Speciation of Al-III in blood serum - The Al-III-citrate-phosphate ternary system

Time-dependent speciation studies have been carried out on the ternary Al-III-citrate (A)-phosphate (B) system in order to clarify the solution state of Al-III in blood serum. The potentiometric results indicate that ternary complexes predominate both in freshly prepared mixtures and at equilibrium. The ternary species AlAB and AlABH(-1) and the binary species AlAH(-1) and AlBH-1 are present at physiological pH. As the solution ages the amount of the trinuclear species [Al-3(AH(-1))(3)(OH)](4-) increases at the expense of the mononuclear binary and ternary complexes. P-31 NMR spectra measured at neutral pH provide corroborating evidence of the formation of the ternary complexes ALAB and AlABH(-1) and binary species AlBH-1 and Al2BH-3 found potentiometrically. Time-dependent H-1 NMR measurements show that monodentate phosphate can slowly displace citrate from the otherwise very stable structure of the trinuclear species [Al-3(AH(-1))(3)(OH)](4-). At blood serum concentrations [c(Al-III) = 3 muM], Al-III is mostly bound to citrate either in binary species or in the ternary species formed with phosphate. However, with increasing Al-III concentrations binding to phosphate becomes more important.