Binding of 3,5-diiodotyrosine to serum proteins.

The reversible binding of 3,5-diiodotyrosine (DIT) to human and bovine serum protein and to purified human serum prealbumin and human and bovine albumin has been studied by equilibrium dialysis. Maximum binding occurred at pH 8.6-9.0. Human serum bound DIT less than did bovine serum. Adult ox and fetal calf sera showed similar binding. The main DIT-binding protein of human serum was prealbumin. It showed a single affinity site with a Ka of 0.85 X 10(6) M-1 at pH 8.6 and 0.40 X 10(6) M-1 at pH 7.4. The affinity constant of serum albumin for DIT was 2.8 X 10(3) M-1 at pH 8.6. The elevated binding of DIT to bovine serum is essentially due to albumin whose affinity constant for DIT is 16-times higher than that of human serum albumin. Fetuin was not responsible for any noticeable DIT binding in fetal calf serum.