Vanadate complexes in serum: a speciation modeling study

The speciations of two drug candidate ligands, 2-hydroxypyridine-N-oxide (Hhpno) and 2-mercaptopyridine-N-oxide (Hmpno), with vanadate (V^V) were determined at 25.0 °C and 0.20 mol dm⁻³ KCl by pH-metric and ⁵¹V-NMR methods. At pH 7.4, the two predominant compounds with both ligands are the VO₂L₂ and VO₂L(OH). NH₄[VO₂(hpno)₂]·3H₂O was prepared in solid form, and its crystal structure was determined by X-ray diffraction. The stabilities of the complexes VO₂L₂ of five drug candidate ligands were compared at pH 7.4. In view of the stability sequence hpno > maltol hdp (Hhdp: 3-hydroxy-1,2-dimethyl-4-pyridinone) ≫ mpno > picolinic acid, the first two of these ligands were chosen for equilibrium studies with apotransferrin (apoTf) competition. The V^V-apoTf stability constants (log K₁ = 6.03 ± 0.10; log K₂ = 5.46 ± 0.18) determined by ⁵¹V-NMR spectroscopy were confirmed by ultrafiltration. Both methods proved that there seems to be no hydrogencarbonate–vanadate competition for the apoTf anion-binding positions. The other potential high molecular mass V^V binder in the serum is human serum albumin (HSA). As no interaction was detected by ⁵¹V-NMR spectroscopy or fluorimetry, the binding properties of HSA were quantified on the basis of literature data. As a final conclusion, speciation modeling calculations suggest that, under serum conditions, apoTf is probably the primary metal ion binder, even in the presence of the most stable V^V carrier ligands hpno and maltol and HSA plays a negligible role in V^V binding.