The chemical speciation of aluminium in milk

Competition between transferrin and the serum ligands citrate and phosphate for the binding of aluminum

A key issue regarding the speciation of Al(3+) in serum is how well the ligands citric acid and phosphate can compete with the iron transport protein serum transferrin for the aluminum. Previous studies have attempted to measure binding constants for each ligand separately, but experimental problems make it very difficult to obtain stability constants with the accuracy required to make a meaningful comparison between these ligands. In this study, effective binding constants for Al-citrate and Al-phosphate at pH 7.4 have been determined using difference UV spectroscopy to monitor the direct competition between these ligands and transferrin. The analysis of this competition equilibrium also includes the binding of citrate and phosphate as anions to apotransferrin. The effective binding constants are 10(11.59) for the 1:1 Al-citrate complexes and 10(14.90) for the 1:2 Al-citrate complexes. The effective binding constant for the 1:2 Al-phosphate complex is 10(12.02). No 1:1 Al-phosphate complex was detected. Speciation calculations based on these effective binding constants indicate that, at serum concentrations of citrate and phosphate, citrate will be the primary low-molecular-mass ligand for aluminum. Formal stability constants for the Al-citrate system have also been determined by potentiometric methods. This equilibrium system is quite complex, and information from both electrospray mass spectrometry and difference UV experiments has been used to select the best model for fitting the potentiometric data. The mass spectra contain peaks that have been assigned to complexes having aluminum:citrate stoichiometries of 1:1, 1:2, 2:2, 2:3, and 3:3. The difference UV results were used to determine the stability constant for Al(H(-1)cta)-, which was then used in the least-squares fitting of the potentiometric data to determine stability constants for Al(Hcta)+, Al(cta), Al(cta)2(3-), Al(H(-1)cta)(cta)(4-), Al2(H(-1)cta)2(2-), and Al3(H(-1)cta)3(OH)(4-).

Aluminum speciation studies in biological fluids. Part 4. A new investigation of aluminum-succinate complex formation under physiological conditions, and possible implications for aluminum metabolism and toxicity

Previous in vivo studies devoted to the capacity of succinate to influence aluminum metabolism have led to apparent contradictory results. Understanding the mechanisms that lie behind such discrepancies requires a knowledge of aluminum-succinate interactions at the molecular level. In the absence of possible direct analysis of the ultrafiltrable fraction of aluminum in vivo, computer simulations can help quantify the mobilizing power of succinate towards aluminum in the main biofluids. Based on this technique, a first attempt to elucidate the above issue was made using especially determined aluminum-succinate formation constants. However, further investigations have led to reconsider the stoichiometry of the aluminum-succinate complexes characterized on that occasion. The present work deals with these new investigations. The results obtained confirm the great complexity of the aluminum-succinate system. No less than seven species, among which five polynuclear complexes, have been characterized in two series of independent experiments. New simulations indicate that succinate is expected to facilitate aluminum gastrointestinal absorption to a greater extent than initially predicted when the metal is administered as its trihydroxide, especially at high concentrations of the metal. In contrast, succinate is not able to significantly increase aluminum absorption when ingested concomitantly with aluminum phosphate. It is also confirmed that succinate cannot influence the fate of aluminum in blood plasma, which supports the view that the protective effect of succinate against aluminum toxicity in mice is not due to aluminum complexation.