Competition among low molecular mass ligands and serum transferrin for the binding of aluminum

摘要

There is a substantial agreement that aluminum is associated with a variety of neurological disorders. Considerable attention has been given to defining the chemical speciation of aluminum ion in serum. One of the key issues with respect to aluminum speciation is how well the low molecular mass ligands in serum 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 among these low molecular mass ligands. In this study, the protein transferrin is used as the reference competitive ligand and effective binding constants for citrate-transferrin, phosphate-transferrin, Al-citrate and Al-phosphate at pH 7.4 are measured by difference-ultraviolet spectroscopy. These effective binding constants are compared with formal aluminum stability constants determined by potentiometric methods. This comparison suggests that the dominant monomeric species at neutral pH is Al(H-1Cit)-, rather than Al(H-1Cit)(OH)2-. The effective binding constants for the 1:2 Al-phosphate complex is 1012.02. No 1:1 Al-phosphate complex was detected. These effective binding constants together with the binding constants for Al-fluoride complexes have been used to perform speciation calculations. The results indicate that 98% of total aluminum is bound to transferrin. Within the low molecular mass pool, more than 46% of low molecular mass aluminum is present as Al(H-1Cit)-, and only 8% as Al(PO4)2 3-. Clearly, citrate is the primary low molecular mass ligand for aluminum in serum. The kinetics of aluminum removal from transferrin by citrate has been investigated. Aluminum removal from transferrin shows saturation kinetics, suggesting that a conformation change pathway is involved. For comparison, the kinetics of gallium and indium removal from transferrin has also been investigated. Similar to aluminum, removal of these two metals from transferrin shows saturation kinetics. Gallium release requires more citrate than aluminum and indium release requires less citrate than aluminum to reach saturation.