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A stability constant at pH 7.4 for the 1:1 zinc-human serum albumin complex was determined. The effectiveness with which amino acids compete with human serum albumin for zinc was shown to depend upon the relevant metal complex stability constants. Studies by gel chromatography of model systems and computations involving stability constants indicate that cysteine and histidine are the only important amino acid ligands of zinc in plasma. The amount of amino acid-complexed zinc in normal human serum is calculated to be 1 μg zinc per 100 ml. The small ligand-complexed fraction in plasma may be of importance in zinc transport and probably accounts for most urinary zinc excreted in normal and pathological conditions.

Dissociation of metal-protein complexes of copper, zinc, and cadmium in human serum by histidine and glycine was studied. Distribution of metal between protein-complexed and amino acid-complexed species as a function of amino acid concentration is similar for solutions of human serum albumin and native human serum, indicating that human serum albumin and the macromolecular ligands which complex loosely bound serum copper, zinc, and cadmium have equal avidity for these metal ions. Copper-albumin complex is the least stable, cadmium-albumin complex the most stable, with respect to dissociation by histidine.