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 degrees C and 0.20 mol dm(-3) KCl by pH-metric and V-51-NMR methods. At pH 7.4, the two predominant compounds with both ligands are the VO2L2 and VO2L(OH). NH4[VO2(hpno)(2)]center dot 3H(2)O 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 K-1 = 6.03 +/- 0.10; log K-2 = 5.46 +/- 0.18) determined by V-51-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 V-51-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.