Uptake of potential anti-diabetic VIVO compounds of picolinate ligands by red blood cells

The interaction of three potential anti-diabetic VIVO compounds formed by picolinate (pic), 3-methylpicolinate (3-mepic) and 6-methylpicolinate (6-mepic) with hemoglobin (Hb) and red blood cells was studied with the combined application of spectroscopic (EPR), spectrophotometric (UV–Vis) and computational (DFT methods) techniques. In the ternary systems with hemoglobin, pic and 3-mepic (L) form mixed species cis-VOL2(Hb), with the equatorial binding of an accessible His residue, whereas 6-mepic forms VO(6-mepic)(OH)(Hb). The experiments about the uptake of VIVO complexes by red blood cells indicate that only [VO(pic)2(H2O)] penetrates the erythrocyte membrane in a significant amount, whereas for [VO(3-mepic)2] and [VO(6-mepic)2] the hydrolytic reactions at physiological pH hinder the diffusion in the intracellular medium. Inside the red blood cells, the biotransformations depend mainly on the strength of the ligand. Pic and 3-mepic form cis-VOL2(Hb) and cis-VOL2(Cys-S−) with the equatorial coordination of a thiolate-S− stemming from GSH or a membrane protein. Instead, the less thermodynamically stable compound, [VO(6-mepic)2], loses the two ligands after the interaction with the membrane or inside the erythrocytes to give the same species formed by free VIVO2+ ion: (VO)Hbβ and (VO)Hbγ, with VIVO2+ coordinated to the sites β and γ of hemoglobin, and VO(L1,L2) and VO(L3,L4), where L1, L2, L3 and L4 are generic red blood cell bioligands, such as proteins (for example, Hb) or low molecular mass (l.m.m.) components. The distribution of an insulin-enhancing V compound between the serum and the red blood cells may influence the mechanism of action and the activity of a V drug and explain the different effectiveness observed in the literature.