Citrate as the main small molecule binding Al3+ in serum.

A recent proposal (Clin Chem 1992;38:1809-18) that inorganic phosphate is the predominant small molecule binding Al3+ in blood serum is shown to be incorrect. Experiments in which Al3+ is added to solutions containing phosphate and citrate in the same proportions as in serum clearly show that citrate prevents formation of any aluminum phosphate precipitate, and analysis of these data indicates that the assumed binding constants for soluble phosphate are two orders of magnitude too strong. Consideration of the effect of competitive binding of alkaline earth metal ions to citrate on Al3+ binding and of a set of transferrin-Al3+ stability constants leads to the conclusion that the proportions are in close agreement with previously published results. We conclude on the basis of stability constants that, of the Al3+ in blood serum, approximately 89% (+/- 5%) binds to transferrin and approximately 11% (+/- 5%) to citrate.