ALUMINUM TRANSPORT IN BLOOD-SERUM - BINDING OF ALUMINUM BY HUMAN TRANSFERRIN IN THE PRESENCE OF HUMAN ALBUMIN AND CITRATE

The binding of Al3+ by human serum transferrin has been investigated by u.v.-visible difference spectroscopy. In the presence of 25 mM-HCO3- at pH 7.4, the apparent association constants were found to be 1.69 x 10(12) M-1 and 5.36 x 10(11) M-1. These association constants are pH-dependent, reducing with both increasing and decreasing pH. The apparent pK(a) values were found to be 6.7 and 8.2. Competitive assays of binding of Al3+ to transferrin in the presence of citrate and human serum albumin at molar ratios corresponding to those found in normal plasma showed that a considerable amount of Al3+ was not bound to transferrin. Taking a concentration of 5-mu-M as a typical value observed for the plasma of patients on haemodialysis [Harris & Sheldon (1990) Inorg. Chem. 29, 119-124] the competitive binding assays indicate that approximately 60% of it is bound to transferrin, approximately 34% to albumin and the remainder to citrate. These results therefore suggest that, although transferrin at pH 7.4 is the major Al3+-binding component of plasma, an appreciable amount of A13+ present in patients on haemodialysis may be bound to albumin.