic effects. The observed hypoglycemic effects indicated that POM with the Dawson structure [[alpha-P(2)W(18)O(62)](6-) (W-2), [alpha-P(2)W(17)V(V)O(62)](7-) (V-19) and [alpha-1,2,3-P(2)W(15)V(V)(3)O(62)](9-) (V-04)] are more effective than those with the Keggin structure [[alpha-PW(12)O(40)](3-) (W-1), [alpha-PW(11)V(V)O(40)](4-) (V-01), [alpha-1,2-PW(10)V(V)(2)O(40)](5-) (V-02), [alpha-1,2,3-PW(9)V(V)(3)O(40)](6-) (V-03) and [alpha-1,4,9-PW(9)V(V)(3)O(40)](6-) (V-13)]. The vanadate cluster [V(10)O(28)](6-) (V-15) also showed a hypoglycemic effect. (31)P and (51)V NMR measurements showed that the Dawson POM (W-2, V-04 and V-19) are stable in aqueous solution under the conditions used. The effect of all POM on the body weight of STZ mice was also examined. The decrease in body weight after administration of W-2 was much less than for V-19, V-04 and V-15. This suggests that not only monomeric tungstate and vanadate, but also the structure factors of tungstate and vanadate clusters, can play a significant role in their biological action.