Interactions between the histidine stimulation of cadmium and zinc influx into human erythrocytes.

1. Histidine (2‐40 mM) stimulated cadmium uptake into human erythrocytes incubated in the presence of 1% bovine serum albumin to ensure that the free, ionic cadmium concentration was low. 2. The histidine‐stimulated cadmium uptake correlated with the calculated concentration of the cadmium‐bis‐histidine complex rather than the cadmium‐mono‐histidine complex or free ionic cadmium. 3. The histidine stimulation of cadmium uptake was saturable and stereospecific. D‐Histidine (10 mM) had no effect. 4. Cadmium and zinc were both able to inhibit 65Zn2+ uptake into erythrocytes incubated in the presence of 40 mM L‐histidine. The relationships between the percentage inhibition of 65Zn2+ uptake and the calculated concentrations of cadmium‐bis‐histidine and zinc‐bis‐histidine were very similar, which suggests that the metal histidine complexes compete for a common transport mechanism. 5. Pretreatment of the erythrocytes with N‐ethylmaleimide using a protocol which is known to inhibit the system y+ amino acid transport mechanism had no effect on the histidine stimulation of metal transport.