Studies on the zinc binding site to the serum thymic factor.

Gel filtration studies of 65Zn2+ binding to thymulin show that the nonapeptide can strongly bind one zinc metal ion. At pH 7.5, thymulin binds one zinc ion with an apparent affinity constant Kd of 5 +/- 2 X 10(-7) M. Binding is pH dependent. No binding is observed below pH 6.0. Ga3+, Al3+, Mn2+ and Cu2+ can compete with the binding of Zn2+ at pH 7.5. A good correlation between the competition potencies of metal ions used and the extent of biological activity of thymulin in the presence of these metal ions in an in vitro rosette assay is observed. Structural analogs of thymulin and non-thymulin-related peptides were used in a gel filtration technique to tentatively define the nature of amino acids present in the Zn2+-binding site of thymulin.