Role of Ligands in the Uptake and Reduction of V(V) Complexes in Red Blood Cells

The interaction with erythrocytes of four [V(V)O2L2]- complexes, with L = picolinate (pic), 5-cyanopicolinate (picCN), 3-aminopyrazine-2-carboxylate (przNH2), and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinonate (dhp), was studied. The thermodynamic stability at physiological pH is: [V(V)O2(dhp)2]- > [V(V)O2(przNH2)2]- > [V(V)O2(pic)2]- > [V(V)O2(picCN)2]-. With picCN and pic, V exists at physiological pH as H2V(V)O4-, with przNH2 as a mixture of H2V(V)O4- and [V(V)O2(przNH2)2]- and with dhp as [V(V)O2(dhp)2]-. In the systems with pic and picCN, H2V(V)O4- and the ligands cross the erythrocyte membrane independently, with dhp the uptake occurs by diffusion, whereas with przNH2 both the mechanisms are active. Inside erythrocytes stable V(IV)OL2 complexes are formed, indicating that there is no relationship with the stability and redox state of the administered compounds and that, if the metal ion changes its oxidation state in the cytosol as V does, unstable complexes in the extracellular medium could become stable inside the cells and contribute to the pharmacological action.