A novel family of vanadyl ion (VO2+, oxidation state +4) carriers is introduced. These carriers possess C2 symmetry, utilize two hydroxamate groups as ion binding sites, and optionally possess asymmetric carbons. Binding efficiencies and hydrophobicities are regulated by the use of a modular assembly. When applied to rat adipocytes, these carriers augment the potency of vanadyl ions to stimulate glucose metabolism. The complexes shift the dose-response curve to the left. Also, the maximal effect of vanadyl ions which is in the order of 20-30% of that of insulin is shifted toward maximal (100-115%) stimulation. Among several chelators studied, the order of synergistic potency was RL-252 greater-than-or-equal-to RL-262 > 1367. RL-239, RL-280, and RL-261 had smaller effects, whereas RL-282 had a negligible effect. The synergistic action of RL-252 (and other chelators as well) on VO2+ was already observed at a molar ratio of 1:0.01 of VO2+ to RL-252, respectively, and maximal augmentation occurred at a molar ratio of 1:0.1. The superiority of the hydrophobic chelators relative to the hydrophilic ones, together with the low molar ratio of chelator to VO2+ to achieve maximal effect, strongly suggests that these chelators act as vanadyl ionophores. This notion was confirmed by carrier-facilitated extraction of VO2+ from water into CHCl3 with the following order of decreasing efficacy: RL-262 > RL-252 > 1367 > RL-261. The chelators' potentiating effect may therefore be related to facilitated transport of VO2+ ions into the cells' interiors. The potency of vanadate ions (VO3-, oxidation state +5) was not increased by RL-252, although RL-252 proved to extract vanadate effectively from water into chloroform. This observation is in line with earlier findings that vanadyl ions, rather than vanadate ions, are the activating principle, and suggests that the effectiveness of vanadate is dependent on the cells' capability to reduce it to vanadyl ions. Vanadyl ions, on the other hand, do not require intracellular reduction events. Their limited solubility at neutral pH value and low permeability is now full overcome by the ionophores which facilitate their permeation at low concentrations of the cation. Moreover, the substantially lower potentiating effect of the D-isomer, RL-262 (D), than of the L-isomer, RL-262 (L), suggests that the vanadyl ions exert their function by interactions with chiral recognition sites. The clinical significance of this study is also discussed.
