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.