Cadmium transport in blood serum

The binding of Cd2+ to human serum transferrin in 0.1 M N-(2-hydroxyethyl) piperazine-N'-2-ethanesulfonic acid and 5 mM sodium bicarbonate at pH 7.4 has been studied by difference ultraviolet spectrophotometry. The apparent association constants were found to be 2.61 x 10(5)M(-1) and 8.51 x 10(4)M(-1), respectively. These association constants are pH-dependent, reducing with both increasing and decreasing pH. The apparent pK(a) values were found to be 4.93 and 5.42. Competitive assays of binding of Cd2+ to transferrin in the presence of citrate and human serum albumin at molar ratios corresponding to those found in normal plasma showed that a considerable amount of Cd2+ was not bound to transferrin. The competitive binding assays indicate that similar to 50% of Cd2+ is bound to transferrin, similar to 37% to albumin and reminder to citrate. These results therefore suggest that, although transferrin at pH 7.4 is the major Cd2+-binding component of plasma, an appreciable amount of Cd2+ may be bound to albumin.