Binding of zinc to amino acids and serum proteins in vitro

Binding of zinc to amino acids and serum proteins was studied in vitro. Following incubation of zinc-65 with pooled native human serum in vitro, ultrafiltrable zinc was determined to be 2 to 8 per cent of the total serum zinc, when the zinc:albumin molar ratio was varied from 0.33 to 2.5. Under similar conditions 0.2 to 1.2 per cent of zinc was ultrafiltrable when predialyzed serum was used. In physiological concentrations, additions of amino acids to predialyzed serum increased ultrafiltrable zinc-65 severalfold. Histidine, glutamine, threonine, cystine, and lysine showed the most marked effects in this regard. It is suggested that the amino acid-bound fraction of zinc may have an important role in biologic transport of this element. By means of starch-block electrophoresis of predialyzed serum, the stable zinc content was determined to be highest in the albumin fraction although smaller concentrations of zinc were found in the gamma, beta, and alpha globulins as well. Our results obtained by using ⁶⁵Zn-incubated predialyzed serum, however, indicated a difference in the behavior of exogenous zinc as compared with the endogenous zinc bound to various serum proteins. In vitro studies with the use of predialyzed albumin, haptoglobin, ceruloplasmin, alpha-2 macroglobulin, transferrin, and IgG, incubated with zinc-65, revealed that zinc was bound to all the above proteins and that the binding of zinc to IgG was electrostatic in nature. Whereas amino acids competed effectively with albumin, haptoglobin, transferrin, and IgG for binding of zinc, a similar phenomenon was not observed with respect to ceruloplasmin and alpha-2 macroglobulin, suggesting that the latter two proteins exhibited special binding property for zinc.