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

The interaction of three potential anti-diabetic (VO)-O-IV 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 (VO)-O-IV 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 (VO2+)-O-IV ion: (VO) Hb(beta) and (VO)Hbc gamma, with (VO2+)-O-IV coordinated to the sites beta and gamma of hemoglobin, and VO(L-1, L-2) and VO(L-3, L-4), where L-1, L-2, L-3 and L-4 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.