Human alpha-fetoprotein and albumin: differences in zinc binding.

The binding of zinc to both human alpha-fetoprotein (AFP) and albumin isolated from cord serum was studied by Sephadex G-50 gel-filtration chromatography. We found that the total number of binding sites for zinc on AFP and albumin were approximately 16 and 12, respectively. Both graphical analysis and the computer program 'LIGAND' indicate that there are at least two major classes of binding sites for both proteins. Both methods of analysis suggested that there are four to five high-affinity sites for zinc on AFP and only two to three similar sites on albumin. The affinity of zinc for AFP (dissociation constant, Kd, 6-8 X 10(-6) mol/l) was higher than for albumin (Kd, 1-3 X 10(-5) mol/l) for the high-affinity sites. The estimates for the zinc low-affinity binding sites were more uncertain, and several classes of low-affinity binding sites of different affinities might be present in both proteins. The results of our inhibition studies suggest that calcium, copper and lead might also bind with AFP at the zinc-binding sites.